Shuffling an array with a function in VBA

Question

I'm using a very old version of VBA that can't return arrays.

I copied most of this from another post and modified it.

x = Array(679, 680, 683, 781, 790, 792, 800, 801, 809, 818, 822, 871, 897, 911, 913, 924, 927, 929, 930, 934, 936, 946, 951, 952, 956, 970, 971)

Dim i As Long
For i = UBound(x) To LBound(x) Step -1
    Dim t As Variant
    t = x(i)
    Dim j As Long
    j = CLng((25 - 0 + 1) * Rnd + 0)
    x(i) = x(j)
    x(j) = t
Next i

When I loop through the outcome and the old array it never overlaps numbers.

The J = CLng((Max - min + 1) * Rnd + Min) but I just moved it the way I had it.

Why does the randomness never overlap to create duplication? I looped the outcomes over 100k times.

I'm happy it works great but I have no clue why it works LOL — G G Synaptic
– G G Synaptic, Commented Jan 25, 2023 at 23:42
The function you provided is using a technique called Fisher-Yates shuffle algorithm. It is a way to randomly shuffle the elements of an array. The idea behind the algorithm is to iterate through the array backwards, and for each element, swap it with a randomly chosen element that comes after it. This way, each element in the array will have an equal probability of being in any position. — Horaciux
– Horaciux, Commented Jan 25, 2023 at 23:43
So no matter how many numbers are used in the array it'll never overlap? If I had a list of a million numbers they would always be shuffled perfectly? What kind of sorcery is this? LOL — G G Synaptic
– G G Synaptic, Commented Jan 25, 2023 at 23:54
Let me add an answer for a any number or items, your code is hardcoded to 27 items — Horaciux
– Horaciux, Commented Jan 25, 2023 at 23:59
Is this actually more like Durstenfeld's version? Since it looks like the array position is switched with the random number generator number. OOOOO I see now. This is really cool. — G G Synaptic
– G G Synaptic, Commented Jan 26, 2023 at 0:01

Horaciux · Accepted Answer · 2023-01-25 23:58:46Z

1

Notice the line I change to dynamically increase the chosen subindex according to array size.

Dim i As Long
For i = UBound(x) To LBound(x) Step -1
    Dim t As Variant
    t = x(i)
    Dim j As Long
    j = CLng(LBound(x) * Rnd )
    x(i) = x(j)
    x(j) = t
Next i

answered Jan 25, 2023 at 23:58

Horaciux

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