// binary search of an integer array, this search is efficient for large arrays
// tested with PellesC       vegaseat     24jan2005
 
#include <stdio.h>
 
int main()
{
  int a[20] = {0}; 
  int n, i, j, temp;
  int *beg, *end, *mid, target;
 
  printf(" enter the total integers you want to enter (make it less then 20):\n");
  scanf("%d", &n);
  if (n >= 20) return 0;   // ouch!
  printf(" enter the integer array elements:\n" );
  for(i = 0; i < n; i++)
  {
    scanf("%d", &a[i]);
  }
 
  // sort the loaded array, a must for binary search! 
  // you can apply qsort or other algorithms here
  for(i = 0; i < n-1; i++)
  {  
    for(j = 0; j < n-i-1; j++)
    {
      if (a[j+1] < a[j])
      {
        temp = a[j];
        a[j] = a[j+1];
        a[j+1] = temp;
      }
    }
  }
  printf(" the sorted numbers are:");
  for(i = 0; i < n; i++)
  {
    printf("%d ", a[i]);
  }
 
  // point to beginning and end of the array
  beg = &a[0];
  end = &a[n];  // use n = one element past the loaded array!
  printf("\n beg points to address %d and end to %d",beg, end);  // test
 
  // mid should point somewhere in the middle of these addresses
  mid = beg += n/2;
  printf("\n mid points to address %d", mid);  // test
 
  printf("\n enter the number to be searched:");
  scanf("%d",&target);
 
  // binary search, there is an AND in the middle of while()!!!
  while((beg <= end) && (*mid != target))
  {
    // is the target in lower or upper half?
	  if (target < *mid)
	  {
      end = mid - 1;     // new end
      n = n/2;
      mid = beg += n/2;  // new middle
    }
    else
    {
      beg = mid + 1;     // new beginning
      n = n/2;
      mid = beg += n/2;  // new middle      
    }
  }
 
  // did you find the target?
  if (*mid == target)
  {
    printf("\n %d found!", target);
  }
  else
  {
    printf("\n %d not found!", target);
  }
 
  getchar();  // trap enter
  getchar();  // wait
  return 0;
}
 
 

Thanks a lot. This program was a real help.

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