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. It returns 16 and prints.

          4 16

    The function stops as soon as the first return statement is reached.

 3. It takes 3 integer arguments and returns 1 (true) if all three
    are equal, and 0 otherwise. Veteran C program might write this
    more succinctly as the following since the result of the && operation
    is always 0 or 1.

          int eq3(int a, int b, int c) {
             return (a == b) && (a == c);
          }


 4.       int max(int a, int b) {
             if (a >= b)
                return a;
             else
                return b;
          }

     The keyword "else" is not essential since once a return statement
     is reached, no further statements are executed.

          int max(int a, int b) {
             if (a >= b)
                return a;
             return b;
          }

 5.
          int max3(int a, int b, int c) {
             if (a >= b && a >= c)
                return a;
             else if (b >= c)
                return b;
             else
                return c;
          }


 6.       double absolute_value(double x) {
             if (x > 0)
                return x;
             else
                return -x;
          }

       Alternatively, use the fabs() function from the math library.
       Be sure to #include <math.h>.



 7.       int f(int n) {
             int i = 1; 
             while (i < n)
                i += i;
             return i;
          }

 8. We take a brute force approach, and check integers less than N
    to see if they divide evenly into N.

          int isPrime(int N) {
             int divisor;
             if (N <= 1)
                return 0;
             for (divisor = 2; divisor < N; divisor++)
                if (N % divisor == 0)    /* N is a multiple of divisor */
                   return 0;
             return 1;
          }

 9. One solution is to copy the above code and exchange 0 and 1 in the three
    return statements. A better solution is to call the above function. Recall !
    means logical NOT in C.

          int isNotPrime(int N) {
             return !isPrime(N);
          }


10. To remove the least significant digit, we divide by 10 and throw
    away the remainder.

         int leadingdigit(int x) {
            while (x >= 10)
               x = x / 10;
            return x;
         }

11. Prints out "9 11".


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

    % cc bad.c
    "bad.c", line 3: syntax error before or at: {
    "bad.c", line 3: undefined or not a type: x
    "bad.c", line 3: warning: old-style declaration or incorrect type for: x
    "bad.c", line 3: identifier redeclared: x
           current : pointer to int
           previous: function() returning int : "bad.c", line 3
    "bad.c", line 3: syntax error before or at: }
    "bad.c", line 8: syntax error before or at: b
    "bad.c", line 11: syntax error before or at: else
    cc: acomp failed for bad.c





    % lcc bad.c
    bad.c:3: unrecognized declaration
    bad.c:3: unrecognized declaration
    bad.c:3: syntax error; found `*' expecting `;'
    bad.c:3: redeclaration of `x' previously declared at bad.c:3
    bad.c:3: unrecognized declaration
    bad.c:8: syntax error; found `b' expecting `;'
    bad.c:11: syntax error; found `else' expecting `;'

    % gcc126 bad.c
    bad.c:3: parse error before `{'
    bad.c: In function `main':
    bad.c:8: parse error before `b'
    bad.c:11: parse error before `else'

    % gcc126 bad.c
    bad.c:3: parse error before `{'
    bad.c: In function `main':
    bad.c:8: parse error before `b'
    bad.c:11: parse error before `else'
    bad.c:6: warning: `a' might be used uninitialized in this function
    bad.c:6: warning: `b' might be used uninitialized in this function


    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.