Binary Search Recursion Not Functioning Correctly

Question

I coded a simple binary search to print out the position of what was searched. It correctly recognizes when a searched element isn't in the array and prints out "error". However, when a searched element is actually in the array, the value is printed, rather than the position. Could you please let me know what i'm missing? Thanks in advance

#include <stdio.h>
#include <stdlib.h>

int binarysearch(int array[], int low, int max, int search);
int main(void) {
    int array[10]={10,11,12,13,14,15,16,17,18,19};

    int count,search;
    count=sizeof(array)/sizeof(array[0]);

    printf("Enter the number you would like to search\n");
    scanf("%d",&search);

    int result=binarysearch(array,0,count,search);

    if (result>0){
        printf("Element in position %d",result);
    }
    else{
        printf("Error");
    }

}

int binarysearch(int array[], int low, int max, int search){
    if(low<=max){
        int middle=(low+max)/2;

        if(search>array[middle]){
            low=middle+1;
            return binarysearch(array,low,max,search);
        }

        else if(search<array[middle]){
            max=middle-1;
            return binarysearch(array,low,max,search);

        }

        else {
            return search;
        }

    }
    else
    return -1;
}

return middle maybe ?

Tony Tannous
– Tony Tannous

2017-12-17 07:03:15 +00:00
Commented Dec 17, 2017 at 7:03 — Tony Tannous
– Tony Tannous, Commented Dec 17, 2017 at 7:03
Shouldn't you be returning middle?

kiner_shah
– kiner_shah

2017-12-17 07:03:23 +00:00
Commented Dec 17, 2017 at 7:03 — kiner_shah
– kiner_shah, Commented Dec 17, 2017 at 7:03

ikegami · Accepted Answer · 2017-12-17 08:14:35Z

2

Apart from the return middle; (in place of return search) change you should also change the

if (result>=0){
         ^^^^
        printf("Element in position %d",result);
    }

Otherwise you will always miss the 0-th element even if found. (By miss I mean the result won't be printed). Binary search will work as intended.

One another thing to notice is while calculating mid to avoid overflow use this

int middle = low + ((max - low) / 2);

Finally, the use of recursion is purely wasteful.

int binarysearch(int array[], int low, int max, int search) {
    while (low <= max) {
        int middle = low + ((max - low) / 2);
        if (search > array[middle]) {
            low = middle + 1;
        }
        else if (search < array[middle]) {
            max = middle - 1;
        }
        else {
            return middle;
        }
    }

    return -1;
}

edited Dec 17, 2017 at 8:14

ikegami

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answered Dec 17, 2017 at 7:12

user2736738

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4 Comments

user2736738 Over a year ago

@ikegami.: Thanks for the awesome addition. Appreciate your help.

Roomi Over a year ago

Thanks for the answers. Is it best to avoid using a recursion when possible, would it be better memory-wise?

user2736738 Over a year ago

@Roomi.: Yes reursion is limited by the amount of memory the system allows you to use when storing the stack frames due to each function call. Iteration avoids that. So yes it's preferrable to use iterative soluton.

ikegami Over a year ago

It's also more expensive to push the arguments on the stack and make the call than to simply jump back to the start of the loop.

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