Random but unique

[miscdas at boxfrog.com]

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. 

miscdas