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)
        }
    }
}

score 5 · Accepted Answer · edited May 17 '20 at 19:09

5

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.

edited May 17 '20 at 19:09

pkamb

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answered Jul 11 '16 at 06:20

Nimit Kalra

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You can improve your code quality by removing statements like `if prime == true` and replacing them with `if prime`, for easier readability and maintainability of your code. – return true Jul 11 '16 at 06:31
Altho it's nice to teach the `break` keyword, in this case it would be a bad usage. `while true` should be set to `while n != 1000` – Antzi Jul 11 '16 at 06:31

ebby94 · Answer 2 · 2016-07-11T06:28:03.357

1

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
    }
}

edited Jul 11 '16 at 06:28

answered Jul 11 '16 at 06:17

ebby94

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1

Please expand your answer. – return true Jul 11 '16 at 06:22
1

They would need to increment `n`. Eventually they will get an integer overflow – Paulw11 Jul 11 '16 at 07:08
@Paulw11: On a 64-bit platform that would take several years if you process 10^6 integers per second :) – Martin R Jul 11 '16 at 07:13
1

Yeah, but that isn't "forever" ;). The question itself is fundamentally flawed. – Paulw11 Jul 11 '16 at 07:17

score 0 · Answer 3 · answered Jul 29 '20 at 04:44

0

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

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

answered Jul 29 '20 at 04:44

eonil

83,476
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it looks much better than `while true { }` but how to break such cycle if necessary? – Gargo Jul 14 '23 at 12:17

score -4 · Answer 4 · answered Jul 11 '16 at 06:52

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.

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

4 Answers4