EXERCISES ON C FUNCTIONS

READING 
-------
Notes for lecture 2.
Kernighan and Ritchie, 1.7, 1.8
Sedgewick, p. 73
King, Chapter 9.1 - 9.5 (Functions)

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


EXERCISES
---------


 1. What is wrong with the following C function?

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

 2. What is the effect of calling show(4)?

              int show(int x) {
                 int i;
                 for (i = 0; i < x; i++)
                    printf(%d %d\n", i, i*i);
                 return x*x;
                 for (i = 0; i < x; i++)
                    printf(%d %d\n", i, i*i);
                 return 0;
              }

 2. What does the following C function do?

              int eq3(int a, int b, int c) {
                 if ((a == b) && (a == c))
                    return 1;
                 else
                    return 0;
              }

 2. Write a C function that takes two integers as arguments and
    returns the value of the larger one.

 3. Write a C function that takes three integers as arguments and
    returns the value of the largest one.

 4. Write a C function that takes a real number as an argument
    and returns the absolute value of that number.

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

 6. Write a C function that takes a positive integer N as an argument and
    returns 1 if N is prime, and 0 otherwise.

 7. Write a C function that takes a positive integer N as an argument and
    returns 0 if N is prime, and 1 otherwise.

 8. What values are printed out by the following C program? 

             #include <stdio.h>

             int f(int x) {
                return x+2;
             }
 
            int main(void) {
                int x = 5;
                printf("%d %d\n", f(x+2), f(f(x+2)));
                return 0;
             }

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

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

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















.
.
.
.
		    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.

          0 0
          1 1
          2 4
          3 9

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

 2. It takes 3 integer arguments and returns 1 (true) if all three
    are equal, and 0 otherwise. Veteran C program might write this
    more succintly 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);
              }


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

   The "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;
          }

3.
          int max3(int a, int b, int c) {
             if (a >= b && a >= c)
                return a;
             else if (b >= c)
                return b;
             else
                return c;
          }


 4.          int 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>.

 5.
             int f(int N) {
                int j; 
                for (j = 1; j <= N; j += j)
                   ;
                return j/2;
             }

 6. 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 an integral multiple of divisor */
                      return 0;
                return 1;
             }

 7. 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);
             }


 8. Prints out "9 11".

 9. 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.