Loops in Swift (for-in, while, repeat-while)

Analogy categoryAnalogies

Introduction

Swift provides three loop constructs for repeating code: for-in, while, and repeat-while. The for-in loop iterates over sequences such as ranges, arrays, and dictionaries. The while loop checks its condition before each iteration, running zero or more times. The repeat-while loop is Swift's equivalent of the classic do-while loop — it checks its condition after each iteration, guaranteeing the body runs at least once. Notably, Swift removed the traditional C-style for (initialization; condition; increment) loop back in Swift 3, favoring the more expressive for-in loop combined with ranges and the stride functions instead.

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Cricket analogy: A bowler's over is a for-in loop through six fixed deliveries, a captain's field review is a while loop that checks the pitch condition before each change, and repeat-while is the umpire's rain inspection, which always happens once before deciding whether to inspect again — Swift dropped the old ball-by-ball tally counter back in Swift 3.

Syntax

swift

for item in sequence {
    // use item
}

while condition {
    // runs while condition is true
}

repeat {
    // runs at least once
} while condition

Explanation

In a for-in loop, sequence can be a range (1...5), an array, a dictionary, a string's characters, or any type conforming to the Sequence protocol. If you don't need the loop variable, you can use an underscore, like for _ in 1...3. The while loop evaluates condition before running the body each time, so the body may never execute if the condition starts false. The repeat-while loop evaluates the body first and checks condition afterward, guaranteeing at least one execution — this is directly analogous to do-while in C or Java, just with different keywords.

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Cricket analogy: for _ in 1...3 is running three unnamed practice deliveries without tracking the count, while resets a batsman's stance only while the bowler's run-up condition stays true and may leave him standing still, and repeat-while mirrors a no-ball review that always happens once before the umpire re-checks — the do-while of the crease.

Example

swift

for number in 1...3 {
    print("for-in: \(number)")
}

var count = 0
while count < 3 {
    print("while: \(count)")
    count += 1
}

var attempts = 0
repeat {
    print("repeat-while: \(attempts)")
    attempts += 1
} while attempts < 3

Output

swift

for-in: 1
for-in: 2
for-in: 3
while: 0
while: 1
while: 2
repeat-while: 0
repeat-while: 1
repeat-while: 2

Key Takeaways

Swift has three loop types: for-in, while, and repeat-while.
The traditional C-style for (init; condition; increment) loop was removed in Swift 3.
for-in iterates over ranges, arrays, dictionaries, and any Sequence-conforming type.
while checks its condition before each iteration, so it may run zero times.
repeat-while checks its condition after the body, so it always runs at least once.

Practice what you learned

1. Which traditional loop construct was removed from Swift starting with Swift 3?

2. Which loop is guaranteed to execute its body at least once?

3. What can a for-in loop iterate over in Swift?

4. When does a while loop check its condition?

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Introduction

Syntax

Explanation

Example

Output

Key Takeaways

Practice what you learned

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