Swift while and repeat-while Loops: Controlling Iteration with Conditions

In Swift, while and repeat-while loops provide an alternative to for loops when you need to repeat a block of code until a specific condition is met. Unlike for loops, which are generally used when the number of iterations is known ahead of time, while and repeat-while loops are used when the number of iterations is unknown, but you want the loop to run based on a condition.

In this article, we’ll explore how while and repeat-while loops work in Swift, the differences between them, and when you should use each loop. We’ll also cover common use cases, examples, and best practices for using these loops effectively.

Swift while Loop

A while loop repeats a block of code as long as a condition remains true. The condition is evaluated before the loop executes, so if the condition is false at the start, the loop does not execute at all.

The syntax for a while loop is:

while condition {
    // Code to execute
}

The loop continues running as long as the condition evaluates to true.

Example 1: Basic while Loop

var count = 1

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

Output:

Count is 1
Count is 2
Count is 3
Count is 4
Count is 5

In this example, the while loop runs as long as count is less than or equal to 5. After each iteration, the count variable is incremented, eventually causing the loop to exit when count reaches 6.

Using while Loops for Unknown Iterations

The while loop is especially useful when you don’t know ahead of time how many iterations are needed, but you want the loop to continue until a certain condition is met.

Example 2: while Loop with User Input

var input: String?

while input != "exit" {
    print("Type 'exit' to stop")
    input = readLine()
}

In this example, the while loop repeatedly prompts the user for input and stops only when the user types "exit". The number of iterations is unknown, and the loop continues until the condition input != "exit" is false.

Swift repeat-while Loop

The repeat-while loop is similar to the while loop, but with one key difference: the condition is evaluated after the loop body has executed. This means the loop always runs at least once, even if the condition is false on the first check.

The syntax for a repeat-while loop is:

repeat {
    // Code to execute
} while condition

Example 3: Basic repeat-while Loop

var count = 1

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

Output:

Count is 1
Count is 2
Count is 3
Count is 4
Count is 5

In this example, the repeat-while loop behaves similarly to the while loop, but the condition is checked after each iteration. This guarantees that the code inside the loop runs at least once, regardless of the initial value of count.

Key Differences Between while and repeat-while

The primary difference between while and repeat-while loops is when the condition is checked:

• In a while loop, the condition is checked before the loop body executes. If the condition is false from the start, the loop body does not execute.
• In a repeat-while loop, the condition is checked after the loop body executes. The loop body is guaranteed to run at least once, even if the condition is initially false.

Example 4: Comparing while and repeat-while

var a = 10
var b = 10

// while loop
while a < 5 {
    print("This won't print because a is already 10.")
}

// repeat-while loop
repeat {
    print("This will print once, even though b is 10.")
} while b < 5

Output:

This will print once, even though b is 10.

In this example, the while loop does not execute because the condition is false from the start. However, the repeat-while loop executes once, even though the condition is false, because the condition is checked after the first iteration.

Practical Uses of while and repeat-while Loops

Example 5: Using a while Loop to Simulate a Countdown Timer

var seconds = 5

while seconds > 0 {
    print("\(seconds) seconds left...")
    seconds -= 1
}
print("Time's up!")

Output:

5 seconds left...
4 seconds left...
3 seconds left...
2 seconds left...
1 second left...
Time's up!

In this example, the while loop simulates a countdown timer. The loop continues running until seconds reaches zero.

Example 6: Using a repeat-while Loop to Ensure Valid Input

var password: String?

repeat {
    print("Enter your password:")
    password = readLine()
} while password == nil || password!.isEmpty

print("Password accepted.")

In this example, the repeat-while loop ensures that the user is prompted to enter a password at least once, and it continues to prompt the user until a valid password (non-empty input) is provided.

Breaking Out of Loops Early

You can use the break statement to exit a while or repeat-while loop early if a certain condition is met.

Example 7: Breaking Out of a while Loop

var number = 1

while number <= 10 {
    if number == 5 {
        print("Exiting the loop at number \(number)")
        break
    }
    number += 1
}

Output:

Exiting the loop at number 5

In this example, the while loop exits early when number reaches 5 due to the break statement.

Skipping Iterations

You can use the continue statement to skip the current iteration of a while or repeat-while loop and move on to the next iteration.

Example 8: Skipping Iterations in a while Loop

var i = 0

while i < 10 {
    i += 1
    if i % 2 == 0 {
        continue  // Skip even numbers
    }
    print(i)
}

Output:

1
3
5
7
9

In this example, the continue statement skips the even numbers, allowing only the odd numbers to be printed.

Infinite Loops

A while loop or repeat-while loop can become an infinite loop if the condition never evaluates to false. This can be intentional in some cases (e.g., waiting for an external event), but you should always ensure that there is a way to exit the loop eventually.

Example 9: Intentional Infinite Loop

while true {
    print("This loop will run forever unless stopped manually.")
    break  // Prevent infinite loop
}

In this example, the while true loop is intentionally infinite, but the break statement is used to exit the loop and prevent it from running forever.

Best Practices for Using while and repeat-while Loops

1. Use while when the number of iterations is unknown: If you need a loop that runs based on a condition rather than a fixed number of iterations, use a while loop.
2. Use repeat-while when you need the loop to run at least once: If you want to ensure that the loop body executes at least once before checking the condition, use a repeat-while loop.
3. Avoid infinite loops unless necessary: Be careful when using loops with conditions that might never become false. Always ensure there’s a clear way for the loop to exit to avoid infinite loops.
4. Use break and continue wisely: Use break to exit a loop early when necessary and continue to skip specific iterations.

Conclusion

Swift’s while and repeat-while loops provide flexibility when you need to repeat a block of code based on a condition. While loops are ideal when the number of iterations is unknown, and the condition is checked before the loop runs, whereas repeat-while loops are perfect for situations where you need the loop to execute at least once before checking the condition.

In this article, we covered:

• The syntax and usage of while and repeat-while loops.
• Key differences between while and repeat-while.
• Practical use cases for each loop type.
• Breaking out of loops early with break and skipping iterations with continue.

In the next article, we’ll explore Swift Nested Loops—how to use loops within loops

to process multidimensional data and perform complex iterations.

Happy coding!