L-18 MCS 275 Mon 19 Feb 2001

/* L-18 MCS 275 : binary search to locate element in integer array */

#include <stdio.h>

int rec_search (int start, int end, int a[], int i);
/* returns k, if a[k] = i, and start <= k <= end;
          -1, otherwise */
int itr_search (int start, int end, int a[], int i);
/* iterative version of the recursive search */
void bin_search (int n);
/* prompts the user for n integer numbers and an element */

int main()
{
   int n;

   printf("Locating a number in an integer array with binary search.\n");
   printf("Give number of elements : ");
   scanf("%d", &n);
   bin_search(n);

   return 0;
}

void bin_search (int n)
{
   int i, a[n], pos;
   char cont = 'y';

   printf("Give %d SORTED integers : ", n);
   for (i=0; i < n; i++) scanf("%d", &a[i]);
   printf("Your array of %d integers : ", n); 
   printf("%d", a[0]);
   for (i=1; i < n; i++)
   {
      printf(" %d", a[i]);
      if (a[i] < a[i-1]) 
      {
         printf("THE ARRAY MUST BE SORTED IN ASCENDING ORDER!!!\n");
         return;
      }
        
   }
   printf("\n"); 
   while (cont == 'y')
   {
      printf("Give a number to look for : ");
      scanf("%d",&i);
      printf("Looking for %d in the array recursively...\n",i);
      pos = rec_search(0,n-1,a,i);
      printf("  the number %d ",i);
      if (pos == -1)
         printf("does not occur in the array\n");
      else
         printf("occurs at position %d\n", pos);
      printf("Looking for %d in the array iteratively...\n",i);
      pos = itr_search(0,n-1,a,i);
      printf("  the number %d ",i);
      if (pos == -1)
         printf("does not occur in the array\n");
      else
         printf("occurs at position %d\n", pos);
      printf("You want more searches ? (y/n) ");
      cont = getchar();
      cont = getchar();
   }
}

int rec_search (int start, int end, int a[], int i)
{
   int middle;

   if (start == end)
      if (a[start] == i)
         return start;
      else
         return -1;
   else
   {
      middle = (start + end)/2;
      if (a[middle] == i)
         return middle;
      else if (a[middle] < i)
         return rec_search(middle+1,end,a,i);
      else
         return rec_search(start,middle-1,a,i);
   }
}

int itr_search (int start, int end, int a[], int i)
{
   int my_start = start;
   int my_end = end;
   int middle;

   while (my_start != my_end)
   {
      middle = (my_start+my_end)/2;
      if (a[middle] == i)
         return middle;
      else if (a[middle] < i)
         my_start = middle+1;
      else
         my_end = middle-1;
   }
   if (a[my_start] == i)
      return my_start;
   else
      return -1;
}