How can I seed a random number in Swift 3 [duplicate] - swift

Question

I need to start the same random number list over every execution of my app.
srand/rand do not exist anymore. What should I do then?
private extension Array {
private func randomValues(_ seed: UInt32, num: Int) -> [Element] {
srand (seed)
var indices = [Int]()
indices.reserveCapacity(num)
let range = 0..<self.count
for _ in 0..<num {
var random = 0
repeat {
random = randomNumberInRange(range)
} while indices.contains(random)
indices.append(random)
}
return indices.map { self[$0] }
}

Answer 1

You can use
srand48(seed) and drand48() in Swift3.

Answer 2

Unless you're developing with Swift for non-Apple platforms, you can get a much better randomization API in GameplayKit: several algorithms (trade randomness vs speed), seedable, distribution control, etc.

Answer 3

I can't find a way to use seeded random in Swift 3 Beta 1. Had to write a silly wrapper function in C:
// ----------------------------------------------
// my_random.h
// ----------------------------------------------
#ifndef my_random_h
#define my_random_h
#include <stdio.h>
#endif /* my_random_h */
long next_random();
// ----------------------------------------------
// my_random.c
// ----------------------------------------------
#include <stdlib.h>
#include "my_random.h"
long next_random() {
return random();
}
You can use the bridging header to import it into Swift. Then you can call it in Swift like this:
srandom(42)
for _ in 0..<10 {
let x = next_random()
print(x)
}
random is better than rand. Read the man pages for discussion on these 2 functions.
Edit:
A workaround, as #riskter suggested, is to use GameKit:
import GameKit
let seed = Data(bytes: [42]) // Use any array of [UInt8]
let source = GKARC4RandomSource(seed: seed)
for _ in 0..<10 {
let x = source.nextInt()
print(x)
}

Answer 4

For a simple repeatable random list try using a Linear Congruential Generator:
import Foundation
class LinearCongruntialGenerator
{
var state = 0 //seed of 0 by default
let a, c, m, shift: Int
//we will use microsoft random by default
init() {
self.a = 214013
self.c = 2531011
self.m = Int(pow(2.0, 31.0)) //2^31 or 2147483648
self.shift = 16
}
init(a: Int, c: Int, m: Int, shift: Int) {
self.a = a
self.c = c
self.m = m //2^31 or 2147483648
self.shift = shift
}
func seed(seed: Int) -> Void {
state = seed;
}
func random() -> Int {
state = (a * state + c) % m
return state >> shift
}
}
let microsoftLinearCongruntialGenerator = LinearCongruntialGenerator()
print("Microsft Rand:")
for i in 0...10
{
print(microsoftLinearCongruntialGenerator.random())
}
More info here:
https://rosettacode.org/wiki/Linear_congruential_generator

Answer 5

I just happened to put this together for Swift 4. I am aware Swift 4.2 has new random extensions that are different from this, but like the OP, I needed them to be seedable during testing. Maybe someone will find it helpful. If you don't seed it, it will use arc4random, otherwise it will use drand48. It avoids mod bias both ways.
import Foundation
class Random {
static var number = unseededGenerator // the current generator
/**
* returns a random Int 0..<n
**/
func get(anIntLessThan n: Int) -> Int {
return generatingFunction(n)
}
class func set(seed: Int) {
number = seedableGenerator
srand48(seed)
}
// Don't normally need to call the rest
typealias GeneratingFunction = (Int) -> Int
static let unseededGenerator = Random(){
Int(arc4random_uniform(UInt32($0)))
}
static let seedableGenerator = Random(){
Int(drand48() * Double($0))
}
init(_ gf: #escaping GeneratingFunction) {
self.generatingFunction = gf
}
private let generatingFunction: GeneratingFunction
}
func randomTest() {
Random.set(seed: 65) // comment this line out for unseeded
for _ in 0..<10 {
print(
Random.number.get(anIntLessThan: 2),
terminator: " "
)
}
}
// Run
randomTest()