After you choose your index randomly, you could then just increment it by one to determine the next value.

However since you are doing a website, you may not want the same two ads to always appear next to each other. Therefore, I would do the following:

1. Read all the images in from the file
2. Shuffle the array
3. Pop one image off the end of the array
4. Repeat (3) until array is empty, then goto (1)

Check out (ar0n: Algorithm::Numerical::Shuffle) Re: Randomize an array and Answer: How do I shuffle an array? for code on how to shuffle an array.

Cheers,
Gryn

In reply to Random and sequential for the price of one! by gryng
in thread Reading files sequentially by damian

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