二分查找在 C 中不起作用

Question

I have the following binary search algorithmn:

int search_binary(int* tab,int size, int a)
{
    int lower=0,upper=size-1,middle=(lower + upper)/2;
    while(lower < upper)
    {
        if(tab[middle] == a)
            break;
        else if(tab[middle] < a)
        {
            upper = middle - 1;
            middle = (lower + upper)/2;
        } else {
            lower = middle + 1;
            middle = (lower + upper)/2;
        }

    }

    if(tab[middle] == a)
        return middle;
    else
        return -1;

}

It either return -1 if I insert a number, that exists or it returns the wrong index.

exampledata:

table: 2 2 4 5 7 7 8 8 8 9

searched number: 7

result: The index is: 4

table: 1 2 3 4 6 6 6 7 8 9

searched number: 4

result: The index is: -1

Answer 1

else if(tab[middle] < a)
    {
        upper = middle - 1;
        middle = (lower + upper)/2;
    }

From here it seems that you expect the array to be ordered in decreasing order. In fact, giving it an array ordered that way, it works for me.

just change the if to:

if(tab[middle] > a)

To have it working for an array ordered in increasing order

Answer 2

A slight change in you loop in function -

int search_binary(int* tab,int size, int a)
{
   int lower=0,upper=size-1,middle=(lower + upper)/2;
   while(lower < upper)
  {
    if(tab[middle] == a){               // if found 
        return middle;                  // return index 
      }
    else if(tab[middle] < a)            // if a is greater 
    {
        lower= middle ;          //lower is updated to middle,search in part above middle 
        middle = (lower + upper)/2;
    } else {
        upper = middle - 1;     //else upper is updated search in part lower than middle
        middle = (lower + upper)/2;
    }

  }
 return -1;
}

What mistake your loop was doing that when tab[middle]<a then due to updation of upper it would search number in part 0 to middle -1 whereas , number was at higher index.I have updated your code with correct changes in vairables.