Do-While Loop

After this lesson you can

• Run code at least once, then repeat it, with a do-while loop

Key takeaways

• A do-while loop runs its block first and checks the condition after, so the block always runs at least once.
• The condition comes at the end, followed by a semicolon: while (roll > 4);.
• Use do-while when something must happen once before you can ask "again?", such as showing a menu.

A do-while loop is a while loop turned upside down. It runs its block first, and checks the condition after. So its block always runs at least once.

The shape

int roll = 6;

do
{
    Console.WriteLine("You rolled a " + roll);
    roll--;
}
while (roll > 4);

You rolled a 6
You rolled a 5

The condition comes at the end, after while, and is followed by a semicolon.

At least once

Compare the two loops when the condition is false from the start:

int ticketsLeft = 0;

while (ticketsLeft > 0)
{
    Console.WriteLine("while: selling a ticket");
    ticketsLeft--;
}

do
{
    Console.WriteLine("do-while: selling a ticket");
    ticketsLeft--;
}
while (ticketsLeft > 0);

do-while: selling a ticket

The while loop checks first and never runs. The do-while runs once before it checks.

A menu that shows at least once

do-while fits whenever something must happen once before you can decide whether to repeat it. A menu is the classic case: you have to show it before the user can choose to quit.

string? choice;

do
{
    Console.WriteLine("1) Show scores");
    Console.WriteLine("2) Add a score");
    Console.WriteLine("Q) Quit");
    Console.Write("Choose: ");
    choice = Console.ReadLine();

    if (choice == "1")
    {
        Console.WriteLine("Showing scores...");
    }
    else if (choice == "2")
    {
        Console.WriteLine("Adding a score...");
    }
}
while (choice != "Q" && choice != "q" && choice != null);

Console.WriteLine("Goodbye.");

The menu always shows once; it repeats until the user types Q. (The choice != null part stops the loop if there's no more input at all.)

Check yourself

2 questions to try on the lesson page.

So what

Choose do-while when the block has to run before the question "again?" makes sense: menus, games, and asking for input until it's valid.

Key terms

• do-while Loop

Cheat sheets

• Do-While Loop