Lesson 17Control Flow

Repeat-While

Do First, Check Later

The repeat-while loop runs the code first, then checks the condition. It always runs at least once!

Basic Syntax

repeat {
    // code to run
} while condition

While vs Repeat-While

while

Check → Run

Checks condition first
May run 0 times

repeat-while

Run → Check

Runs code first
Runs at least 1 time

When to Use Repeat-While

📋

Menu Systems

Show menu at least once

✅

Input Validation

Get input, then validate

🎲

Games

Roll dice at least once

🔄

Retry Logic

Try at least once

Key Difference

Remember: repeat-while is called do-while in other languages (C, Java, JavaScript). Swift uses repeat to avoid confusion with the do keyword used for error handling!

main.swift

// Basic repeat-while loop
var count = 0

repeat {
    print("Count: \(count)")
    count += 1
} while count < 5

// Always runs at least once!
var number = 10

repeat {
    print("This runs even though number is already 10!")
} while number < 5

// Menu selection example
var choice = 0

repeat {
    print("Menu:")
    print("1. Play Game")
    print("2. Settings")
    print("3. Exit")

    choice = Int.random(in: 1...3)  // Simulate user input
    print("You selected: \(choice)")

    switch choice {
    case 1:
        print("Starting game...")
    case 2:
        print("Opening settings...")
    case 3:
        print("Goodbye!")
    default:
        print("Invalid choice")
    }
} while choice != 3

// Dice game - roll until you get 6
var roll: Int

repeat {
    roll = Int.random(in: 1...6)
    print("You rolled: \(roll)")
} while roll != 6
print("You got 6! Game over!")

// Input validation simulation
var validInput = false
var attempts = 0

repeat {
    attempts += 1
    validInput = (attempts == 3)  // Simulate valid on 3rd try

    if !validInput {
        print("Invalid input, try again...")
    }
} while !validInput
print("Input accepted after \(attempts) attempts!")

Try It Yourself!

Create a simple menu that shows options and keeps asking the user until they choose "Exit"!