How do you generate a random number in swift?

Question

tl:dr; How do I generate a random number, because the method in the book picks the same numbers every time.

This seems to be the way in Swift to generate a random number, based on the book released from Apple.

protocol RandomNumberGenerator {
    func random() -> Double
}
class LinearCongruentialGenerator: RandomNumberGenerator {
    var lastRandom = 42.0
    let m = 139968.0
    let a = 3877.0
    let c = 29573.0
    func random() -> Double {
        lastRandom = ((lastRandom * a + c) % m)
        return lastRandom / m
    }
}
let generator = LinearCongruentialGenerator()

for _ in 1..10 {
    // Generate "random" number from 1-10
    println(Int(generator.random() * 10)+1)
}

The problem is that in that for loop I put at the bottom, the output looks like this:

The output is the same every time, no matter how many times I run it.

I looked at the other comments, and they were actually just copying and pasting what was in the book. — slooker, Jun 04 '14 at 00:42
look at the answers in the other question. they are a lot simpler than this and don't require a seed — Connor Pearson, Jun 04 '14 at 00:44
Yup. I'm deleting this question since it was duplicated. arc4random() is much simpler. Sorry! -- edit, can't delete because there are answers. Oh well. — slooker, Jun 04 '14 at 00:45

Anubian Noob · Accepted Answer · 2014-11-15T21:13:54.830

5

The random number generator you created is not truly random, it's psueodorandom.

With a psuedorandom random number generator, the sequence depends on the seed. Change the seed, you change the sequence.

One common usage is to set the seed as the current time, which usually makes it random enough.

You can also use the standard libraries: arc4random(). Don't forget to import Foundation.

edited Nov 15 '14 at 21:13

answered Jun 04 '14 at 00:42

Anubian Noob

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It's not really pseudorandom. It's pretty darn deterministic. – Michael Petrotta Jun 04 '14 at 00:43
I get "Use of unresolved identifier 'arc4random'" when I try to call arc4random() in the playground. How do I use the standard lib? – Dan Hixon Jun 04 '14 at 00:44
Seems to work if you `import Foundation` – Austin Jun 04 '14 at 00:44
1

@MichaelPetrotta, I'm not sure that you could find a pseudorandom number generator that isn't "pretty darn deterministic". – zneak Jun 04 '14 at 00:45
@DanHixon You'd have to import it... Setting the seed to the current time would work better. – Anubian Noob Jun 04 '14 at 00:45
@MichaelPetrotta This is how most random number functions are implemented. Surprised me once, but not anymore. – Anubian Noob Jun 04 '14 at 00:46
@zneak: Well, ok, I'm talking about the code the OP has posted, which is lacking a (non-fixed) seed. – Michael Petrotta Jun 04 '14 at 00:46
@AnubianNoob: random number functions require a non-fixed seed to work properly. – Michael Petrotta Jun 04 '14 at 00:47
@MichaelPetrotta That is a problem... You can change one of M, A, and C... – Anubian Noob Jun 04 '14 at 00:47

score 0 · Answer 2 · answered Jun 04 '14 at 00:41

0

Pseudorandom number generators need a "seed" value. In your case, if you change lastRandom with any number, you'll get a different sequence.

answered Jun 04 '14 at 00:41

zneak

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How do you generate a random number in swift?

2 Answers2