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二分法查找

   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: