What is wrong with my binary search implementation? - c

Question

#include <stdio.h>
int bsearch(int a[], int n, int lo, int hi) {
int mid;
mid = (hi + lo) / 2;
if(a[mid] == n)
return 1;
else if(a[mid] > n)
bsearch(a, n, lo, mid);
else
bsearch(a, n, mid, hi);
return 0;
}
int main(void) {
int n, a[7] = {2, 4, 5, 67, 70, 80, 81};
int hi = 6, lo = 0, j;
scanf("%d", &n);
j = bsearch(a, n, lo, hi);
if(j)
printf("Found");
else
printf("Not Found");
return 0;
}
input : 5 output: Not Found
Can anyone tell me why I'm getting this result?

Answer 1

You need to fix several big issues to make it work (see details in following code comments).
Change your binary search function to the following:
int bsearch(int a[], int n, int lo, int hi)
{
// add base case
if (high < low)
return 0; // not found
int mid;
mid=(hi+lo)/2;
if(a[mid]==n)
return 1;
else if(a[mid]>n)
return bsearch(a,n,lo,mid-1); // add return
else
return bsearch(a,n,mid+1,hi); // add return
}
P.S.: And based on your usage in the main() body, you actually only need to return 0/1 to indicate contains the value or not. I will suggest you to use bool return type to make it more clear.

Answer 2

Add "return" to the recursive calls, e.g.:
return bsearch(a,n,lo,mid);
Otherwise, when you return 1 in bsearch, it does not get returned all the way to main.
That will make it work for 5. You have other bugs, so try with many values and use an IDE and/or printf to see what's happening. Good luck and have fun!

Answer 3

That's because the return 0; statement in your bsearch function is always executed because you are simply discarding the values returned by the recursive calls. In a recursive function, you must first decide the base case. Here in your bsearch, the base case should be
low <= hi
This is the first condition which must be true to start the search for the sought value. If this condition is not fulfilled, then you must return false, i.e., 0.
Next, a value returning function call is an expression, i.e., it evaluates to a value. When you simply call the function and do nothing with the result, you will always fall down to the last return statement in your function. Here I list some points in comments alongside the statements in your bsearch function.
int bsearch(int a[], int n, int lo, int hi) {
// first check for the base condition here
// if(hi < low) return 0;
int mid;
// can cause integer overflow. should be
// mid = lo + (hi - lo) / 2;
mid = (hi + lo) / 2;
if(a[mid] == n)
return 1;
else if(a[mid] > n)
// you are doing nothing with the value returned
// think of the function call as an expression
// return the value of the expression
// should be
// return besearch(a, n, lo, hi);
bsearch(a, n, lo, mid);
else
// same follows here
// should be
// return bsearch(a, n, mid, hi);
bsearch(a, n, mid, hi);
// finally you will always return 0 because this statement is always executed
// all cases have been taken care of.
// no return statement needed here
return 0;
}