Table of Contents

## Issue

## Info

**Hi everyone**

I was searching an efficient way to check if a number is multiple of 5. So I searched on google and found this solution on geeksforgeeks.org.

**There were 3 solutions of my problem.**

- First solution was to subtract 5 until reaching zero,
- Second solution was to convert the number to string and check last character to be 5 or 0,
- Third solution was by doing some interesting operations on bitwise level.

I’m interested in third solution as I can fully understand the first and the second.

**Here’s the code from geeksforgeeks.**

```
bool isMultipleof5(int n)
{
// If n is a multiple of 5 then we
// make sure that last digit of n is 0
if ( (n & 1) == 1 )
n <<= 1;
float x = n;
x = ( (int)(x * 0.1) ) * 10;
// If last digit of n is 0 then n
// will be equal to (int)x
if ( (int)x == n )
return true;
return false;
}
```

I understand only some parts of the logic. I haven’t even tested this code. So I need to understand it to use freely.

As said in mentioned article this function is multiplying number by 2 if last bit is set and then checking last bit to be 0 and returns true in that case. But after checking binary representations of numbers I got confused as last bit is 1 in case of any odd number and last bit is 0 in case of any even number. So…

## Actual question is

What’s the logic of this function?

## Any answer is appreciated!

## Thanks for all!

## Solution

- The most straightforward way to check if a number is a multiple of 5 is to simply

```
if (n % 5 == 0) {
// logic...
}
```

What the bit manipulation code does is:

- If the number is odd, multiply it by two. Notice that for multiples of 5, the ones digit will end in either 0 or 5, and doubling the number will make it end in 0.
- We create a number x that is set to n, but with a ones digit set to 0. (We do this by multiplying n by 0.1, which removes the ones digit, and then multiply by 10 in order to add a 0, which has a total effect of just changing the ones digit to 0).
- We know that originally, if n was a multiple of 5, it would have a ones digit of 0 after step 1. So we check if x is equal to it, and if so, then we can say n was a multiple of 5.

Answered By – jh316

Answer Checked By – Candace Johnson (BugsFixing Volunteer)