Tutorial Exercises: Set 7

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Exercise 1

What values does the following program print?

   #include <stdio.h>

   void doit ( int A[] )
   {
      int *p;

      p = A;
      ++p;
      printf("%d\n", *p);
   }

   int main ()
   {
      int A[] = {1,2,3,4,5};
      int *p;

      p = A;
      doit(p);
      printf("%d"\n", *p);
   }

Solution 1

   2
   1

Exercise 2

Write a function that takes three two-dimensional arrays A,B,C and three positive integers r,s,t as input. A is treated as an r x s matrix, and B as an s x t matrix. The function computes and stores in C the product matrix AB.

Solution 2

   void matmul ( int A[][100], int B[][100], int C[][100], int r, int s, int t )
   {
      int i, j, k;

      for (i=0; i<r; ++i) {
         for (j=0; j<t; ++j) {
            C[i][j] = 0;
            for (k=0; k<s; ++k) C[i][j] += A[i][k] * B[k][j];
         }
      }
   }

Exercise 3

Write a recursive function that computes the maximum of an array A of size n as follows. It first computes the maximum of the first n/2 elements and stores this maximum in a temporary variable a. It then recursively computes the maximum of the remaining elements of the array and stores this second maximum in a temporary variable b. The function then compares a and b and returns the larger of these values. Recursion stops when the size of the array is reduced to one.

Solution 3

   #define ERROR -2147483648

   int findmax ( int A[], int n )
   {
      int a,b;

      if (n == 0) return ERROR;
      if (n == 1) return A[0];
      a = findmax(A,n/2);
      b = findmax(A+n/2,n-(n/2));
      return ((a >= b) ? a : b);
   }

Exercise 4

Write a function that accepts two strings s and t as parameters. The function checks whether t is a substring of s. If so, a pointer to the leftmost match is returned, otherwise the NULL pointer is returned.

Solution 4

   char *match ( char *s, char *t )
   {
      int lens, lent, i;

      lens = stdlen(s);
      lent = stdlen(t);
      for (i=0; i<=lens-lent; ++i) {
         if (!strncmp(s+i,t,lent)) return s+i;
      }
      return NULL;
   }

Exercise 5

Given a 2-dimensional matrix as input, the following function returns an array storing a specified column of the matrix.

   #define MAXDIM 100

   int *getcol ( int (*A)[MAXDIM] , int m , int n , int j )
   {
      int v[MAXDIM];
      int i;

      for (i=0; i<m; ++i) v[i] = A[i][j];
      return v;
   }

If you compile this function, the compiler issues a warning message. What is wrong with this function? How can you remove this problem?

Solution 5

A statically allocated array inside a function is local to that function. After the function returns, the memory goes back to the run-time system pool. Returning a pointer to a temporary memory is the source of the problem.

This error can be removed by dynamically allocating memory to the vector v. A dynamically allocated memory does not go back to the system pool, until you expicitly free it (or the program terminates). The following implementation removes this error.

   #define MAXDIM 100

   int *getcol ( int (*A)[MAXDIM] , int m , int n , int j )
   {
      int *v;
      int i;

      v = (int *)malloc(m * sizeof(int));
      for (i=0; i<m; ++i) v[i] = A[i][j];
      return v;
   }

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