binary search of an integer array

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   1:  // binary search of an integer array, this search is efficient for large arrays
   2:  // tested with PellesC       vegaseat     24jan2005
   3:   
   4:  #include <stdio.h>
   5:   
   6:  int main()
   7:  {
   8:    int a[20] = {0}; 
   9:    int n, i, j, temp;
  10:    int *beg, *end, *mid, target;
  11:    
  12:    printf(" enter the total integers you want to enter (make it less then 20):\n");
  13:    scanf("%d", &n);
  14:    if (n >= 20) return 0;   // ouch!
  15:    printf(" enter the integer array elements:\n" );
  16:    for(i = 0; i < n; i++)
  17:    {
  18:      scanf("%d", &a[i]);
  19:    }
  20:      
  21:    // sort the loaded array, a must for binary search! 
  22:    // you can apply qsort or other algorithms here
  23:    for(i = 0; i < n-1; i++)
  24:    {  
  25:      for(j = 0; j < n-i-1; j++)
  26:      {
  27:        if (a[j+1] < a[j])
  28:        {
  29:          temp = a[j];
  30:          a[j] = a[j+1];
  31:          a[j+1] = temp;
  32:        }
  33:      }
  34:    }
  35:    printf(" the sorted numbers are:");
  36:    for(i = 0; i < n; i++)
  37:    {
  38:      printf("%d ", a[i]);
  39:    }
  40:      
  41:    // point to beginning and end of the array
  42:    beg = &a[0];
  43:    end = &a[n];  // use n = one element past the loaded array!
  44:    printf("\n beg points to address %d and end to %d",beg, end);  // test
  45:   
  46:    // mid should point somewhere in the middle of these addresses
  47:    mid = beg += n/2;
  48:    printf("\n mid points to address %d", mid);  // test
  49:    
  50:    printf("\n enter the number to be searched:");
  51:    scanf("%d",&target);
  52:    
  53:    // binary search, there is an AND in the middle of while()!!!
  54:    while((beg <= end) && (*mid != target))
  55:    {
  56:      // is the target in lower or upper half?
  57:        if (target < *mid)
  58:        {
  59:        end = mid - 1;     // new end
  60:        n = n/2;
  61:        mid = beg += n/2;  // new middle
  62:      }
  63:      else
  64:      {
  65:        beg = mid + 1;     // new beginning
  66:        n = n/2;
  67:        mid = beg += n/2;  // new middle      
  68:      }
  69:    }
  70:    
  71:    // did you find the target?
  72:    if (*mid == target)
  73:    {
  74:      printf("\n %d found!", target);
  75:    }
  76:    else
  77:    {
  78:      printf("\n %d not found!", target);
  79:    }
  80:    
  81:    getchar();  // trap enter
  82:    getchar();  // wait
  83:    return 0;
  84:  }
  85:   

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