for-in Loops and Ranges

A loop repeats code. A for-in loop runs once for each item in a sequence — a range of numbers, the elements of an array, or the characters of a string.

Looping over a range

for i in 1...3 {
    print("Pass \(i)")
}

Pass 1
Pass 2
Pass 3

1...3 includes 3; 0..<3 would give 0, 1, 2 — the natural choice when counting positions, which start at 0.

Looping over a collection

let fruits = ["apple", "banana", "cherry"]

for fruit in fruits {
    print("I like \(fruit)")
}

I like apple
I like banana
I like cherry

enumerated() also gives each position:

let fruits = ["apple", "banana"]

for (index, fruit) in fruits.enumerated() {
    print("\(index + 1). \(fruit)")
}

1. apple
2. banana

Steps and reverse order

for n in stride(from: 0, through: 10, by: 5) {
    print(n, terminator: " ")
}
print()

for n in (1...3).reversed() {
    print(n, terminator: " ")
}
print()

0 5 10
3 2 1

stride(from:to:by:) excludes the end; stride(from:through:by:) includes it.

When you don't need the value

Use _ if the loop variable isn't used:

for _ in 1...3 {
    print("Hip hip hooray!")
}

Common mistakes

• Writing 1..3 — that's not Swift syntax. Use 1...3 or 1..<3.
• Creating a range where the start is bigger than the end, like 5...1. It crashes at runtime; use (1...5).reversed().

Practice

Print the 5 times table from 5 x 1 = 5 to 5 x 10 = 50.

for n in 1...10 {
    print("5 x \(n) = \(5 * n)")
}