Loops: while and for

Computers are good at doing the same thing over and over without getting bored. Instead of writing a statement out a hundred times, you write a loop: a block that is repeated until a condition no longer holds.

while: as long as something holds

// countdown.cs
int number = 5;
while (number > 0)
{
    Console.WriteLine(number);
    number = number - 1;
}
Console.WriteLine("Liftoff!");

5
4
3
2
1
Liftoff!

Before every round, while checks the condition. If it is true, the block runs, and then the condition is checked again. If it is false, the program goes on after the block. Inside the block, number gets one smaller each time, until the condition number > 0 no longer holds.

For “one smaller” and similar calculations, there are shorthand forms you will see often:

When a loop doesn’t stop

As a test, delete the line number = number - 1; in countdown.cs and run the program. Now number stays 5, the condition stays true, and the program writes 5 on line after line, forever. That’s an infinite loop. Nothing to worry about: press Ctrl+C in the terminal. That stops any running program, on Windows, macOS and Linux alike (on a Mac, too, it’s the Control key, not Command). Then put the line back.

For every loop, ask yourself: what in the block makes sure the condition turns false at some point?

for: counting

Very often, a loop simply counts from one number to another. That’s what for is made for:

// times-table.cs
for (int i = 1; i <= 10; i++)
{
    Console.WriteLine($"{i} x 7 = {i * 7}");
}

1 x 7 = 7
2 x 7 = 14
3 x 7 = 21
4 x 7 = 28
5 x 7 = 35
6 x 7 = 42
7 x 7 = 49
8 x 7 = 56
9 x 7 = 63
10 x 7 = 70

The parentheses after for hold three parts, separated by semicolons:

1. int i = 1 runs once at the start and creates the counter.
2. i <= 10 is checked before every round, just like with while.
3. i++ runs after every round and counts on.

The name i is the usual one for counters. But you can also call it row or number.

Collecting values

A loop can collect results across all its rounds. To do that, you create a variable before the loop and change it in every round:

// sum.cs
int sum = 0;
for (int i = 1; i <= 100; i++)
{
    sum += i;
}
Console.WriteLine($"The sum of 1 to 100 is {sum}.");

The sum of 1 to 100 is 5050.

If int sum = 0; were inside the block, the sum would start over at 0 in every round, and after the block it wouldn’t exist at all. The compiler would reject the last line. A variable only lives in the block where it was created.

Input until someone stops

Often a program doesn’t know in advance how much input will come. Then a loop runs “forever,” with while (true), and is left from the inside with break as soon as a certain input comes, here an empty line:

// add-numbers.cs
int sum = 0;
int count = 0;
Console.WriteLine("Enter whole numbers. An empty line ends the input.");
while (true)
{
    Console.Write("Number: ");
    string input = Console.ReadLine() ?? "";
    if (input == "")
    {
        break;
    }

    if (int.TryParse(input, out int number))
    {
        sum += number;
        count++;
    }
    else
    {
        Console.WriteLine("That was not a whole number, I'll skip it.");
    }
}
Console.WriteLine($"{count} numbers, sum {sum}.");

Enter whole numbers. An empty line ends the input.
Number: 12
Number: 30
Number: two
That was not a whole number, I'll skip it.
Number: 8
Number:
3 numbers, sum 50.

break ends the loop right away, and the program goes on after it. Because Console.ReadLine() ?? "" gives an empty text when the input has ended, the loop also ends cleanly when no more input comes at all.

// ten-to-zero.cs
for (int i = 10; i >= 0; i -= 2)
{
    Console.WriteLine(i);
}

10
8
6
4
2
0

// guess-the-number.cs
int secret = 42;
int tries = 0;
Console.WriteLine("I'm thinking of a number between 1 and 100.");
while (true)
{
    Console.Write("Your guess: ");
    string input = Console.ReadLine() ?? "";
    if (input == "")
    {
        Console.WriteLine($"Too bad. The number was {secret}.");
        break;
    }

    if (!int.TryParse(input, out int guess))
    {
        Console.WriteLine("Please enter a whole number.");
        continue;
    }

    tries++;
    if (guess < secret)
    {
        Console.WriteLine("Too low.");
    }
    else if (guess > secret)
    {
        Console.WriteLine("Too high.");
    }
    else
    {
        Console.WriteLine($"Correct! Number of tries: {tries}");
        break;
    }
}

I'm thinking of a number between 1 and 100.
Your guess: 50
Too high.
Your guess: 25
Too low.
Your guess: 37
Too low.
Your guess: 43
Too high.
Your guess: 42
Correct! Number of tries: 5

*
**
***
****
*****

// stairs.cs
for (int row = 1; row <= 5; row++)
{
    for (int star = 1; star <= row; star++)
    {
        Console.Write("*");
    }
    Console.WriteLine();
}

What you can do now

• You repeat statements with while as long as a condition holds.
• You count with for and choose the start, the end and the step yourself.
• You leave a loop with break, skip a round with continue and stop a stuck program with Ctrl+C.
• You collect values in a variable created before the loop.
• You put one loop inside another.

Read more

• Branches and loops: loops in Microsoft’s tutorial series, including the do loop and nested loops.
• Iterate through a code block using for statement in C#: a module with exercises on for.
• Add looping logic to your code using the do-while and while statements in C#: a module on while and do.