Repeat

Some things in Swift have no known endpoint—we stop them (break the loop) on demand. For a simple loop, we can invoke the "repeat" keyword.

The repeat loop requires a "repeat" and a "while" keyword. To have an infinitely repeating loop, we can use a "while true" part at the end.

First example

Here we have a repeat-while loop. We have a local variable named "i" and we increment this in the body of the repeat-while loop.

Note The loop ends when our variable reaches the value 6. We print the statement "I have N cats" on each iteration.

var i = 0

// Use repeat loop.
// ... No initial condition is checked.
repeat {
    print("I have \(i) cats.")
    i += 1
} while i < 5I have 0 cats.
I have 1 cats.
I have 2 cats.
I have 3 cats.
I have 4 cats.

Repeat, random numbers

Here is a simple program that repeats a function call. It terminates the repeat-while loop only when the function call returns a specific number.

Note Arc4random returns a random number. In the following if-statement, we break if the number is even (not odd).

Tip Consider using a "while-true" loop when an infinite (or indeterminate) loop is needed. A break can stop the loop.

import Foundation

// Initialize a seed number.
sranddev()

// Get random number in repeat loop.
// ... Print number.
//     If number is even then break the loop.
repeat {
    let number = arc4random()
    print(number)
    // Check for even number.
    if number % 2 == 0 {
        print("Even number, stopping loop.")
        break
    }
} while true546090829
4220801392
Even number, stopping loop.

Repeat, example 3

The repeat-while loop does not check its condition before the loop body is entered. So a variable (like "x" in this program) can have any value.

var x = 999
repeat {
    // This block is entered on any value of x.
    // ... The value is not checked before the block is executed.
    print(x)
} while x < 10
999

The repeat-while loop is the same as a "do-while" loop in C-like languages. The word "repeat" may be easier to read for programmers. Its while part is required.