Mod or Modulo operator (`%`) usually refers to an operator that returns the remainder of the division.

In Swift, this operator is called Remainder Operator (`%`).

## Remainder Formula

The Remainder operator has the following formula.

Dividend = Divisor x Quotient + Remainder

Therefore,

Remainder = Dividend – (Divisor x Quotient)

Dividend is a value that is to be divided by another value.
Divisor is the number which divides the dividend.
Quotient is the result of the division process.
Remainder is what remains after the division.

It is easier to understand with an example.

Example: `9 % 4`.

``let quotient = 9 / 4// 2let remainder = 9 % 4// 1``

### Remainder Operator with negative numbers

The remainder operator can be used with negative numbers, but the calculation isn't straightforward.

Here is the rule:

• Perform the operation as if both operands were positive.
• If the left operand (dividend) is negative, then make the result negative.
• If the left operand is positive, then make the result positive.
• Ignore the sign of the right operand (divisor) in all cases.

For example:

``9 % 4 // 19 % -4 // 1-9 % 4 // -1-9 % -4 // -1``

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