**C# Modulo Examples**Apply the modulo division operator to get remainders from dividing numbers.

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**Modulo.** This operator gets a remainder. It provides a way to execute code once every several iterations of a loop. To use modulo, we specify the percentage sign character.

**Modulo notes.** Modulo has some unique properties. As with all low-level operations, it has a specific cost. We analyze modulo division in the C# language.

Divide

Odd, Even

Every Nth Element

**An example.** Modulo division is expressed with the percentage sign. It is implemented with "rem" in the intermediate language. Rem takes the top 2 values on the evaluation stack.

**Then** Rem performs the computation that returns the remainder of the division. It pushes that value onto the evaluation stack.

**Here** This example demonstrates the math behind modulo. The expressions here are turned into constants during the C# compilation step.

**5 modulo 3** When 5 is divided by 3, we have 2 left over—only one 3 can be part of 5. The modulo result is 2.

C# program that uses modulo operator

using System;
class Program
{
static void Main()
{

*
// When 5 is divided by 3, the remainder is 2.
*Console.WriteLine(

5 % 3);

*
// When 1000 is divided by 90, the remainder is 10.
*Console.WriteLine(1000 % 90);

*
// When 100 is divided by 90, the remainder is also 10.
*Console.WriteLine(100 % 90);

*
// When 81 is divided by 80, the remainder is 1.
*Console.WriteLine(81 % 80);

*
// When 1 is divided by 1, the remainder is zero.
*Console.WriteLine(1 % 1);
}
}

2
10
10
1
0

**Loop example.** We can use modulo in a loop for an interval or step effect. If we use a modulo operation on the loop index variable, we can execute code at an interval.

**Note** This example shows how to write to the screen every 10 iterations in the for-loop.

For

**Important** The 3 numbers in the condition in the if-statement can have any values, but we cannot divide by 0.

C# program that uses modulo division in loop

using System;
class Program
{
static void Main()
{

*
//
// Prints every tenth number from 0 to 200.
// Includes the first iteration.
//
***for** (int i = 0; i < 200; i++)
{
if (

(i % 10) == 0)
{
Console.WriteLine(i);
}
}
}
}

0
10
20
30
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90
100
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**Error, divide by zero.** If you use modulo by 0, you will get a compile error or a runtime exception. The denominator must never be zero.

DivideByZeroException

C# program that causes divide by zero error

class Program
{
static void Main()
{
int zero = int.Parse(

*"0"*);
int result = 100 % zero;
}
}

Unhandled exception.
System.DivideByZeroException: Attempted to divide by zero.
at Program.Main()...

**Performance.** Modulo is slower than other arithmetic operators such as increment and decrement or even multiply. This is a hardware limitation on computers.

**But** The total time required for individual modulo operations is tiny compared to other tasks such as disk reads or network accesses.

**So** If you can reduce those operations with modulo division, you can improve overall performance.

**Table** Some analysis from Microsoft is helpful. This page provides a table listing times required.

Writing Faster Managed Code: Microsoft Docs

Estimated costs of instructions:

Add:

1 ns
Subtract:

1 ns
Multiply:

2.7 ns
Divide:

35.9 ns**Discussion.** Modulo has several common uses in programs. You can use modulo division in loops to only execute code every several iterations. This can improve real code.

**A summary.** Modulo division returns the remainder of the 2 operands. We use the "percent" symbol for modulo in the C# language. This is a powerful operator, but it has its nuances.

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