Random excel function with no duplicates and unfixed data - excel

Question

I need to translate an old program working on AS/400 that was picking random students to work for my city. I can use any program, as long as it works. To make it simple and fast, i chose excel.
However, i come over a small problem. I need to have no duplicates, because the same student can't do 2 jobs over one summer. Also, i need this to be flexible, since every year, new students will be added and some will be deleted.
This function works almost as much as i would want it:
=INDEX($A:$A,RANDBETWEEN(1,COUNTA($A:$A)),1)
The index $A:$A gets all the lines in the column A. So even if i add 20 names, it will take them into consideration. Then it choose randomly a value (the name) between the line 1 and the number of total lines (COUNTA) in the column $A. The problem with this method is that it allows duplicates.
Another function i found was to create a colum full of =ALEA() and then rank these by the numbers. This is not very pretty, but at least, there is no duplicates. The problem comes from my formula, that is static, and that i can't make flexible:
=INDEX($A$2:$A$74,RANK(B2,$B$2:$B$74))
My names are in the colum $A and my random values in colum $B. What i say is, rank the value in B2 (then B3, then B4, etc.) that is found in the column B.
What i would like is to integrate the COUNTA into the second function and (IF POSSIBLE) take the RANDBETWEEN instead of the rank function so that i don't have ugly numbers.
I am opened to use the first function with some kind of duplicate check. As long as the secretary doesn't have to do a lot of manipulation, it should be fine.
Thanks a lot for your help xox

Answer 1

I've created something in VBA that does what I think you want. Now keep in mind I'm very new with VBA, so it probably isn't the prettiest thing. To be clear, I had 10 names in column A from rows 1 to 10 and then simply ran this subroutine and it generated a list of unique names in column F. Here's my code:
Sub getRandom()
Do While Application.WorksheetFunction.CountA(Range("A:A")) > 0
Dim count As Integer
count = Application.WorksheetFunction.CountA(Range("A:A"))
Dim name As String
name = Application.WorksheetFunction.Index(Range("A:A"), Application.WorksheetFunction.RandBetween(1, count))
Dim row As Integer
row = Application.WorksheetFunction.Match(name, Range("A:A"), 0)
Range("F11").Select
Selection = name
Rows(row).EntireRow.Delete
Loop
End Sub
If you wanted to get the names one at a time, just remove the loop. How it works is it does exactly what you did with the INDEX and RANDBETWEEN functions, grabs the name in column A with that generated number, then deletes that row entirely, and thus no unique name is generated.
I chose column F arbitrarily and cell F11 specifically since that cell will not be affected when the rows are deleted.
I hope this helps, and if it's not what you were looking for I'll see if I can enhance it a bit.

Answer 2

There are many examples available for doing this in VBA (google Excel Random Unique) One of the best is from CPearson.Com which includes (among others):
Random Elements From A Range Of Worksheet Cells
You can also create a function that returns a number of elements in random order and without duplicates from a range of worksheet cells. The function below does just this. You can call as an array formula with a formula like
=RandsFromRange(A1:A10,5)
where A1:A10 is the list of elements from which to pull the values and 5 is the number of values to return. Enter the formula in a range with as many cells as you specify to return, and press CTRL SHIFT ENTER rather than just ENTER.
If you don't want to use VBA then then it can be done with a few simple steps:
Insert a column next to your student names, with the formula =RAND()
Sort the list of names and random numbers on the number
Pick as many students as you want fromm the top of the list
Each time you sort you get a new random ordering