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
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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);