# What's the difference between a random number and a pseudo-random number?

**URL:** <https://boards.straightdope.com/t/whats-the-difference-between-a-random-number-and-a-pseudo-random-number/605004>\
**Category:** Factual Questions\
**Created:** [December 4, 2011, 4:00am UTC](https://boards.straightdope.com/t/whats-the-difference-between-a-random-number-and-a-pseudo-random-number/605004 "2011-12-04T04:00:32Z")\
**Posts on this page:** 3\
**Page:** 4

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**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [December 8, 2011, 3:48am UTC](https://boards.straightdope.com/t/whats-the-difference-between-a-random-number-and-a-pseudo-random-number/605004/61 "2011-12-08T03:48:12Z")

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An aperiodic PRNG is impossible on any real computer. It’s perfectly possible on a computer with infinite storage; alternately, a real computer can run a PRNG with a long enough period that it’s for all practical purposes aperiodic.

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**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\
**Post date:** [December 11, 2011, 11:08pm UTC](https://boards.straightdope.com/t/whats-the-difference-between-a-random-number-and-a-pseudo-random-number/605004/62 "2011-12-11T23:08:51Z")

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Note that there are a lot of PRNG that _do not repeat_. That has nothing to do with being a real or pseudo RNG. (You just don’t want to think in terms of memory limits.)

One difference is between having a secure crypto system and not having a secure cryptosystem. That seems very practical to me.

“Any one who considers arithmetical methods of producing random digits is, of course, in a state of sin.” John von Neumann

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**Author:** ![Gagundathar](https://avatars.discourse-cdn.com/v4/letter/g/a183cd/32.png) [@Gagundathar](https://boards.straightdope.com/u/Gagundathar)\
**Post date:** [December 27, 2011, 12:25pm UTC](https://boards.straightdope.com/t/whats-the-difference-between-a-random-number-and-a-pseudo-random-number/605004/63 "2011-12-27T12:25:09Z")

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> [@panamajack](#):
>
> May as well bring up the [best Amazon book review ever](http://www.amazon.com/Million-Random-Digits-Normal-Deviates/dp/0833030477).

Love this comment: "Such a terrific reference work! But with so many terrific random digits, it’s a shame they didn’t sort them, to make it easier to find the one you’re looking for. "

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