Recognizing when to use the modulus operator

Question

I know the modulus (%) operator calculates the remainder of a division. How can I identify a situation where I would need to use the modulus operator?

I know I can u

Answer 1

The modulo can be useful to convert and split total minutes to "hours and minutes":

hours = minutes / 60

minutes_left = minutes % 60

In the hours bit we need to strip the decimal portion and that will depend on the language you are using.

We can then rearrange the output accordingly.

Answer 2

One use case I saw recently was when you need to reverse a number. So that 123456 becomes 654321 for example.

int number   = 123456;
int reversed = 0;

while ( number > 0 ) {
    # The modulus here retrieves the last digit in the specified number
    # In the first iteration of this loop it's going to be 6, then 5, ...
    # We are multiplying reversed by 10 first, to move the number one decimal place to the left.
    # For example, if we are at the second iteration of this loop, 
    #  reversed gonna be 6, so 6 * 10 + 12345 % 10 => 60 + 5
    reversed = reversed * 10 + number % 10;
    number = number / 10;
}

Answer 3

It's an easy way to tell if a number is even or odd. Just do # mod 2, if it is 0 it is even, 1 it is odd.

Answer 4

• Wrapping values (like a clock).
• Provide finite fields to symmetric key algorithms.
• Bitwise operations.

And so on.

Answer 5

My favorite use is for iteration.

Say you have a counter you are incrementing and want to then grab from a known list a corresponding items, but you only have n items to choose from and you want to repeat a cycle.

var indexFromB = (counter-1)%n+1;

Results (counter=indexFromB) given n=3:

`1=1`
`2=2`
`3=3`
`4=1`
`5=2`
`6=3`
...

Answer 6

Often, in a loop, you want to do something every k'th iteration, where k is 0 < k < n, assuming 0 is the start index and n is the length of the loop.

So, you'd do something like:

int k = 5;
int n = 50;
for(int i = 0;i < n;++i)
{
    if(i % k == 0)  // true at 0, 5, 10, 15..
    {
        // do something
    }
}

Or, you want to keep something whitin a certain bound. Remember, when you take an arbitrary number mod something, it must produce a value between 0 and that number - 1.