EXERCISES ON C FUNCTIONS

READING 
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Notes for lecture 2.
Kernighan and Ritchie, 1.7, 1.8
Sedgewick, p. 73

SUPPLEMENTAL READING
--------------------
Deitel and Deitel, 5.1-5.5


EXERCISES
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 1. What is wrong with the following C function?

              int square (int x);
                { return x*x; }

 2. Write a C function that takes a positive integer N as argument and
    returns the largest power of two less than or equal to N.

 3. What value is printed out by the following program? 

             #include <stdio.h>
             int f(int x) { return x+2; }
             main()
               {
                 int x = 5;
                 printf("%d \n", f(x+2));
               }

 4. Run the following program through the compiler to see the
    error messages that lcc (or cc) produces for semicolon errors.

             int square (int x);
               { return x*x; }
             main()
               {
                  int a, b, c;
                  c = 0 
                  b = 0;
                  if (a > b) c = 0 else b = 0;        
               }















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		    ANSWERS TO EXERCISES ON C FUNCTIONS



 1. Extra semicolon on the first line.  Only function prototypes
    have semicolons after the parenthesis, not function definitions.

 2.
             int f(int N)
               {
                 int j; 
                 for (j = 1; j <= N; j += j) ;
                 return j/2;
               }

 3. Prints out "9".

 4. Different compilers have different ways of trying to tell you about
    the problems. 

    % cc bad.c
    "bad.c", line 2: syntax error before or at: {
    "bad.c", line 2: undefined or not a type: x
    "bad.c", line 2: warning: old-style declaration or incorrect type for: x
    "bad.c", line 2: identifier redeclared: x
            current : pointer to int
            previous: function(void) returning int : "bad.c", line 2
    "bad.c", line 2: syntax error before or at: }
    "bad.c", line 7: syntax error before or at: b
    "bad.c", line 8: syntax error before or at: else
    cc: acomp failed for bad.c

    % lcc bad.c
    bad.c:2: unrecognized declaration
    bad.c:2: unrecognized declaration
    bad.c:2: syntax error; found `*' expecting `;'
    bad.c:2: redeclaration of `x' previously declared at bad.c:2
    bad.c:2: unrecognized declaration
    bad.c:7: syntax error; found `b' expecting `;'
    bad.c:8: syntax error; found `else' expecting `;'
    bad.c:9: warning: missing return value

    % gcc bad.c
    bad.c:2: parse error before `{'
    bad.c: In function `main':
    bad.c:7: parse error before `b'
    bad.c:8: parse error before `else'
    % 

    Compilers do the best they can to try to understand what we give
    them to be legal C programs.  When we make errors, their interpretation
    may diverge from our intention, to the point that the error messages
    may not make sense to us.  Lesson: fix the *first* error, then try
    again.