C语言中递归二进制搜索算法中的分割错误

Question

This is a C program with the recursive binary search algorithm, however when I run it, the debugger says there is an access segmentation fault in the binary search function. Why is this and how do I fix this?

Here is the recursive binary search function:

int binSearch(int val, int numbers[], int low, int high)                 
{
     int mid;

     mid=(low+high)/2;
     if(val==numbers[mid])
     {  
                return(mid);          
     }   
     else if(val<numbers[mid])
     {
                return(binSearch(val, numbers, low, mid-1));               
     }            
     else if(val>numbers[mid])
     { 
                return(binSearch(val, numbers, mid+1, high));  
     }    
     else if(low==high)
     {
                return(-1);    
     }
}

Thank you :)

Answer 1

您必须在val < ...和val > ...之前检查low == high ，因为否则high可能变得小于low ，因此您的下一个递归可能会计算出无效的mid

Answer 2

Your edge cases are off: specifically, when your low and high indices pass, you continue to call recursively before you reach the low == high test. Rearrange the tests:

int binSearch(int val, int numbers[], int low, int high) {
    int mid = (low + high) / 2;

    if (val == numbers[mid]) return mid;

    if (val < numbers[mid]) {
        if (mid > low) return binSearch(val, numbers, low, mid-1);
    } else if (val > numbers[mid]) {
        if (mid < high) return binSearch(val, numbers, mid+1, high);
    }
    return -1;
}

Answer 3

Try this:

Fixed if constructs in your code

int binSearch(int val, int numbers[], int low, int high)                 
{
     int mid;

     mid=(low+high)/2;

     if(low<=high)
     {
         if(val==numbers[mid])
           return mid;          

         else if(val<numbers[mid])
           return binSearch(val, numbers, low, mid-1);

        else 
          return binSearch(val, numbers, mid+1, high);  
     }
     else
           return -1;    

}

Answer 4

low < high is not ensure. If that is not the case your are going to search out of the array bound.

Add a sanity check for that.

if (low < high)
     return -1;

EDIT: as other point out you can also check if low == high at the beginning but that does not ensure that the first call of the function have sound value.