Re: Random but unique

[miscdas]

Here is the technique that I used in another app. It goes through the list only one time, swapping a randomly chosen value from your list with the last element of the list, where the last element is decremented in each pass.
I decided to make a swap on EVERY pass, rather than checking to see if a swap is required, because until the very last pass the probablity of needing to swap is always greater than 50:50.
For a list L comprising n elements.
Begin loop at i=n and decrement i by 1.
Generate integer random number R in the range 1 through i.
Set Temp = L(i)
Set L(i) = L(R)
Set L(R) = Temp
Next i
So, on the first pass, the value of last element is swapped with the value of the randomly generated element. On the final pass, the value of element 2 is swapped with the value of element 1. Because a swap is made with the element L(i), you never have to worry about a duplicate value.
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