C# while

Summary: in this tutorial, you’ll learn how to use the C# while statement to execute a block while a boolean expression is true.

Introduction to the C# while statement

The while statement evaluates a boolean expression and executes a block repeatedly as long as the expression is true. Here’s the syntax of the while statement:

while (expression)
{
    // statement
}Code language: C# (cs)

How it works.

The expression, which follows the while keyword, must be a boolean expression that evaluates to true or false.

The while statement evaluates the expression first. If the expression evaluates to true, it’ll execute the block inside the curly braces.

Once completed executing the block, the while statement checks the expression again. And it’ll execute the block again as long as the expression is true.

If the expression is false, the while statement exits and passes the control to the statement after it.

Therefore, you need to change some variables inside the block to make the expression false at some point. Otherwise, you’ll have an indefinite loop.

Since the expression is checked at the beginning of each iteration, the while statement is often called a pretest loop.

The following flowchart illustrates how the C# while statement works.

C# while statement examples

Let’s take some examples of using the while statement.

1) Simple C# while statement example

The following example uses the while loop statement to output five numbers from 1 to 5 to the console:

int counter = 0;

while(counter < 5)
{
    counter++;
    Console.WriteLine(counter);
}Code language: C# (cs)

Output:

1
2
3
4
5Code language: C# (cs)

How it works.

First, declare a counter variable and initialize it to zero.

Second, enter the while loop because the following expression is true:

counter < 5Code language: C# (cs)

Third, increase the counter by one and print it out to the console; repeat this step as long as the counter is less than 5.

2) Using the C# while statement to calculate the average

The following program prompts users to enter a list of numbers and calculate the average:

double number = 0,
    total = 0,
    count = 0,
    average = 0;

string input = "";


Console.WriteLine("Enter a list of numbers to calculate the average (Q - quit):");

while (input != "Q" && input != "q")
{
    input = Console.ReadLine();
    if (input != "Q" && input != "q")
    {
        number = Convert.ToDouble(input);
        total += number;
        count++;
    }
}

if (count > 0)
{
    average = total / count;
}

Console.WriteLine($"Average:{average}");Code language: C# (cs)

How it works.

First, declare variables and initialize them:

double number = 0,
    total = 0,
    count = 0,
    average = 0;

string input = "";Code language: C# (cs)

Second, print out the instruction:

Console.WriteLine("Enter a list of numbers to calculate the average (Q - quit):");Code language: C# (cs)

Third, prompt users to enter a number until they enter the letter Q or q. In each iteration, calculate the total and count the entered numbers:

while (input != "Q" && input != "q")
{
    input = Console.ReadLine();
    if (input != "Q" && input != "q")
    {
        number = Convert.ToDouble(input);
        total += number;
        count++;
    }
}Code language: C# (cs)

Finally, calculate the average if users enter at least one number and output it to the console:

if (count > 0)
{
    average = total / count;
}
Console.WriteLine($"Average:{average}");Code language: C# (cs)

In the following output, we enter three numbers 10, 20, and 30. And the program shows the average as 20:

Enter a list of numbers to calculate the average (Q - quit):
10
20
30
q
Average:20Code language: C# (cs)

Summary

• Use the while statement to execute a block as long as a boolean expression is true.