Question

Duplicate:

Unique random numbers in O(1)?

I want an pseudo random number generator that can generate numbers with no repeats in a random order.

For example:

random(10)

might return 5, 9, 1, 4, 2, 8, 3, 7, 6, 10

Is there a better way to do it other than making the range of numbers and shuffling them about, or checking the generated list for repeats?

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Edit:

Also I want it to be efficient in generating big numbers without the entire range.

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Edit:

I see everyone suggesting shuffle algorithms. But if I want to generate large random number (1024 byte+) then that method would take alot more memory than if I just used a regular RNG and inserted into a Set until it was a specified length, right? Is there no better mathematical algorithm for this.

Answer 1

You may be interested in a linear feedback shift register. We used to build these out of hardware, but I’ve also done them in software. It uses a shift register with some of the bits xor’ed and fed back to the input, and if you pick just the right ‘taps’ you can get a sequence that’s as long as the register size. That is, a 16-bit lfsr can produce a sequence 65535 long with no repeats. It’s statistically random but of course eminently repeatable. Also, if it’s done wrong, you can get some embarrassingly short sequences. If you look up the lfsr, you will find examples of how to construct them properly (which is to say, ‘maximal length’).