What is the difference between if and else if

Question

#include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>
#include <math.h>

int binary(int a[],int target,int l,int r); //return index , needs array, target  left index, right index
//goal: find if the number is in the array
int main(void)
{
    int a[]={1,2,3,4,5,6,7,8,9,10,11,12,13};
    int target = get_int("number:");
    int l = 0;
    int r = 13;
    int index=binary(a,target,l,r);
    if(index>=0)
        printf("number %i is at index %i\n",target,index);
    else
        printf("not found\n");
}
int binary(int a[],int target,int l,int r)
{
    if(r<l)
    {
        return -1;
    }
    int mid=l+(r-l)/2;

    if(a[mid]==target)
    {
        return mid;
    }

    if(a[mid]<target)
    {
        return binary(a,target,mid+1,r);
    }
    if(a[mid]>target)
    {
        return binary(a,target,l,mid-1);
    }
}

Error when I compile this:

non-void function does not return a value in all control paths
[-Werror,-Wreturn-type]

However, I do not understand why I get this error message even though all my if statements have a return value.

However, when compared to the answer key, they used else if. I understand why they are right, but I do not understand why I am wrong though.

Answer 1

The issue lies in your recursive calls to the binary function. Let's take a closer look:

if(a[mid]<target)
{
    return binary(a,target,l+1,r); // This should be mid+1 instead of l+1
}
if(a[mid]>target)
{
    return binary(a,target,l,r-1); // This should be mid-1 instead of r-1
}

In the recursive calls, you're not adjusting the boundaries properly. When you're searching the left half of the array, you should update r to mid - 1, not r - 1. Similarly, when searching the right half, you should update l to mid + 1, not l + 1.

Here's the corrected code:

if(a[mid]<target)
{
    return binary(a,target,mid+1,r);
}
if(a[mid]>target)
{
    return binary(a,target,l,mid-1);
}

Answer 2

i do not understand why i get this error message even though all my if statements have a return value.

Because, although it is obvious to you that one of those three if conditions must be true, the compiler can't work it out and thinks there is a possibility of not executing any of the blocks that return a value and thus falling out of the bottom of the function.

Edit missed a bit of the question.

I understand why they are right, but i do not understand why i am wrong though.

In this case, you are not wrong. Because the statements executed inside the ifs all return from the function, using if or else if is effectively the same.

To fix the error you are seeing, you need to omit the last condition because, by the time you get there, it is always true. You can do this:

    if(a[mid]==target)
    {
        return mid;
    }

    if(a[mid]<target)
    {
        return binary(a,target,l+1,r);
    }
    // a[mid]>target is guaranteed at this point
    return binary(a,target,l,r-1);

Or this:

    if(a[mid]==target)
    {
        return mid;
    }

    if(a[mid]<target)
    {
        return binary(a,target,l+1,r);
    }
    else
    {
        // a[mid]>target is guaranteed at this point
        return binary(a,target,l,r-1);
    }

Or this

    if(a[mid]==target)
    {
        return mid;
    }
    else if(a[mid]<target)
    {
        return binary(a,target,l+1,r);
    }
    else
    {
        // a[mid]>target is guaranteed at this point
        return binary(a,target,l,r-1);
    }

Personally, I go with the last one and I don't like the middle one because it mixes styles, but it's a matter of personal choice because they are all functionally the same.

Edit 2 See also Priya's answer for the other bug in your code.