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== Chapter 1 ==
<pre>
                       Programming in C _ A Tutorial

                             Brian W. Kernighan

                   Bell Laboratories, Murray Hill, N. J.







        1
        _.  Introduction
            ____________

             C is a computer language available on the and operating
        systems  at  Murray Hill and (in preliminary form) on OS/360
        at Holmdel.  C lets you write your programs clearly and sim-
        ply _ it has decent control flow facilities so your code can
        be read straight down the page, without labels or GOTO's; it
        lets  you write code that is compact without being too cryp-
        tic; it encourages modularity and good program organization;
        and it provides good data-structuring facilities.

             This memorandum is a tutorial to  make  learning  C  as
        painless  as  possible.   The first part concentrates on the
        central features of C; the second part discusses those parts
        of  the  language which are useful (usually for getting more
        efficient and smaller code) but which are not necessary  for
        the  new user.  This is not
                                ___ a reference manual.  Details and
        special cases will be skipped  ruthlessly,  and  no  attempt
        will  be made to cover every language feature.  The order of
        presentation is hopefully pedagogical  instead  of  logical.
        Users  who  would  like  the full story should consult the C
                                                                   _
        Reference
        _________ Manual
                  ______ by D. M. Ritchie [1], which should be  read
        for details anyway.  Runtime support is described in [2] and
        [3]; you will have to read one of these to learn how to com-
        pile and run a C program.

             We will assume that you are familiar with the mysteries
        of creating files, text editing, and the like in the operat-
        ing system you run on, and that you have programmed in  some
        language before.

        2
        _.  A
            _ Simple
              ______ C
                     _ Program
                       _______

             main( ) {
             printf("hello, world"); }

             A C program consists of one or  more  functions
                                                   _________,  which
        are  similar  to  the functions and subroutines of a Fortran
        program or the procedures of PL/I, and perhaps some external
        data  definitions.   is  such  a function, and in fact all C
        programs must have a Execution of the program begins at  the
        first  statement  of  will usually invoke other functions to
        perform its job, some coming from the same program, and oth-
        ers from libraries.

             One method of communicating data between  functions  is
        by  arguments.   The parentheses following the function name
        surround the argument list; here is a function of  no  argu-
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                                   - 2 -



        ments, indicated by ( ).  The {} enclose the  statements  of
        the  function.   Individual  statements end with a semicolon
        but are otherwise free-format.

             is a library function which will format and print  out-
        put on the terminal (unless some other destination is speci-
        fied).  In this  case it prints hello, world A  function  is
        invoked  by  naming  it,  followed by a list of arguments in
        parentheses.  There is no statement as in Fortran or

        3
        _.   A
             _  Working
                _______  C
                         _  Program
                            _______;  Variables
                                      _________;  Types
                                                  _____   and
                                                          ___   Type
                                                                ____
        Declarations
        ____________

             Here's a bigger program that adds  three  integers  and
        prints their sum.  main( ) {
             int a, b, c, sum;
             a = 1;  b = 2;  c = 3;
             sum = a + b + c;
             printf("sum is %d", sum); }

             Arithmetic and the assignment statements are  much  the
        same as in Fortran (except for the semicolons) or The format
        of C programs is quite free.  We can put several  statements
        on  a  line  if  we  want, or we can split a statement among
        several lines if  it  seems  desirable.  The  split  may  be
        between  any  of  the operators or variables, but not
                                                          ___ in the
        middle of a name or operator.  As a matter of style, spaces,
        tabs,  and  newlines should be used freely to enhance reada-
        bility.

             C has four fundamental types
                                    _____ of variables:

                int  integer (PDP-11: 16 bits; H6070: 36 bits; IBM360: 32 bits)
                char one byte character (PDP-11, IBM360: 8 bits; H6070: 9 bits)
                float     single-precision floating point
                double    double-precision floating point

        There are also arrays
                       ______ and structures
                                  __________ of these  basic  types,
        pointers
        ________  to  them  and  functions
                                 _________  that return them, all of
        which we will meet shortly.

             All
             ___ variables in a C program must be declared, although
        this  can sometimes be done implicitly by context.  Declara-
        tions must precede executable statements.   The  declaration
        int a, b, c, sum; declares and to be integers.

             Variable names have one  to  eight  characters,  chosen
        from  A-Z,  a-z,  0-9,  and (ul, and start with a non-digit.
        Stylistically, it's much better to use only  a  single  case
        and  give  functions  and  external variables names that are
        unique in the first six characters.  (Function and  external
        variable names are used by various assemblers, some of which
        are limited in the size and case  of  identifiers  they  can
        handle.)  Furthermore,  keywords  and  library functions may



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                                   - 3 -



        only be recognized in one case.

        4
        _.  Constants
            _________

             We have already seen decimal integer constants  in  the
        previous  example  _ 1, 2, and 3.  Since C is often used for
        system programming and bit-manipulation, octal  numbers  are
        an  important  part  of the language.  In C, any number that
        begins with 0 (zero!) is an octal integer (and  hence  can't
        have any 8's or 9's in it).  Thus 0777 is an octal constant,
        with decimal value 511.

             A ``character'' is one  byte  (an  inherently  machine-
        dependent  concept).   Most  often  this  is  expressed as a
        character
        _________ constant
                  ________, which is one character enclosed in  sin-
        gle  quotes.  However, it may be any quantity that fits in a
        byte, as in below: char quest, newline, flags; quest =  '?';
        newline = '\n'; flags = 077;

             The sequence `\n' is C notation for  ``newline  charac-
        ter'', which, when printed, skips the terminal to the begin-
        ning of the next line.  Notice that `\n' represents  only  a
        single  character.  There are several other ``escapes'' like
        `\n'  for representing hard-to-get or invisible  characters,
        such  as  `\t'  for tab, `\b' for backspace, `\0' for end of
        file, and `\\' for the backslash itself.

             and constants are discussed in section 26.

        5
        _.  Simple
            ______ I
                   _/O
                     _ _ getchar
                         _______, putchar
                                  _______, printf
                                           ______

             main( ) {
             char c;
             c = getchar( );
             putchar(c); }

             and are the basic I/O library functions in C.   fetches
        one character from the standard input (usually the terminal)
        each time it is called, and returns that  character  as  the
        value  of the function.  When it reaches the end of whatever
        file it is reading,  thereafter  it  returns  the  character
        represented  by  `\0' (ascii which has value zero).  We will
        see how to use this very shortly.

             puts one character out on the standard output  (usually
        the  terminal) each time it is called.  So the program above
        reads one character and writes it back out.  By itself, this
        isn't  very  interesting,  but observe that if we put a loop
        around this, and add a test for end of file, we have a  com-
        plete program for copying one file to another.

             is a more complicated function for producing  formatted
        output.   We  will  talk  about only the simplest use of it.
        Basically, uses its first argument  as  formatting  informa-



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                                   - 4 -



        tion, and any successive arguments as variables to  be  out-
        put.   Thus printf ("hello, world\n"); is the simplest use _
        the string ``hello, world\n'' is printed out.  No formatting
        information,  no variables, so the string is dumped out ver-
        batim.  The newline is necessary to put this out on  a  line
        by  itself.  (The construction "hello, world\n" is really an
        array of More about this shortly.)

             More complicated, if is 6, printf ("sum is %d\n", sum);
        prints  sum is 6 Within the first argument of the characters
        ``%d'' signify that the next argument in the  argument  list
        is to be printed as a base 10 number.

             Other useful formatting commands are  ``%c''  to  print
        out  a  single  character,  ``%s''  to  print  out an entire
        string, and ``%o'' to print a number as octal instead of de-
        cimal  (no  leading  zero).   For  example,  n = 511; printf
        ("What is the value of %d in octal?", n); printf ("  %s!  %d
        decimal is %o octal\n", "Right", n, n); prints
        What is the value of 511 in octal?  Right!  511  decimal  is
        777  octal Notice that there is no newline at the end of the
        first output line.  Successive calls  to  (and/or  for  that
        matter)  simply put out characters.  No newlines are printed
        unless you ask for them.  Similarly,  on  input,  characters
        are  read  one  at a time as you ask for them.  Each line is
        generally terminated by a newline (\n), but there is  other-
        wise no concept of record.












                              October 10, 1975



</pre>
== Chapter 2 ==
<pre>
        1
        _.  If
            __; relational
                __________ operators
                           _________; compound
                                      ________ statements
                                               __________

             The basic conditional-testing statement  in  C  is  the
        statement: c = getchar( ); if( c  '?' )
             printf("why did you type a question mark?\n"); The sim-
        plest form of is if (expression) statement

             The condition to be tested is any  expression  enclosed
        in parentheses.  It is followed by a statement.  The expres-
        sion is evaluated, and if its value is non-zero, the  state-
        ment  is  executed.   There's  an  optional  clause,  to  be
        described soon.

             The character sequence `=='  is one of  the  relational
        operators in C; here is the complete set:
             equal  to  (EQ  to  Fortraners)   !=   not   equal   to
        >    greater  than <    less than >=   greater than or equal
        to <=   less than or equal to

             The value of is 1 if the relation is  true,  and  0  if
        false.   Don't forget that the equality test is `=='; a sin-
        gle `=' causes an assignment, not  a  test,  and  invariably
        leads to disaster.

             Tests can be combined with the operators and For  exam-
        ple, we can test whether a character is blank or tab or new-
        line with if( c' '  c'\t'  c'\n' ) C guarantees that and are
        evaluated left to right _ we shall soon see cases where this
        matters.

             One of the nice things about C is that the part  of  an
        can  be  made  arbitrarily complicated by enclosing a set of
        statements in {}.  As a simple example, suppose we  want  to
        ensure  that  is bigger than as part of a sort routine.  The
        interchange of and takes three statements in C, grouped  to-
        gether by {}: if (a < b) {
             t = a;
             a = b;
             b = t; }

             As a general rule in C, anywhere you can use  a  simple
        statement, you can use any compound statement, which is just
        a number of simple or compound ones enclosed in  {}.   There
        is  no  semicolon  after  the } of a compound statement, but
        there is
              __ a semicolon after the last  non-compound  statement
        inside the {}.

             The ability to replace  single  statements  by  complex
        ones  at will is one feature that makes C much more pleasant
        to use than Fortran.  Logic (like the exchange in the previ-
        ous  example)  which would require several GOTO's and labels
        in Fortran can and should be done in C  without  any,  using
        compound statements.





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                                   - 2 -



        2
        _.  While
            _____ Statement
                  _________; Assignment
                             __________ within
                                        ______ an
                                               __  Expression
                                                   __________;  Null
                                                                ____
        Statement
        _________

             The basic looping mechanism  in  C  is  the  statement.
        Here's a program that copies its input to its output a char-
        acter at a time.  Remember that `\0' marks the end of  file.
        main( ) {
             char c;
             while( (c=getchar( )) != '\0' )
                  putchar(c); } The statement is a loop, whose  gen-
        eral form is while (expression) statement Its meaning is (a)
        evaluate the expression (b) if its value is  true  (ie,  not
        zero)
                  do the statement, and go back to (a)  Because  the
        expression  is  tested before the statement is executed, the
        statement part can be executed zero times,  which  is  often
        desirable.   As  in  the  statement,  the expression and the
        statement can both be arbitrarily complicated,  although  we
        haven't  seen that yet.  Our example gets the character, as-
        signs it to and then tests if it's a `\0''.  If it is not  a
        `\0',  the  statement  part of the is executed, printing the
        character.  The then repeats.  When the input  character  is
        finally a `\0', the terminates, and so does

             Notice  that  we  used  an  assignment  statement  c  =
        getchar( ) within an expression.  This is a handy notational
        shortcut which often produces clearer code.  (In fact it  is
        often  the  only way to write the code cleanly.  As an exer-
        cise, re-write the file-copy without using an assignment in-
        side  an  expression.) It works because an assignment state-
        ment has a value, just as any other  expression  does.   Its
        value  is  the  value of the right hand side.  This also im-
        plies that we can use multiple assignments like x = y = z  =
        0; Evaluation goes from right to left.

             By the way, the extra  parentheses  in  the  assignment
        statement  within  the conditional were really necessary: if
        we had said c = getchar( ) != '\0' would be set to  0  or  1
        depending  on  whether  the  character fetched was an end of
        file or not.  This is because in the absence of  parentheses
        the assignment operator `=' is evaluated after the relation-
        al operator `!='.  When in doubt, or  even  if  not,  paren-
        thesize.

             Since returns as its function value, we could also copy
        the  input to the output by nesting the calls to and main( )
        {
             while( putchar(getchar( )) != '\0' ) ; } What statement
        is  being  repeated?   None, or technically, the null
                                                         ____ state-
        ment, because all the work is really done  within  the  test
        part of the This version is slightly different from the pre-
        vious one, because the final `\0' is copied  to  the  output
        before we decide to stop.



                              October 10, 1975




                                   - 3 -



        3
        _.  Arithmetic
            __________

             The arithmetic operators are the usual `+', `(mi', `*',
        and  `/'  (truncating  integer  division if the operands are
        both and the remainder or mod operator `%': x = a%b; sets to
        the  remainder  after  is  divided by (i.e., The results are
        machine dependent unless and are both positive.

             In arithmetic, variables can usually  be  treated  like
        variables.  Arithmetic on characters is quite legal, and of-
        ten makes sense: c = c + 'A' - 'a'; converts a single  lower
        case  ascii character stored in to upper case, making use of
        the fact that corresponding ascii letters are a  fixed  dis-
        tance apart.  The rule governing this arithmetic is that all
        are converted to before the arithmetic is done.  Beware that
        conversion  may involve sign-extension _ if the leftmost bit
        of a character is 1, the resulting integer  might  be  nega-
        tive.   (This  doesn't happen with genuine characters on any
        current machine.)

             So to convert a file into lower case: main( ) {
             char c;
             while( (c=getchar( )) != '\0' )
                  if( 'A'<=c && c<='Z' )
                       putchar(c+'a'-'A');
                  else
                       putchar(c); } Characters have different sizes
        on  different machines.  Further, this code won't work on an
        IBM machine, because the letters in the ebcdic alphabet  are
        not contiguous.

        4
        _.  Else
            ____ Clause
                 ______; Conditional
                         ___________ Expressions
                                     ___________

             We just used an after an The most general form of is if
        (expression)  statement1 else statement2 the part is option-
        al, but often useful.  The canonical  example  sets  to  the
        minimum of and if (a < b)
             x = a; else
             x = b; Observe that there's a semicolon after

             C provides an alternate form of  conditional  which  is
        often  more concise.  It is called the ``conditional expres-
        sion'' because it is a  conditional  which  actually  has  a
        value and can be used anywhere an expression can.  The value
        of a<b ? a : b; is if is less than it is otherwise.  In gen-
        eral,  the form expr1 ? expr2 : expr3 means ``evaluate If it
        is not zero, the value of the whole thing is  otherwise  the
        value is

             To set to the minimum of and then: x = (a<b ? a  :  b);
        The parentheses aren't necessary because is evaluated before
        `=', but safety first.

             Going a step further, we could write the  loop  in  the



                              October 10, 1975




                                   - 4 -



        lower-case program as while( (c=getchar( )) != '\0' )
             putchar( ('A'<=c && c<='Z') ? c-'A'+'a' : c );

             and can be used to construct logic that branches one of
        several  ways  and then rejoins, a common programming struc-
        ture, in this way: if()
             {} else if()
             {} else if()
             {} else
             {} The conditions are tested in order, and exactly  one
        block is executed _ either the first one whose is satisfied,
        or the one for the last When this  block  is  finished,  the
        next  statement executed is the one after the last If no ac-
        tion is to be taken for the ``default'' case, omit the last

             For example, to count letters, digits and others  in  a
        file, we could write main( ) {
             int let, dig, other, c;
             let = dig = other = 0;
             while( (c=getchar( )) != '\0' )
                  if( ('A'<=c && c<='Z')  ('a'<=c && c<='z') )  let;
                  else if( '0'<=c && c<='9' )  dig;
                  else  other;
             printf("%d letters, %d digits, %d others\n", let,  dig,
        other);  }  The  `++'  operator means ``increment by 1''; we
        will get to it in the next section.



















                              October 10, 1975



</pre>
== Chapter 3 ==
<pre>
        1
        _.  Increment
            _________ and
                      ___ Decrement
                          _________ Operators
                                    _________

             In addition to the usual `(mi', C also  has  two  other
        interesting  unary  operators, `++' (increment) and `(mi(mi'
        (decrement).  Suppose we want to count the lines in a  file.
        main( ) {
             int c,n;
             n = 0;
             while( (c=getchar( )) != '\0' )
                  if( c  '\n' )
                       n;
             printf("%d lines\n", n); } is equivalent to but  clear-
        er, particularly when is a complicated expression.  `++' and
        `(mi(mi' can be applied only to and (and  which  we  haven't
        got to yet).

             The unusual feature of `++' and `(mi(mi' is  that  they
        can be used either before or after a variable.  The value of
        is the value of after
                        _____ it has been incremented.  The value of
        is before
           ______ it is incremented.  Suppose is 5.  Then x = k; in-
        crements to 6 and then sets to the resulting value, i.e., to
        6.  But x = k; first sets to to 5, and then
                                               ____ increments to 6.
        The incrementing effect of and is the same, but their values
        are  respectively 5 and 6.  We shall soon see examples where
        both of these uses are important.

        2
        _.  Arrays
            ______

             In C, as in Fortran or PL/I, it is possible to make ar-
        rays  whose  elements  are basic types.  Thus we can make an
        array of 10 integers with the  declaration  int  x[10];  The
        square brackets mean subscripting
                             ____________; parentheses are used only
        for function references.  Array indexes begin  at  zero
                                                           ____,  so
        the elements of are x[0], x[1], x[2], , x[9] If an array has
        elements, the largest subscript is

             Multiple-dimension arrays are provided, though not much
        used  above  two  dimensions.   The declaration and use look
        like int name[10] [20]; n = name[i+j]  [1]  +  name[k]  [2];
        Subscripts  can  be  arbitrary  integer expressions.  Multi-
        dimension arrays are stored by row (opposite to Fortran), so
        the  rightmost  subscript varies fastest; has 10 rows and 20
        columns.

             Here is a program which reads a line, stores  it  in  a
        buffer,  and prints its length (excluding the newline at the
        end).  main( ) {
             int n, c;
             char line[100];
             n = 0;
             while( (c=getchar( )) != '\n' ) {
                  if( n < 100 )
                       line[n] = c;
                  n;
             }



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                                   - 2 -



             printf("length = %d\n", n); }

             As a more complicated problem, suppose we want to print
        the  count  for  each  line  in the input, still storing the
        first 100 characters of each line.  Try it  as  an  exercise
        before looking at the solution: main( ) {
             int n, c; char line[100];
             n = 0;
             while( (c=getchar( )) != '\0' )
                  if( c  '\n' ) {
                       printf("%d0, n);
                       n = 0;
                  }
                  else {
                       if( n < 100 ) line[n] = c;
                       n;
                  } }

        3
        _.  Character
            _________ Arrays
                      ______; Strings
                              _______

             Text is usually kept as an array of characters,  as  we
        did with in the example above.  By convention in C, the last
        character in a character array should be a `\0' because most
        programs  that  manipulate  character arrays expect it.  For
        example, uses the `\0' to detect the end of a character  ar-
        ray when printing it out with a `%s'.

             We can copy a character array into another like this:
             i = 0;
             while( (t[i]=s[i]) != '\0' )
                  i;

             Most of the time we have to put in our own `\0' at  the
        end  of  a  string;  if  we want to print the line with it's
        necessary.  This code prints the character count before  the
        line: main( ) {
             int n;
             char line[100];
             n = 0;
             while( (line[n]=getchar( )) != '\n' );
             line[n] = '\0';
             printf("%d:\t%s", n, line); } Here we increment in  the
        subscript itself, but only after the previous value has been
        used.  The character is read, placed in and only then is in-
        cremented.

             There is one place and one place only where C  puts  in
        the  `\0'  at the end of a character array for you, and that
        is in the construction "stuff  between  double  quotes"  The
        compiler  puts  a  `\0'  at the end automatically.  Text en-
        closed in double quotes is called a string
                                            ______;  its  properties
        are precisely those of an (initialized) array of characters.




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                                   - 3 -



        4
        _.  For
            ___ Statement
                _________

             The statement is a somewhat generalized  that  lets  us
        put  the initialization and increment parts of a loop into a
        single statement along with the test.  The general  form  of
        the is for( initialization; expression; increment )
             statement The meaning is exactly
             initialization;
             while( expression ) {
                  statement
                  increment;
             } Thus, the following code does the same array copy  as
        the example in the previous section:
             for( i=0; (t[i]=s[i]) != '\0'; i ); This slightly  more
        ornate example adds up the elements of an array:
             sum = 0;
             for( i=0; i<n; i)
                  sum = sum + array[i];

             In the statement, the initialization can be left out if
        you  want, but the semicolon has to be there.  The increment
        is also optional.  It is not
                                 ___ followed by a  semicolon.   The
        second clause, the test, works the same way as in the if the
        expression is true (not zero) do another loop, otherwise get
        on  with  the  next  statement.  As with the the loop may be
        done zero times.  If the expression is left out, it is taken
        to be always true, so for( ; ; ) and while( 1 ) are both in-
        finite loops.

             You might ask why we use a since it's so  much  like  a
        (You  might also ask why we use a because...) The is usually
        preferable because it keeps the code  where  it's  used  and
        sometimes eliminates the need for compound statements, as in
        this code that zeros a two-dimensional array: for( i=0; i<n;
        i )
             for( j=0; j<m; j )
                  array[i][j] = 0;

        5
        _.  Functions
            _________; Comments
                       ________

             Suppose we want, as part of a larger program, to  count
        the  occurrences of the ascii characters in some input text.
        Let us also map illegal characters (those with value>127  or
        <0)  into  one  pile.   Since this is presumably an isolated
        part of the program, good  practice  dictates  making  it  a
        separate function.  Here is one way: main( ) {
             int hist[129];      / 128 legal chars + 1 illegal group
        /

             count(hist, 128);   / count the letters into hist /
             printf(  );         / comments look like this; use them
        /
                       / anywhere blanks, tabs or newlines could ap-
        pear / } count(buf, size)



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                                   - 4 -



           int size, buf[ ]; {
             int i, c;
             for( i=0; i<=size; i )
                  buf[i] = 0;              / set buf to zero /
             while( (c=getchar( )) != '\0' ) {  / read til eof /
                  if( c > size  c < 0 )
                       c = size;      / fix illegal input /
                  buf[c];
             }
             return; } We have already seen many examples of calling
        a  function,  so  let  us  concentrate on how to define
                                                         ______ one.
        Since has two arguments, we need to declare them, as  shown,
        giving  their  types, and in the case of the fact that it is
        an array.  The declarations of arguments go between
                                                    _______ the  ar-
        gument list and the opening `{'.  There is no need to speci-
        fy the size of the array for it is defined outside of

             The statement simply says to go  back  to  the  calling
        routine.   In fact, we could have omitted it, since a return
        is implied at the end of a function.

             What if we wanted to return a value, say the number  of
        characters read?  The statement allows for this too:
             int i, c, nchar;
             nchar = 0;

             while( (c=getchar( )) != '\0' ) {
                  if( c > size  c < 0 )
                       c = size;
                  buf[c];
                  nchar;
             }
             return(nchar); Any expression  can  appear  within  the
        parentheses.   Here  is a function to compute the minimum of
        two integers: min(a, b)
           int a, b; {
             return( a < b ? a : b ); }


             To copy a character array, we could write the  function
        strcopy(s1, s2)          / copies s1 to s2 /
           char s1[ ], s2[ ]; {
             int i;
             for( i = 0; (s2[i] = s1[i]) != '\0'; i ); } As is often
        the  case,  all the work is done by the assignment statement
        embedded in the test part of the Again, the declarations  of
        the  arguments and omit the sizes, because they don't matter
        to (In the section on pointers, we will see a more efficient
        way to do a string copy.)

             There is a subtlety in function usage  which  can  trap
        the  unsuspecting Fortran programmer.  Simple variables (not
        arrays) are passed in C by ``call by  value'',  which  means
        that  the  called function is given a copy of its arguments,



                              October 10, 1975




                                   - 5 -



        and doesn't know their addresses.  This makes it  impossible
        to change the value of one of the actual input arguments.

             There are two ways out of this dilemma.  One is to make
        special  arrangements to pass to the function the address of
        a variable instead of its value.  The other is to  make  the
        variable  a  global  or external variable, which is known to
        each function by its name.  We will discuss both  possibili-
        ties in the next few sections.

        6
        _.  Local
            _____ and
                  ___ External
                      ________ Variables
                               _________

             If we say f( ) {
             int x;

        } g( ) {
             int x;

        } each is local
                  _____ to its own routine _ the in is unrelated  to
        the  in  (Local  variables  are  also called ``automatic''.)
        Furthermore each local variable in a  routine  appears  only
        when  the  function is called, and disappears
                                           __________ when the func-
        tion is exited.  Local variables have  no  memory  from  one
        call  to  the  next  and must be explicitly initialized upon
        each entry.  (There is a  storage  class  for  making  local
        variables with memory; we won't discuss it.)

             As opposed to local variables, external
                                            ________  variables
                                                      _________  are
        defined  external  to  all  functions, and are (potentially)
        available to all functions.  External storage always remains
        in  existence.  To make variables external we have to define
                                                              ______
        them external to all functions, and, wherever we want to use
        them, make a declaration
                     ___________.  main( ) {
             extern int nchar, hist[ ];

             count( );

        } count( ) {
             extern int nchar, hist[ ];
             int i, c;

        }    int  hist[129];     /    space    for    histogram    /
        int  nchar;         /  character  count  / Roughly speaking,
        any function that wishes to access an external variable must
        contain  an declaration for it.  The declaration is the same
        as others, except for the added keyword  Furthermore,  there
        must  somewhere  be  a  definition
                                __________ of the external variables
        external to all functions.

             External variables can be initialized; they are set  to
        zero  if  not explicitly initialized.  In its simplest form,
        initialization is done by putting the value (which must be a
        constant)    after    the    definition:   int  nchar     0;
        char flag 'f';



                              October 10, 1975




                                   - 6 -



          etc This is discussed further in a later section.

             This ends our discussion of what might  be  called  the
        central  core of C.  You now have enough to write quite sub-
        stantial C programs, and it would probably be a good idea if
        you  paused long enough to do so.  The rest of this tutorial
        will describe some more ornate constructions, useful but not
        essential.

























                              October 10, 1975



</pre>
== Chapter 4 ==
<pre>
        1
        _.  Pointers
            ________

             A pointer
               _______ in C is the address of something.   It  is  a
        rare  case indeed when we care what the specific address it-
        self is, but pointers are a quite common way to get  at  the
        contents  of  something.   The unary operator `&' is used to
        produce the address of an object, if it has one. Thus
             int a, b;
             b = &a; puts the address of into We can't do much  with
        it except print it or pass it to some other routine, because
        we haven't given the right kind of declaration.  But  if  we
        declare  that  is  indeed  a pointer
                                     _______ to an integer, we're in
        good shape:
             int a, b, c;
             b = &a;
             c = b; contains the address of and  means  to  use  the
        value  in  as an address, i.e., as a pointer.  The effect is
        that we get back the contents of albeit  rather  indirectly.
        (It's  always  the  case  that  is the same as if has an ad-
        dress.)

             The most frequent use of pointers in C is  for  walking
        efficiently along arrays.  In fact, in the implementation of
        an array, the array  name  represents  the  address  of  the
        zeroth element of the array, so you can't use it on the left
        side of an expression.  (You can't  change  the  address  of
        something  by  assigning  to  it.)  If  we  say char y; char
        x[100]; is of type pointer to character (although it doesn't
        yet  point anywhere).  We can make point to an element of by
        either of y = &x[0]; y = x; Since is the address of this  is
        legal and consistent.

             Now  gives  More  importantly,   (y+1)     gives   x[1]
        (y+i)     gives x[i] and the sequence
             y = &x[0];
             y; leaves pointing at

             Let's use pointers in a function that computes how long
        a character array is.  Remember that by convention all char-
        acter arrays are terminated  with  a  `\0'.   (And  if  they
        aren't,  this program will blow up inevitably.) The old way:
        length(s)
           char s[ ]; {
             int n;
             for( n=0; s[n] != '\0'; )
                  n;
             return(n); } Rewriting with pointers gives length(s)
           char s; {
             int n;
             for( n=0; s != '\0'; s )
                  n;
             return(n); } You can now see why we have  to  say  what
        kind  of  thing points to _ if we're to increment it with we
        have to increment it by the right amount.




                              October 10, 1975

                                   - 2 -



             The pointer version is more efficient (this  is  almost
        always true) but even more compact is
             for( n=0; s != '\0'; n ); The returns a character;  the
        increments  the pointer so we'll get the next character next
        time around.  As you can see, as we make things  more  effi-
        cient,  we  also  make  them less clear.  But is an idiom so
        common that you have to know it.

             Going a step further, here's our function that copies a
        character array to another strcopy(s,t)
           char s, t; {
             while(t = s); } We have omitted the test against  `\0',
        because  `\0'  is  identically  zero; you will often see the
        code this way.  (You must
                             ____ have a space after  the  `=':  see
        section 25.)

             For arguments to a function, and there  only,  the  de-
        clarations  char s[ ]; char s; are equivalent _ a pointer to
        a type, or an array of unspecified size of  that  type,  are
        the same thing.

             If this all seems mysterious, copy  these  forms  until
        they  become  second  nature.  You don't often need anything
        more complicated.

        2
        _.  Function
            ________ Arguments
                     _________

             Look back at the function in the previous section.   We
        passed  it  two string names as arguments, then proceeded to
        clobber both of them by  incrementation.   So  how  come  we
        don't lose the original strings in the function that called

             As we said before, C is a ``call by  value''  language:
        when  you  make a function call like the value
                                                 _____ of is passed,
        not its address.  So there's no way to alter
                                               _____ from inside  If
        is an array this isn't a problem, because is
                                                  __ an address any-
        way, and you're not trying to change it, just  what  it  ad-
        dresses.  This is why works as it does.  And it's convenient
        not to have to worry about making temporary  copies  of  the
        input arguments.

             But what if is a scalar and you do want to  change  it?
        In  that  case,  you have to pass the address
                                              _______ of to and then
        use it as a pointer.  Thus for example, to  interchange  two
        integers, we must write flip(x, y)
           int x, y; {
             int temp;
             temp = x;
             x = y;
             y = temp; } and to call we have to pass  the  addresses
        of the variables: flip (&a, &b);





                              October 10, 1975




                                   - 3 -



        3
        _.  Multiple
            ________ Levels
                     ______ of
                            __ Pointers
                               ________; Program
                                         _______ Arguments
                                                 _________

             When a C program is called, the arguments on  the  com-
        mand line are made available to the main program as an argu-
        ment count and an array of character strings containing  the
        arguments.   Manipulating these arguments is one of the most
        common uses of multiple levels  of  pointers  (``pointer  to
        pointer to ...'').  By convention, is greater than zero; the
        first argument (in is the command name itself.

             Here is a program that  simply  echoes  its  arguments.
        main(argc, argv)
           int argc;
           char argv; {
             int i;
             for( i=1; i < argc; i )
                  printf("%s ", argv[i]);
             putchar('\n'); } Step by step: is called with two argu-
        ments,  the argument count and the array of arguments.  is a
        pointer to an array, whose individual elements are  pointers
        to arrays of characters.  The zeroth argument is the name of
        the command itself, so we start to print with the first  ar-
        gument,  until  we've printed them all.  Each is a character
        array, so we use a in the

             You will sometimes see the declaration  of  written  as
        char argv[ ]; which is equivalent.  But we can't use because
        both dimensions are variable and there would be  no  way  to
        figure out how big the array is.

             Here's a bigger example using and A  common  convention
        in C programs is that if the first argument is `(mi', it in-
        dicates a flag of some sort.  For example, suppose we want a
        program  to  be  callable  as  prog -abc arg1 arg2 where the
        `(mi' argument is optional; if it is present, it may be fol-
        lowed by any combination of a, b, and c.  main(argc, argv)
           int argc;
           char argv; {

             aflag = bflag = cflag  = 0;
             if( argc > 1 && argv[1][0]  '-' ) {
                  for( i=1; (c=argv[1][i]) != '\0'; i )
                       if( c'a' )
                            aflag;
                       else if( c'b' )
                            bflag;
                       else if( c'c' )
                            cflag;
                       else
                            printf("%c?\n", c);
                  --argc;
                  argv;
             }




                              October 10, 1975




                                   - 4 -



             There are several  things  worth  noticing  about  this
        code.   First,  there  is  a real need for the left-to-right
        evaluation that && provides; we don't want to look at unless
        we know it's there.  Second, the statements
             --argc;
             argv; let us march along the argument list by one posi-
        tion,  so  we  can  skip over the flag argument as if it had
        never existed _ the rest of the program  is  independent  of
        whether  or  not there was a flag argument.  This only works
        because is a pointer which can be incremented.

        4
        _.  The
            ___ Switch
                ______ Statement
                       _________; Break
                                  _____; Continue
                                         ________

             The statement can be used to replace the multi-way test
        we  used in the last example.  When the tests are like this:
        if( c  'a' ) else if( c  'b' ) else if( c  'c' ) else  test-
        ing a value against a series of constants
                                        _________, the switch state-
        ment is often clearer and usually gives better code.  Use it
        like this: switch( c ) {

        case 'a':
             aflag;
             break; case 'b':
             bflag;
             break; case 'c':
             cflag;
             break; default:
             printf("%c?\n", c);
             break; } The statements label the  various  actions  we
        want;  gets  done  if none of the other cases are satisfied.
        (A is optional; if it isn't there, and  none  of  the  cases
        match, you just fall out the bottom.)

             The statement in this example is new.  It is there  be-
        cause  the  cases  are  just labels, and after you do one of
        them, you fall
                  ____ through
                       _______ to the next unless you take some  ex-
        plicit  action to escape.  This is a mixed blessing.  On the
        positive side, you can  have  multiple  cases  on  a  single
        statement;  we might want to allow both upper and lower case
        letters in our flag field, so we could say case  'a':   case
        'A':      case 'b':  case 'B':
         etc But what if we just want to get out after doing  ?   We
        could  get  out  of  a of the with a label and a but this is
        really ugly.  The statement lets us exit without  either  or
        label.  switch( c ) {
        case 'a':
             aflag;
             break; case 'b':
             bflag;
             break;

        } / the break statements get us here directly /  The  state-
        ment  also  works in and statements _ it causes an immediate



                              October 10, 1975




                                   - 5 -



        exit from the loop.

             The statement works only
                                 ____ inside and it causes the  next
        iteration  of the loop to be started.  This means it goes to
        the increment part of the and the test part of the We  could
        have used a in our example to get on with the next iteration
        of the but it seems clearer to use instead.

































                              October 10, 1975



</pre>
== Chapter 5 ==
<pre>
        1
        _.  Structures
            __________

             The main use of structures is to lump together  collec-
        tions  of disparate variable types, so they can conveniently
        be treated as a unit.  For example, if  we  were  writing  a
        compiler or assembler, we might need for each identifier in-
        formation like its name (a character array), its source line
        number  (an  integer),  some  type information (a character,
        perhaps), and probably a usage count (another integer).
             char id[10];
             int  line;
             char type;
             int  usage;

             We can make a structure out of this quite  easily.   We
        first  tell  C  what  the structure will look like, that is,
        what kinds of things it contains; after that we can actually
        reserve  storage  for  it,  either  in the same statement or
        separately.  The simplest thing is to define it and allocate
        storage all at once: struct {
             char id[10];
             int  line;
             char type;
             int  usage; } sym;

             This defines to  be  a  structure  with  the  specified
        shape;  and  are members
                         _______ of the structure.  The way we refer
        to   any   particular   member   of   the    structure    is
        structure(hyname  member as in
             symtype = 077;
             if( symusage  0 )
             while( symid[j] )
                etc Although the names of  structure  members  never
        stand  alone,  they still have to be unique _ there can't be
        another or in some other structure.

             So far we  haven't  gained  much.   The  advantages  of
        structures  start to come when we have arrays of structures,
        or when we want to pass  complicated  data  layouts  between
        functions.   Suppose we wanted to make a symbol table for up
        to 100 identifiers.  We could extend our definitions like
             char id[100][10];
             int  line[100];
             char type[100];
             int  usage[100]; but a structure lets us rearrange this
        spread-out  information so all the data about a single iden-
        tifer is collected into one lump: struct {
             char id[10];
             int  line;
             char type;
             int  usage; } sym[100]; This makes an array  of  struc-
        tures;  each  array element has the specified shape.  Now we
        can refer to members as
             sym[i]usage;   / increment usage of i(hyth identifier /
             for( j=0; sym[i]id[j] != '\0'; )



                              October 10, 1975

                                   - 2 -



                etc Thus to print a list  of  all  identifiers  that
        haven't been used, together with their line number,
             for( i=0; i<nsym; i )
                  if( sym[i]usage  0 )
                       printf("%d\t%s\n", sym[i]line, sym[i]id);

             Suppose we now want to write a function which will tell
        us  if  already  exists  in  by giving its index, or that it
        doesn't, by returning a (mi1.  We can't pass a structure  to
        a  function directly _ we have to either define it external-
        ly, or pass a pointer to it.  Let's try the first way first.
        int  nsym 0;   / current length of symbol table / struct {
             char id[10];
             int  line;
             char type;
             int  usage; } sym[100];         / symbol table /  main(
        ) {

             if( (index = lookup(newname)) >= 0 )
                  sym[index]usage;         / already there  /
             else
                  install(newname, newline, newtype);

        } lookup(s)
           char s; {
             int i;
             extern struct {
                  char id[10];
                  int  line;
                  char type;
                  int  usage;
             } sym[ ];
             for( i=0; i<nsym; i )
                  if( compar(s, sym[i]id) > 0 )
                       return(i);
             return(-1); }

        compar(s1,s2)       /  return 1 if s1s2, 0 otherwise /
           char s1, s2; {
             while( s1  s2 )
                  if( s2  '\0' )
                       return(1);
             return(0); } The declaration of the structure in  isn't
        needed  if  the  external definition precedes its use in the
        same source file, as we shall see in a moment.

             Now what if we want to use pointers?  struct  symtag {
             char id[10];
             int  line;
             char type;
             int  usage; } sym[100], psym;

             psym = &sym[0];     / or  p  =  sym;  /  This  makes  a
        pointer  to  our  kind of structure (the symbol table), then


                              October 10, 1975




                                   - 3 -



        initializes it to point to the first element of

             Notice that we added something after the word a ``tag''
        called  This  puts  a name on our structure definition so we
        can refer to it  later  without  repeating  the  definition.
        It's  not  necessary but useful.  In fact we could have said
        struct    symtag {
              structure definition }; which wouldn't  have  assigned
        any      storage      at      all,     and     then     said
        struct    symtag    sym[100];      struct    symtag    psym;
        which would define the array and the pointer.  This could be
        condensed further, to struct    symtag    sym[100], psym;

             The way we actually refer to an member of  a  structure
        by a pointer is like this:
             ptr -> structure(hymember The symbol `(mi>' means we're
        pointing  at a member of a structure; `(mi>' is only used in
        that context.  is a pointer to the  (base  of)  a  structure
        that  contains  the structure member.  The expression refers
        to the indicated member of the pointed-to  structure.   Thus
        we  have  constructions  like: psym->type = 1; psym->id[0] =
        'a'; and so on.

             For more complicated pointer expressions, it's wise  to
        use  parentheses  to  make it clear who goes with what.  For
        example, struct { int x, y; } p; p->x increments  x  p->x so
        does   this!    (p)->x    increments   p  before  getting  x
        p->y uses y as a pointer, then  increments  it  (p->y)    so
        does  this  (p)->y    uses y as a pointer, then increments p
        The way to remember these  is  that  (dot),  and  bind  very
        tightly.  An expression involving one of these is treated as
        a unit.  and are names exactly as is.

             If is a pointer to a structure, any arithmetic on takes
        into  account  the  acutal  size  of the structure.  For in-
        stance, increments by the correct amount  to  get  the  next
        element  of  the array of structures.  But don't assume that
        the size of a structure is the  sum  of  the  sizes  of  its
        members  _ because of alignments of different sized objects,
        there may be ``holes'' in a structure.

             Enough theory. Here is the lookup  example,  this  time
        with pointers.  struct symtag {
             char id[10];
             int  line;
             char type;
             int  usage; } sym[100]; main( ) {
             struct symtag lookup( );
             struct symtag psym;

             if( (psym = lookup(newname)) )     / non-zero pointer /
                  psym -> usage;      / means already there /
             else
                  install(newname, newline, newtype);



                              October 10, 1975




                                   - 4 -



        } struct symtag lookup(s)
           char s; {
             struct symtag p;
             for( p=sym; p < &sym[nsym]; p )
                  if( compar(s, p->id) > 0)
                       return(p);
             return(0); } The function doesn't change: refers  to  a
        string.

             In we test the pointer returned by against zero,  rely-
        ing  on  the fact that a pointer is by definition never zero
        when it really points at something.  The other pointer mani-
        pulations are trivial.

             The only complexity is the set  of  lines  like  struct
        symtag  lookup(  );  This  brings us to an area that we will
        treat only hurriedly _ the question of function  types.   So
        far, all of our functions have returned integers (or charac-
        ters, which are much the same).  What  do  we  do  when  the
        function  returns something else, like a pointer to a struc-
        ture?  The rule is that any function that doesn't return  an
        has  to say explicitly what it does return.  The type infor-
        mation goes before the function name  (which  can  make  the
        name hard to see).  Examples: char f(a)
           int a; {

        }

        int g( ) {  }

        struct symtag lookup(s) char s; {  } The function returns  a
        character,  returns  a  pointer to an integer, and returns a
        pointer to a structure that looks like And if we're going to
        use  one  of  these functions, we have to make a declaration
        where we use it, as we did in above.

             Notice th parallelism between the declarations
             struct symtag lookup( );
             struct symtag psym; In effect, this says that  and  are
        both  used  the same way _ as a pointer to a strcture _ even
        though one is a variable and the other is a function.




                              October 10, 1975



</pre>
== Chapter 6 ==
<pre>
        1
        _.  Initialization
            ______________ of
                           __ Variables
                              _________

             An external variable may be initialized at compile time
        by  following its name with an initializing value when it is
        defined.  The initializing value has to be  something  whose
        value   is   known   at   compile  time,  like  a  constant.
        int  x    0;   / "0" could be any constant /  int  a    'a';
        char flag 0177; int  p    &y[1];    / p now points to y[1] /
        An external array can be initialized by following  its  name
        with   a   list   of  initializations  enclosed  in  braces:
        int  x[4] {0,1,2,3};          / makes x[i]  =  i  /  int  y[
        ] {0,1,2,3};          /  makes  y  big enough for 4 values /
        char msg  "syntax error\n";   / braces  unnecessary  here  /
        char keyword[ ]{
             "if",
             "else",
             "for",
             "while",
             "break",
             "continue",
             0 }; This last one is very useful _ it makes  an  array
        of  pointers to character strings, with a zero at the end so
        we can identify the last element easily.   A  simple  lookup
        routine could scan this until it either finds a match or en-
        counters  a  zero  keyword  pointer:   lookup(str)         /
        search for str in keyword[ ] /
           char str; {
             int i,j,r;
             for( i=0; keyword[i] != 0; i) {
                  for( j=0; (r=keyword[i][j])  str[j] && r !=  '\0';
        j );
                  if( r  str[j] )
                       return(i);
             }
             return(-1); }

             Sorry _ neither local variables nor structures  can  be
        initialized.




















                              October 10, 1975


</pre>
== Chapter 7 ==
<pre>
        1
        _.  Scope
            _____ Rules
                  _____: Who
                         ___ Knows
                             _____ About
                                   _____ What
                                         ____

             A complete C program need not be compiled all at  once;
        the source text of the program may be kept in several files,
        and previously compiled routines  may  be  loaded  from  li-
        braries.   How  do we arrange that data gets passed from one
        routine to another?  We have already seen how to  use  func-
        tion arguments and values, so let us talk about external da-
        ta.  Warning: the words declaration
                                ___________ and definition
                                                __________ are  used
        precisely  in  this  section;  don't  treat them as the same
        thing.

             A major shortcut exists for  making  declarations.   If
        the  definition of a variable appears before
                                              ______ its use in some
        function, no declaration  is  needed  within  the  function.
        Thus,  if a file contains f1( ) {  } int foo; f2( ) {  foo =
        1;  } f3( ) {  if ( foo )  } no declaration of is needed  in
        either  or or because the external definition of appears be-
        fore them.  But if wants to use it has to  contain  the  de-
        claration f1( ) {
             extern int foo;

        }

             This is true also of any function that exists on anoth-
        er  file  _ if it wants it has to use an declaration for it.
        (If somewhere there is an declaration for  something,  there
        must  also  eventually  be  an external definition of it, or
        you'll get an ``undefined symbol'' message.)

             There are some hidden pitfalls in external declarations
        and  definitions if you use multiple source files.  To avoid
        them, first, define and initialize  each  external  variable
        only  once  in the entire set of files: int  foo  0; You can
        get away with multiple external definitions on but not on so
        don't ask for trouble.  Multiple initializations are illegal
        everywhere.  Second, at the beginning of any file that  con-
        tains  functions  needing  a variable whose definition is in
        some other file, put in an declaration, outside of any func-
        tion: extern    int  foo; f1( ) {  }
           etc

             The compiler control line,  to  be  discussed  shortly,
        lets you make a single copy of the external declarations for
        a program and then stick them into each of the source  files
        making up the program.

        2
        _.  #define
             ______, #include
                      _______

             C provides a very limited macro facility.  You can  say
        #define   name      something    and   thereafter   anywhere
        ``name'' appears as a token, ``something'' will  be  substi-
        tuted.   This  is  particularly  useful  in parametering the
        sizes of arrays: #define   ARRAYSIZE 100
             int  arr[ARRAYSIZE];



                              October 10, 1975

                                   - 2 -



             while( i < ARRAYSIZE ) (now we  can  alter  the  entire
        program  by  changing  only  the or in setting up mysterious
        constants: #define   SET       01 #define   INTERRUPT 02   /
        interrupt bit / #define   ENABLED   04

        if( x & (SET | INTERRUPT | ENABLED) ) Now we have meaningful
        words  instead  of  mysterious  constants.   (The mysterious
        operators `&' (AND) and `(or' (OR) will be  covered  in  the
        next  section.) It's an excellent practice to write programs
        without any literal constants except in statements.

             There are several warnings about First, there's no sem-
        icolon at the end of a all the text from the name to the end
        of the line (except for comments) is taken to be the ``some-
        thing''.   When  it's  put  into the text, blanks are placed
        around it.  Good style typically makes the name in the upper
        case  _ this makes parameters more visible.  Definitions af-
        fect things only after they occur, and only within the  file
        in  which  they  occur.   Defines can't be nested.  Last, if
        there is a in a file, then the first character of  the  file
        must
        ____  be  a `#', to signal the preprocessor that definitions
        exist.

             The other control word known to C  is  To  include  one
        file  in  your  source  at  compilation  time,  say #include
        "filename" This is useful for putting a lot of heavily  used
        data  definitions  and statements at the beginning of a file
        to be compiled.  As with the first line of a file containing
        a has to begin with a `#'.  And can't be nested _ an includ-
        ed file can't contain another


























                              October 10, 1975



</pre>
== Chapter 8 ==
<pre>
        1
        _.  Bit
            ___ Operators
                _________

             C has several  operators  for  logical  bit-operations.
        For  example,  x = x & 0177; forms the bit-wise of and 0177,
        effectively retaining only the  last  seven  bits  of  Other
        operators  are (or  inclusive OR ^    (circumflex) exclusive
        OR +    (tilde) 1's complement  !    logical  NOT  <<   left
        shift   (as  in  x<<2)  >>   right  shift    (arithmetic  on
        PDP(hy11; logical on H6070, IBM360)

        2
        _.  Assignment
            __________ Operators
                       _________

             An unusual feature of  C  is  that  the  normal  binary
        operators  like  `+',  `(mi', etc.  can be combined with the
        assignment operator `=' to form  new  assignment  operators.
        For example, x =- 10; uses the assignment operator `=(mi' to
        decrement by 10, and x =& 0177 forms the of and 0177.   This
        convention  is a useful notational shortcut, particularly if
        is a complicated expression.  The classic example is summing
        an array: for( sum=i=0; i<n; i )
             sum =+ array[i]; But the spaces around the operator are
        critical!   For instance, x = -10; sets to (mi10, while x =-
        10; subtracts 10 from When no space is present, x=-10;  also
        decreases  by  10.  This is quite contrary to the experience
        of most programmers.  In particular, watch  out  for  things
        like  c=s;  y=&x[0];  both of which are almost certainly not
        what you wanted.  Newer versions of  various  compilers  are
        courteous enough to warn you about the ambiguity.

             Because  all  other  operators  in  an  expression  are
        evaluated  before  the  assignment  operator,  the  order of
        evaluation should be watched carefully: x = x<<y | z;  means
        ``shift left places, then with and store in But x =<< y | z;
        means ``shift left by places'', which is rather different.

        3
        _.  Floating
            ________ Point
                     _____

             We've skipped over  floating  point  so  far,  and  the
        treatment  here will be hasty.  C has single and double pre-
        cision numbers (where the precision depends on  the  machine
        at hand).  For example,
             double sum;
             float avg, y[10];
             sum = 00;
             for( i=0; i<n; i )
                  sum =+ y[i];
             avg = sum/n; forms the sum and average of the array

             All floating arithmetic is done  in  double  precision.
        Mixed mode arithmetic is legal; if an arithmetic operator in
        an expression has both operands or the  arithmetic  done  is
        integer,  but  if one operand is or and the other is or both
        operands  are  converted  to  Thus  if  and   are   and   is
        (x+i)/j        converts i and j to float x + i/j        does
        i/j integer, then converts Type conversion may  be  made  by



                              October 10, 1975

                                   - 2 -



        assignment; for instance,
             int m, n;
             float x, y;
             m = x;
             y = n; converts to integer  (truncating  toward  zero),
        and to floating point.

             Floating constants are just like those  in  Fortran  or
        PL/I, except that the exponent letter is `e' instead of `E'.
        Thus:
             pi = 314159;
             large = 123456789e10;

             will format  floating  point  numbers:  in  the  format
        string  will print the corresponding variable in a field di-
        gits wide, with decimal places.  An instead of an will  pro-
        duce exponential notation.

        4
        _.  Horrors
            _______! goto
                     ____'s
                          _ and
                            ___ labels
                                ______

             C has a statement and labels, so you can  branch  about
        the  way  you  used to.  But most of the time aren't needed.
        (How many have we used up to this point?) The code  can  al-
        most always be more clearly expressed by and compound state-
        ments.

             One use of with some legitimacy is in a  program  which
        contains  a  long loop, where a would be too extended.  Then
        you might write
           mainloop:

             goto mainloop; Another use is to  implement  a  out  of
        more  than  one level of or can only branch to labels within
        the same function.

        5
        _.  Acknowledgements
            ________________

             I am indebted to a veritable host of readers  who  made
        valuable  criticisms  on  several  drafts  of this tutorial.
        They ranged in experience from  complete  beginners  through
        several  implementors  of  C  compilers  to  the  C language
        designer himself.  Needless to say, this is  a  wide  enough
        spectrum of opinion that no one is satisfied (including me);
        comments and suggestions are still welcome, so that some fu-
        ture version might be improved.











                              October 10, 1975



</pre>
== Chapter 9 ==
<pre>
        References
        __________

             C is an extension of B, which was  designed  by  D.  M.
        Ritchie  and  K. L. Thompson [4].  The C language design and
        implementation are the work of D. M. Ritchie.   The  version
        was begun by A. Snyder and B. A. Barres, and completed by S.
        C. Johnson and M. E. Lesk.  The version is primarily due  to
        T. G. Peterson, with the assistance of M. E. Lesk.

        [1]  D. M. Ritchie, C
                            _ Reference
                              _________  Manual
                                         ______.   Bell  Labs,  Jan.
             1974.

        [2]  M. E. Lesk & B. A. Barres, The
                                        ___ GCOS
                                            ____  C
                                                  _  Library
                                                     _______.   Bell
             Labs, Jan. 1974.

        [3]  D. M. Ritchie & K. Thompson, UNIX
                                          ____ Programmer
                                               __________'s
                                                          _  Manual
                                                             ______.
             5th Edition, Bell Labs, 1974.

        [4]  S. C.  Johnson  &  B.  W.  Kernighan,  The
                                                    ___  Programming
                                                         ___________
             Language
             ________  B
                       _.  Computer Science Technical Report 8, Bell
             Labs, 1972.































                              October 10, 1975



</pre>

[[Category: C Language]]