Swift - for Loop Examples

For-loop. In Swift 5.8 programs we use a for-in loop to iterate forward. We repeat statements. We access indexes and elements with enumerated. While is also used.

Loop over string chars. To loop over the characters in a string, we must use indexes. We start at startIndex and stop when endIndex is reached. We cannot just access chars with Ints.

String Length

Example, for-in. This loop is a "for-each" loop. It acts upon a range or a collection like an array. Here we use a range of the numbers 0, 1, 2 and 3.

Tip If we do not need to access the iteration variable, we can use an underscore instead of a proper name.

// Use a for-loop over the range. // ... The range is inclusive so all numbers are included. for i in 0...3 { print(i) } // An underscore can be used in place of an identifier name. for _ in 0...3 { print("Hello") }

0 1 2 3 Hello Hello Hello Hello

Exclusive bound. In Swift 4 the for-in loop is emphasized. Here we see the exclusive bound syntax, where we continue to one minus the maximum.

Tip An exclusive maximum bound can help us avoid some awkward "minus one" expressions.

// Use exclusive maximum in for-loop. for i in 0..<3 { print("EXCLUSIVE LOOP:", i) }

EXCLUSIVE LOOP: 0 EXCLUSIVE LOOP: 1 EXCLUSIVE LOOP: 2

While. The for-loop gets most of the attention in programs. But "while" is also useful. It continues iterating while a condition evaluates to true.

Here We begin the variable "i" at 5, and decrement it in each iteration of the while-loop. The loop stops when "i" is -1.

Info This loop is a decrementing loop—it proceeds from 5 to 0. We must be careful to terminate the loop.

var i = 5 // Decrement i while it is greater than or equal to 0. while i >= 0 { print(i) i -= 1 }

5 4 3 2 1 0

Break. This program uses the break keyword in a while-loop to stop the loop. It tests for even numbers, and after 3 even numbers have been encountered, it breaks.

Info This is a while-true loop. We can use while "true" to continue a loop infinitely until a break (or return) is reached.

Note We use a modulo division to test for even numbers. If a number is evenly divisible by 2, we know it is even.

var i = 5 var evenCount = 0 // Break loop when the third even number is encountered. while true { if i % 2 == 0 { print("Even") evenCount += 1 if evenCount == 3 { break } } print(i) i += 1 }

5 Even 6 7 Even 8 9 Even

Continue. In a loop, the continue statement stops the current iteration, but does not terminate the entire loop. The next iteration is started.

Here The program uses "continue" to avoid printing even numbers. Only odd numbers reach the print call.

// Loop over the numbers 0 through 10. for i in 0...10 { // If number is even, continue to the next iteration. if (i % 2) == 0 { continue } // Print remaining numbers. print(i) }

1 3 5 7 9

String characters. It is possible to loop over the characters in a String in Swift with a for-loop. We can directly access each Character this way.

String

let values = "abc" // Loop over Characters in String. for c in values { print(c) }

a b c

Enumerated. The enumerated() func can be used with a for-loop. A tuple of the index and value is returned for use in each iteration of the loop. Here we enumerate a String array.

Array

let codes = ["XX", "XY", "YZ"] // Loop over each index and value in the array. for (index, value) in codes.enumerated() { print("\(index) \(value)") }

0 XX 1 XY 2 YZ

Nested loop. For more complex algorithms, we can nest loops. In the inner block, we can access both the outer and the inner iteration variables.

// A for-loop can be nested. for i in 0...2 { for y in 0...2 { // Access both iteration variables in inner block. print("\(i), \(y)") } }

0, 0 0, 1 0, 2 1, 0 1, 1 1, 2 2, 0 2, 1 2, 2

Repeat-while. This loop continues iterating over a block while the condition (specified after the block) is true. It is the same as a do-while loop in other languages like C.

repeat while

Tip Sometimes a repeat-while loop can be a performance improvement. If the initial check is expensive and not needed, "repeat" is faster.

var size = 10 repeat { // Increment. size += 1 // Print the value. print(size) } while size < 16

11 12 13 14 15 16

Range error. A range must go from low (the start index) to high (the end index). The short syntax does not support a decrementing range.

// This will not work: a range must go from low to high. for i in 10...3 { print(i) }

fatal error: Can't form Range with end < start (lldb)

Adjacent elements. Here is a loop that accesses adjacent elements based on indexes. It uses enumerated() and ensures each index is the start of a pair.

Tip A for-enumerated() loop is powerful. It can accommodate many requirements (like adjacent accesses).

let positions = [10, 20, 30, 40, 50, 60] // Loop over elements and indexes in array. for (index, element) in positions.enumerated() { // See if index is even and not the last index. if index % 2 == 0 && index + 1 < positions.count { // Get adjacent elements. let next = positions[index + 1] print("\(element), \(next)") } }

10, 20 30, 40 50, 60

For C-style. In Swift 3 the C-style for-statement was removed. We must use enumerated() to iterate over the indexes and elements. A while-loop can also be a substitute.

// This does not work in Swift 3. for var i = 0; i < 4; i++ { print(i) }

main.swift:2:1: C-style for statement has been removed in Swift 3 main.swift:2:24: '++' is unavailable: it has been removed in Swift 3

Summary. Loops are important in most programs. And most of the time spent in programs is spent in loops. This is a good focus for optimization efforts.

Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.

Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.

This page was last updated on Sep 2, 2023 (edit link).

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