Choose between for, while, and do-while loops

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Programs become more useful when they can automate repetitive tasks. Instead of writing the same code over and over, you can use a loop to repeat a block of code automatically.

C# provides three loop types: for loops, while loops, and do-while loops. Choosing the right one depends on whether you know how many times to repeat ahead of time, and whether the loop body must run at least once.

The for loop (known number of repetitions)

Use a for loop when you want to repeat an action a known number of times.

A for loop has three parts, separated by semicolons: a starting value, a condition, and an update step.

for (int i = 0; i < 5; i++)
{
    Console.WriteLine(i);
}

Output:

0
1
2
3
4

The loop starts with i set to 0. Before each pass, C# checks whether i < 5. If it's true, the loop body runs, then i++ increases i by 1. Once i reaches 5, the condition becomes false, and the loop stops. This loop runs exactly 5 times.

The while loop

Use a while loop when you want to repeat something as long as a condition remains true. Choose this loop when you don't know how many times you'll need to repeat the action ahead of time.

int health = 3;

while (health > 0)
{
    Console.WriteLine($"Player is alive. Health: {health}");
    health--;
}

Output:

Player is alive. Health: 3
Player is alive. Health: 2
Player is alive. Health: 1

Avoid infinite loops

A while loop checks its condition before each iteration. If the condition is always true, the loop never stops. This behavior is called an infinite loop, and it freezes your program.

Always make sure your code updates the condition so it eventually becomes false. In the example above, health-- subtracts 1 from health each time through the loop. When health reaches 0, the condition becomes false, and the loop stops.

The do-while loop

Use a do-while loop when you need the loop body to run at least once, no matter what the condition is. Unlike for and while, a do-while loop checks its condition after the body runs.

int attempts = 0;

do
{
    attempts++;
    Console.WriteLine($"Attempt number {attempts}");
}
while (attempts < 3);

Output:

Attempt number 1
Attempt number 2
Attempt number 3

This pattern is useful for menus and retry prompts, where you need to show something to the user at least once before deciding whether to repeat it.

Tip

Choose descriptive loop variable names when they represent something meaningful, like attempts or health. Reserve short names like i for simple counters where the meaning is obvious from context.

Choosing the right loop

Choose a for loop when:

• You know how many times you want to repeat the action.
• You want an automatic exit after a certain number of repetitions.
• You want a counter variable to track how many times the loop has run.

Choose a while loop when:

• The total number of repetitions depends on a variable's state.
• You're waiting for something specific to happen before stopping the loop, such as user input.
• The loop might not need to run at all if the condition starts out false.

Choose a do-while loop when:

• You need the loop body to run at least once, regardless of the condition.
• You're validating user input and want to prompt the user at least one time before checking whether to repeat.