How to keep a Swift loop running indefinitely (forever)?

Question

I have written a function which currently displays all of the prime numbers under 1000.

I can keep making 1000 bigger to generate more numbers, but I do not know how to make it keep going on forever once run.

func generatePrimes() {
    let numbers = 2...1000
    for n in numbers {
        var prime = true

        for i in 2..<n { 
            if n % i == 0 {
                prime = false
                break
            }
        }

        if prime == true {
            print(n)
        }
    }
}

Answer 1

You could use a while loop with the argument being true . That way, the loop would never end.

func generatePrimes() {
    var n = 2
    while true {
        var prime = true

        for i in 2..<n { 
            if n % i == 0 {
                prime = false
                break
            }
        }

        if prime == true {
            print(n)
        }

        n += 1
    }
}

Note that we use a variable ( n ) and increment (add 1 to it) on each iteration of the while loop.

If you want it to end when it hits some n , you can have a conditional that checks against n and break 's out of the loop:

var n = 2
while true {
    if n == 1000 {
        break
    }
    n += 1
}

The above code will stop execution when n hits 1000 and will not print anything for that case.

Answer 2

Use a while loop instead of for loop. It can also be done with a for loop, but for loop is preferred when you know the number or range of the iterations you are performing. If you're using a while loop you just have to set the condition for the loop, in this case you need to specify the condition as true since you want the loop to run forever.

Just replace your for loop in your code with while . It should look something like this

while(true)
{
    if n % i == 0 {
        prime = false
        break
    }
}

Answer 3

You also can build an unconditional infinite loop using do {} block and continue statement.

AGAIN:
do {
    print("step...")
    continue AGAIN
}

Answer 4

func generatePrimes() {

    var prime = true

    let numbers = 2...1000
    for n in numbers {

        prime = true

        if n == 2 || n == 3 {
            print(n)
        } else {
            for i in 2..<n-1 {
                if n % i == 0 {
                    //not prime numbers
                    prime = false
                }
            }
            if prime == true {
                print(n)
            }
        }
    }
}

You can do it like this. Just exclude 1 and n.