## Introduction to Modular Arithmetic

“In mathematics, modular arithmetic is a system of arithmetic for integers, where numbers “wrap around” upon reaching a certain value—the modulus.” – Wiki

The clock is a good example for modular arithmetic. Let’s say $12$’o clock means $0$. Then clock shows the following values, ${0,1,2,3,4,5,6,7,8,9,10,11,0,1,2,3…}$. Every time clock hits $12$, it wraps around to $0$. Since clock wraps around $12$, we say that $12$ is the Modulus.

# General Concepts

In order to understand modular arithmetic, we need to know the following:

## Modulo Operation $\%$

Modular arithmetic deals with Modulo operation. It has the symbol, “$\%$”. The expression $A \% B = C$ means that when we divide $A$ with $B$ we get the remainder $C$. In other words, the modulo operation finds the remainder. For example:

$5 \ \% \ 3 = 2$

$15 \ \% \ 5 = 0$

$24 \ \% \ 30 = 24$

$30 \ \% \ 24 = 6$

$4 \ \% \ 4 = 0$

## Congruence Relation $\equiv$

In mathematics, saying that $A \ \equiv \ B \ \text{(mod M)}$ means that both $A$ and $B$ has same remainder when they are divided by $M$. For example,

$5 \ \equiv \ 10 \text{ (mod 5) }$

$2 \ \equiv \ 7 \text{ (mod 5) }$

$4 \ \equiv \ 10 \text{ (mod 3) }$

## Range

In modular arithmetic with modulus $M$, we only work with values ranging from $0$ to $M-1$. All other values can be converted to the corresponding value from the range using Modulo operation. For example for $M=3$:

Type | |||||||
---|---|---|---|---|---|---|---|

Normal Arithmetic | -3 | -2 | -1 | 0 | 1 | 2 | 3 |

Modular Arithmetic | 0 | 1 | 2 | 0 | 1 | 2 | 0 |

# Properties of Modular Arithmetic

There are four properties of Modular Arithmetic that we need to learn.

## Addition

$$ \text{if,}\ a\ \equiv\ b\ \text{(mod m)} \ \text{and,}\ c\ \equiv\ d\ \text{(mod m)} \ \text{then,}\ a + c\ \equiv\ b + d \ \text{(mod m)}$$

Meaning, if we have to find $(a_1+a_2+a_3…+a_n)\ \%\ m$, we can instead just find $( (a_1\ \% \ m ) + (a_2\ \% \ m ) +… (a_n\ \% \ m ) )\ \% \ m$.

For example, $( 13 + 24 + 44 )\ \%\ 3 = ( 1 + 0 + 2 )\ \%\ 3 = 3\ \%\ 3 = 0$.

## Subtraction

$$ \text{if,}\ a\ \equiv\ b\ \text{(mod m)} \ \text{and,}\ c\ \equiv\ d\ \text{(mod m)} \ \text{then,}\ a – c\ \equiv\ b – d \text{(mod m)}$$

Meaning, if we have to find $(a_1-a_2-a_3…-a_n)\ \%\ m$, we can instead just find $( (a_1\ \%\ m ) – (a_2\ \%\ m ) -… (a_n\ \%\ m ) )\ \%\ m$.

For example, $( -14 – 24 – 44 )\ \%\ 3 = ( -2 – 0 – 2 )\ \%\ 3 = -4\ \%\ 3 = -1 \ \%\ 3 = 2$.

## Multiplication

$$ \text{if,}\ a\ \equiv\ b\ \text{(mod m)} \ \text{and,}\ c\ \equiv\ d\ \text{(mod m)} \ \text{then,}\ a \times c\ \equiv\ b \times d\ \text{(mod m)}$$

Meaning, if we have to find $(a_1 \times a_2 \times a_3… \times a_n)\ \%\ m$, we can instead just find $( (a_1\ \%\ m ) \times (a_2\ \%\ m ) \times … (a_n\ \%\ m ) )\ \%\ m$.

For example, $( 14 \times 25 \times 44 )\ \%\ 3 = ( 2 \times 1 \times 2 )\ \%\ 3 = 4\ \%\ 3 = 1 \ \%\ 3 = 1$.

## Division

Modular Division does not work same as the above three. We have to look into a separate topic known as Modular Inverse in order to find $\frac{a}{b}\ \%\ m$. Modular Inverse will be covered in a separate post.

For now just know that, $\frac{a}{b}\ not\equiv\ \frac{a\ \%\ m} {b\ \%\ m}\ \text{(mod m)}$. For example, $\frac{6}{3}\ \%\ 3$ should be $2$. But using the formula above we get $\frac{0}{0}\ \%\ 3$ which is undefined.

# Coding Tips

## Handling Negative Numbers

In some programming language, when modulo operation is performed on negative numbers the result is also negative. For example in C++ we will get $-5\ \%\ 4 = -1$. But we need to convert this into legal value. We can convert the result into legal value by adding $M$.

Therefore, in C++ it is better to keep a check for a negative number when doing modulo operation.

res = a % m; if ( res < 0 ) res += m;

## Modulo Operation is Expensive

Modulo Operation is as expensive as a division operator. It takes more time to perform division and modulo operations than to perform addition and subtraction. So it is better to avoid using modulo operation where possible.

One possible situation is when we are adding lots of values. Let's say that we have two variables $a$ and $b$ both between $0$ to $m-1$. Then we can write:

res = a + b; if ( res >= m ) res -= m;

This technique will not work when we are multiplying two values.

# Resource

- Wiki - Modular Arithmetic - https://en.wikipedia.org/wiki/Modular_arithmetic
- AoPS - Introduction to Modular Arithmetic - http://www.artofproblemsolving.com/wiki/index.php/Modular_arithmetic/Introduction