for-in / while / repeat-while
Swift provides three types of loops: for-in for iterating over a collection in order, while for repeating while a condition is true, and repeat-while for executing at least once before checking the condition.
Syntax
// for-in (iterate over a collection)
for variable in collection {
// body
}
// stride (iterate over a numeric range)
for variable in stride(from: start, to: end, by: step) {
// body
}
// while (repeat while condition is true)
while condition {
// body
}
// repeat-while (execute at least once)
repeat {
// body
} while condition
Syntax Overview
| Syntax | Description |
|---|---|
| for i in 0..<5 | Iterates from 0 to 4 (half-open range, end not included). |
| for i in 0...4 | Iterates from 0 to 4 (closed range, end included). |
| for _ in range | Uses _ to ignore the loop variable when it is not needed. |
| stride(from:to:by:) | Iterates with a specified step size. A negative step value iterates in reverse. |
| stride(from:through:by:) | Iterates with a specified step size, including the end value. |
| for (index, value) in array.enumerated() | Retrieves both the index and value on each iteration. |
Sample Code
// Iterate over an array
let fruits = ["Apple", "Orange", "Grape"]
for fruit in fruits {
print(fruit)
}
// Retrieve index and value together
for (i, fruit) in fruits.enumerated() {
print("\(i + 1): \(fruit)")
}
// Count up by 2 using stride
for n in stride(from: 0, to: 10, by: 2) {
print(n, terminator: " ") // 0 2 4 6 8
}
print()
// while loop
var count = 3
while count > 0 {
print("Count: \(count)")
count -= 1
}
// repeat-while (runs at least once regardless of the condition)
var num = 0
repeat {
print("repeat: \(num)")
num += 1
} while num < 3
Notes
Swift does not have a C-style for (var i = 0; i < n; i++) loop. To iterate over a numeric range, use the range operator (0..<n) or stride().
Use break to exit a loop early, and continue to skip to the next iteration. See break / continue / return for details.
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