Bsearch function in C not working properly

Question

Basically I'm creating a program that will output the number if it is found in an given array or output -1 if not found. (Sorted Array.)

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

int cmp(const void*a,const void *b){
    if(*(int*)a-*(int*)b>0) return 1;
    if(*(int*)a-*(int*)b<0) return -1;
    return 0;
}

int main() {
    int n; scanf("%d",&n);
    int a[n];

    for(int i =0;i<n;i++) 
        scanf("%d",a+i);

    for(int i =0;i<n;i++){
        int *item;
        item = (int*)bsearch(&i,a,n,sizeof(int),cmp);
        if(item!=NULL) printf("%d ",i);
        else printf("-1 "); 
    }

    return 0;
}

INPUT : 10

-1 -1 6 1 9 3 2 -1 4 -1

OUTPUT : -1 1 2 3 4 -1 6 -1 -1 9

My OUTPUT : -1 -1 -1 3 4 -1 -1 -1 -1 -1

Possible duplicate of [What is a debugger and how can it help me diagnose problems?](https://stackoverflow.com/questions/25385173/what-is-a-debugger-and-how-can-it-help-me-diagnose-problems) — Marco Bonelli, Sep 21 '18 at 11:58
The array needs to be sorted. That's rather obvious (the __b__ in bsearch stands for binary) and the documentation mentions it. — Jabberwocky, Sep 21 '18 at 12:01
@MarcoBonelli this problem could be solved that way. It is not a duplicate of that question. — Thunderwood, Sep 21 '18 at 12:02
[binary search (...) is a search algorithm that finds the position of a target value within a **sorted array**](https://en.wikipedia.org/wiki/Binary_search_algorithm) — Antti Haapala -- Слава Україні, Sep 21 '18 at 12:32
BTW: `scanf("%d",a+i);` is correct but you should rather write `scanf("%d", &a[i]);` — Jabberwocky, Sep 21 '18 at 12:32

score 0 · Accepted Answer · edited Jun 20 '20 at 09:12

https://www.tutorialspoint.com/c_standard_library/c_function_bsearch.htm

The C library function void *bsearch(const void *key, const void *base, size_t nitems, size_t size, int (*compar)(const void *, const void *)) function searches an array of nitems objects, the initial member of which is pointed to by base, for a member that matches the object pointed to, by key. The size of each member of the array is specified by size.

The contents of the array should be in ascending sorted order according to the comparison function referenced by compar.

The whole point of binary searching is that if the array has size size you start in position size/2. If this element is less than what you're looking for, then go to size/2 + size/4, and otherwise go to size/2 - size/4. If this approach should work, the array needs to be sorted.

Read more about binary searching here: https://en.wikipedia.org/wiki/Binary_search_algorithm

And as mentioned in comments, scanf("%d",a+i) is correct, but scanf("%d",&a[i]) is preferable.

Bsearch function in C not working properly

1 Answers1