# Creating a random combination

1. Dec 3, 2012

### reldridge

I am creating a random combination(code) for a client. I need to see what the total number of combinations there can be.

Please let me know if I am using the wrong word Combination vs Permutations.

Formula:
2 letters + 2 numbers + 2 letters
example: TR12FR

The random letters are taken from 24 values:
ABCDEFGHJKLMNPQRSTUVWXYZ

The random Numbers are taken from 8 Values:
23456789

Any help would be greatly appreciated.

Thanks
Ryan.

Last edited by a moderator: Feb 5, 2013
2. Dec 3, 2012

### Staff: Mentor

Re: Combinations

combination is right you're selecting letters, numbers and more letters:

Choices for each character of your 6 charcter code:

24 * 24 * 8 * 8 * 24 * 24 = total number of codes possible

Now if these could be rearranged then you'd have to worry about permutations to elimnate repeats in your counts.

3. Dec 3, 2012

### reldridge

Re: Combinations

Thank you -

So de-duping these down to create 2.5 million unique codes using this formula will not be an issue, sense there are 21,233,664 possibilities.

Last edited: Dec 3, 2012
4. Dec 3, 2012

### Staff: Mentor

Re: Combinations

okay what do you mean by de-duping?

5. Dec 3, 2012

### reldridge

Re: Combinations

Well sense this is just a random generator out of excel the possibility of it creating a duplicate is there, so i will need to just remove dupes, or nth select down to 2.5 million unique codes.

Just needed to make sure that the 6 digit using that formula could create that many unique codes.

6. Dec 3, 2012

### phinds

Re: Combinations

So you have a belief that "true" random would not include any dupes?

How would you create 100000 consecutive random numbers from 1 to 100 ?

7. Dec 3, 2012

### Staff: Mentor

Re: Combinations

this reminds of how new programmers used to construct a deck of randomly sorted cards.

generate a number from 0-51 check if its present in the crd deck array if so then generate another.

The better approach is to generate a sorted sequence of cards into an array and then to randomly shuffle two cards by selecting two indexes at random and swapping the cards.

8. Dec 3, 2012

### reldridge

Re: Combinations

No belief, was just stating that there could be duplicate codes during the random selection and i would need to remove the duplicates until reaching my goal of 2.5 unique codes...

9. Dec 3, 2012

### reldridge

Re: Combinations

Thanks for the help.