如何使数组shuffle函数在后续调用中产生不同的结果

Question

我有以下功能,它可以实现我想要的单个呼叫:

let shuffle (arr : 'a array) =
    let array = Array.copy arr
    let rng = new Random()
    let n = array.Length
    for x in 1..n do
        let i = n-x
        let j = rng.Next(i+1)
        let tmp = array.[i]
        array.[i] <- array.[j]
        array.[j] <- tmp
    array

但是,对于多个调用,如下所示(x不用于任何内容),它会为每个调用产生相同的shuffle.我如何让它每次都产生不同的洗牌？

[for x in 1..3 do yield shuffle [|1;2;3|]]

>
val it : int [] list = [[|1; 3; 2|]; [|1; 3; 2|]; [|1; 3; 2|]]

Answer 1

你想在函数之外移动随机:

let rng = new Random()
let shuffle (arr : 'a array) =
    let array = Array.copy arr
    let n = array.Length
    for x in 1..n do
        let i = n-x
        let j = rng.Next(i+1)
        let tmp = array.[i]
        array.[i] <- array.[j]
        array.[j] <- tmp
    array

原因是RNG默认按时间播种,在紧密循环中没有足够的变化.将rng移到函数外部意味着它会在调用之间持续存在.