# Loops

![](imgs/4%20Loops_0.png)

---

# Overview

While Loop

For Loop

Nested Loops

Breaking Out of Loops

# Loops

Loops are useful when you need to repeat a block of code multiple times and to avoid code like this:

Console.WriteLine(count);

++count;

Console.WriteLine(count);

++count;

Console.WriteLine(count);

// Yuck! Nobody wants to see this.

Loops can also be used for iterating through arrays and lists (more on those later)

# while Loop

_while_  loop keeps executing its code block as long as the condition in its statement is true:

int count = 0;

while(count < 4)

{

Console.WriteLine(count);

++count;

}

// Outputs '0', '1', '2' and '3'

# do..while Loop

_do..while_  works the same way as while, but it executes the code at least once and checks the condition at the end:

int count = 0;

do

{

Console.WriteLine(count);

++count;

} while (count < 4);

// Outputs '0', '1', '2' and '3'

# for Loop

_for _ loops are used when the number of iterations are predefined

for loop executes in three inherent steps:

for (/*initial value*/;/*condition*/;/*increment*/)

{

/* Code to be executed */

}

The initial value is set before the first iteration. The condition is checked before each iteration, and the increment is executed after each iteration.

# for Loop (continued)

Try to understand the following two examples:

for (int i = 0; i < 4; ++i)

{

Console.WriteLine

("Current: " + i);

}

/* Outputs

Current: 0

Current: 1

Current: 2

Current: 3

*/

for (int i = 8; i > 0; i -= 2)

{

Console.WriteLine

("Current: " + i);

}

/* Outputs

Current: 8

Current: 6

Current: 4

Current: 2

*/

We start from zero, print the current value and add one until the value is no longer under four.

We start from eight, print the current value and remove two until the value is no longer larger than zero.

for (int i = 8; i > 0; i -= 2)

{

Console.WriteLine

("Current: " + i);

}

for (int i = 0; i < 4; ++i)

{

Console.WriteLine

("Current: " + i);

}

# Nested Loops

Nothing prevents you from using a loop inside another loop:

int columns = 3;

int rows = 4;

for (int i = 0; i < columns; ++i)

{

for (int j = 0; j < rows; ++j)

{

// Console.Write doesn't add a line break after printout

Console.Write("(" + j.ToString() + "," + i.ToString() + ")");

}

Console.WriteLine("");

}

![](imgs/4%20Loops_1.png)

# Breaking Out of Loops

To stop the execution of a loop completely, use the  _break _ -keyword:

int i = 0;

while(true)

{

++i;

if(i > 3)

break;

}

// i is now 4

To skip the current iteration, use the  _continue _ -keyword

int i = 0;

while(i < 10)

{

++i;

if (i % 2 == 0)

continue;

Console.WriteLine(i);

}

// Prints every odd number from 1 to 10

# Exercise 1: The Main Loop

At the heart of every dynamic program, there is something called the  _event loop _ a.k.a the  _main loop. _ Create a console application which keeps asking the user for an input, until the user inputs "quit".

Expand the program so that if the user inputs "help", a dialog shows up which shows the command and explanation for the command for both quit and help commands.

![](imgs/4%20Loops_2.png)

# Exercise 2: Prime Numbers

Prime number is a number greater than 1 that is not a product of two small numbers, i.e. cannot be divided into a whole number.  (Wikipedia:  [https://en.wikipedia.org/wiki/Prime\_number](https://en.wikipedia.org/wiki/Prime_number) )

Create a console application which prints all the prime numbers between 0 .. 40

![](imgs/4%20Loops_3.png)

# Assignments

[Assignments about this topic can be found here](https://gitea.buutti.com/education/academy-assignments/src/branch/master/C%23%20Basics/4.%20Loops)