about.md

For loops

Looping is a fundamental concept in programming that allows you to execute a block of code multiple times. In Swift, there are two types of loops: for-in loops and while loops. In this chapter, you'll learn about for-in loops.

For loops allows you to iterate over a sequence of values, taking each element in turn, binding it to a variable of your choosing. Swift allows you to iterate over a variety of sequences, such as ranges, arrays, and strings (and more types which will be covered later). When every element of the sequence has been iterated over, the loop exits.

For loops are declared by using the for keyword, followed by a variable name, the in keyword, and a sequence of values to iterate over. The variable given in the for-in loop is inmutable, meaning you can't change its value inside the loop. Here's an example of a for-in loop that iterates over an array of numbers:

let numbers = [3, 10, 7, 11]

for number in numbers {
  print(number)
}
print("Done with numbers")

// prints:
// 3
// 10
// 7
// 11
// Done with numbers

The `number` variable is declared in the `for-in` loop and is only available within the loop's scope.

```swift
let numbers = [3, 10, 7, 11]

for number in numbers {
    number + 1
}
number + 1 // Error: Use of unresolved identifier 'number'
```

Iterating over a range

You can also iterate over a range of numbers using a for-in loop. This allows you to execute a block of code a specific number of times, for example, the range 1...5 will iterate over the numbers 1, 2, 3, 4, and 5, so the loop will execute 5 times. Sometimes you might want to iterate over indexes, in a datastructure like an array, then you can use a 0..<array.count range.

let numbers = [3, 10, 7, 11]

for i in 0..<numbers.count {
  print(numbers[i])
}

// prints:
// 3
// 10
// 7
// 11

Iterating over a string

You can also iterate over a string using a for-in loop. This allows you to iterate over each character in the string, and note specifically that the type given in the loop is a Character.

let message = "Hello!"

for character in message {
  print(character)
}

// prints:
// H
// e
// l
// l
// o
// !

Unusued variables

If you don't need the value of the variable in the loop, you can use an underscore _ to ignore it.

let numbers = [3, 10, 7, 11]

for _ in numbers {
  print("Hello!")
}

stride

Swift also provides a stride function that allows you to create a sequence over a range with a specific step. Which can be then iterated over using a for-in loop. stride is defined as stride(from:to:by:) or stride(from:through:by:), the first one is exclusive and the second one is inclusive.

for i in stride(from: 0, to: 10, by: 2) {
  print(i)
}

// prints:
// 0
// 2
// 4
// 6
// 8

Note that the to parameter is exclusive, so the loop will iterate until the number before the to parameter, while the through parameter is inclusive, so in this case it would also include the 10.