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.

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.

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.

// 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.

// 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.

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.

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.

// 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.

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.

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.

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.

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

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.