The do-while loop

Learn loops that run at least once with do-while.

The do-while loop

The do-while loop runs the body FIRST and checks the condition AFTER. So the body always runs at least once. It is written do { ... } while (n > 0);.

For example, it is ideal for showing a menu at least once and looping until the user asks to stop. Unlike while, the first pass always happens even when the condition is false; you use it when you want to take one step before checking the result.

while vs do-while

Because while checks the condition first, the body may not run at all. do-while runs the body once and then checks the condition; so it always runs at least once.

For example, int n = 10; while (n < 5) { ... } never runs the body. The same condition with do-while would run the body once and then stop. This small difference creates a big behavior gap.

Exercise At least once with do-while

Declare an n variable with the value 0. Use a do-while loop: in the body print n, then increment with n++. The condition is n < 3. The output should be 0, 1, 2 (the body runs at least once).

Show example solution

Write int n = 0; do { Console.WriteLine(n); n++; } while (n < 3);
Example solution:
using System;

class Program
{
    static void Main()
    {
        int n = 0;
        do
        {
            Console.WriteLine(n);
            n++;
        }
        while (n < 3);
    }
}

Test yourself

Match each loop type to its feature

for

Counter, counted repeat
while

Condition first, zero runs possible
do-while

Body first, at least once
foreach

Collection elements
How many times does int n = 10; do { Console.WriteLine(n); } while (n < 5); print?
```
int n = 10;
do { Console.WriteLine(n); } while (n < 5);
```
- Never
- 5 times
- 10 times
- 1 time
Show answer

1 time — Correct. do-while runs the body once, then the condition is false and it stops.

Write the do-while terminator

int n = 0;
do
{
    Console.WriteLine(n);
    n++;
}
____ (n < 3);

Show answer

int n = 0;
do
{
    Console.WriteLine(n);
    n++;
}
while (n < 3);

Next lesson: The foreach loop

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