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