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LESSON 111 OF 415 · FOR LOOPS - LOOPING OVER ARRAYS

Looping Through Arrays in Swift: for-in Loops

Lesson Read the full explanation

A for loop is a special type of loop in Swift which allows us to loop over the items of a collection, such as an array.

Looping through an array:

For example, an array is a sequence of items, so we can use a for loop to iterate over each item and do something with it:

let fruits = ["banana", "orange", "pear", "kivi"]

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

The output of this code is:

I eat banana
I eat orange
I eat pear
I eat kivi

Let's break down this code:

  • The for keyword that indicates the start of a for loop.
  • fruit is the loop variable. It will take the value of each different item in the collection
  • The in keyword separates the loop variable from the collection we will iterate on
  • fruits is the collection to loop over

Assignment
Now let's greet our friends using a for loop!

Hint
Look at the examples above if you get stuck.

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    SWIFT · FOR LOOPS - LOOPING OVER ARRAYS

    Looping Through Arrays in Swift, explained

    The for-in loop

    A for loop repeats a block of code once for each item of a collection, such as an array. Swift's version is called for-in:

    let fruits = ["banana", "orange", "pear", "kiwi"]

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

    I eat banana
I eat orange
I eat pear
I eat kiwi

    The pieces of the loop are:

    • for starts the loop.
    • fruit is the loop variable. On each pass it holds the next item of the array.
    • in separates the loop variable from the collection.
    • fruits is the collection to go through.
    • The braces hold the body, the code that runs once per item.

    You choose the name of the loop variable. A singular name for a plural array, like fruit for fruits, keeps the code easy to read.

    The loop variable is a constant

    You don't write let or var for the loop variable. Swift creates a new constant for each pass, so you can read it but not change it:

    for friend in friends {
    friend = "x"
}

    error: cannot assign to value: 'friend' is a 'let' constant

    The loop variable also exists only inside the braces. Using fruit after the loop is a compile error, cannot find 'fruit' in scope.

    Building a result

    A loop usually collects something. Declare a var before the loop, then update it in the body:

    var total = 0
for n in [3, 4, 5] {
    total += n
}
print(total)

    12

    Indexes, ranges and empty arrays

    When you also need the position of each item, use enumerated(), which pairs each item with its index starting at 0. To loop a fixed number of times, loop over a range such as 0..<3 (3 excluded) or 1...3 (3 included):

    for (i, fruit) in fruits.enumerated() {
    print(i, fruit)
}
for i in 0..<fruits.count {
    print(fruits[i])
}

    0 banana
1 orange
2 pear
3 kiwi
banana
orange
pear
kiwi

    If the array is empty, the body never runs and the loop finishes without an error. Using 0...fruits.count instead of 0..<fruits.count would be an off-by-one mistake: it reaches index 4 and the program crashes with an index out of range error.

    Counting loops and walking backwards

    When the loop variable isn't needed, write _ in its place so the body just repeats. To visit the indexes from last to first, reverse the array's indices:

    for _ in 1...2 {
    print("hi")
}
for i in fruits.indices.reversed() {
    print(i)
}

    hi
hi
3
2
1
0

    The exercise

    The starter code has an array friends. The task has two steps:

    1. Write a for-in loop with the loop variable friend over friends.
    2. Inside the body, print "Hey, \(friend)!". The \(...) inserts the current friend into the text.
    let friends = ["Andy", "Mary", "John"]

for friend in friends {
    print("Hey, \(friend)!")
}

    Hey, Andy!
Hey, Mary!
Hey, John!

    The body runs three times, once per friend, and adding a fourth friend to the array needs no other change.

    Common mistakes

    • Printing outside the braces. A statement after the closing brace runs once, after the loop.
    • Misspelling the loop variable. The name in the body must match the name after for, or the compiler reports that it cannot find the name in scope.
    • Changing the array while looping. The loop goes over the array as it was at the start, so appending inside the loop does not make it longer.

    The same loop works on other collections too. A string gives its characters one at a time, and a range gives its numbers.

    Key takeaways

    • for item in collection { ... } runs the body once per item.
    • The loop variable is a new constant on each pass and is visible only inside the loop.
    • Use enumerated() for indexes and 0..<n for counting loops.
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