# A Sequences order cannot be predicted beforehand, but is deterministic

I need a single word or short phrase that describes the following behavior:

If I, without any further information, ask for the sequence, its order will look random and cannot be predicted.
But if I ask a second time I will get the same order as the first time.

My current explanation is that "the lists order is essentially random, but will stay the same over multiple runs" (coming from an IT-background). This is a bit long and unwieldy and I wonder if there is a shorter version to express this.

Ideally I could say "the lists order is <...>"

• random but static [or fixed] Nov 10, 2017 at 10:57

This is pseudorandom: something that appears random but is actually deterministic and repeatable. Pseudo is a prefix meaning "pretending, or appearing, to be" - as in "pseudonym" for example.

In computer programming pseudorandomness is usually achieved with a seed: an input given to the start of the random number generator. If not given a seed, the generator will create an unpredictable series of numbers, but if given a seed, it will produce a series of numbers which looks random, but will be the same every time (for the same seed).

• With or without a seed, the random number generator is pseudorandom. The seed is only there to choose which pseudorandom sequence should be used. Jan 17, 2018 at 12:35
• @jimm101 that's a technical distinction which I think is unimportant. The key thing is predictability - without a seed, even if it's pseudorandom under the hood, the user has no way of predicting which number will come next, and so to all intents and purposes it can be called "random". Jan 17, 2018 at 13:46
• It's not "a technical distinction", it's wrong. A sequence can be both pseudorandom, and NOT repeatable, and is so whenever a developer doesn't set a random seed. If the OP states that the sequence is pseudorandom, it's misleading people, and doesn't meet the requirements of "essentially random, but will stay the same over multiple runs". It's "essentially random", full stop. Jan 22, 2018 at 15:35
• @jimm101 "essentially random, but will stay the same over multiple runs" isn't a requirement, it's the OP's first attempt to explain what was happening, which they said they were unhappy with (as an explanation). I'm suggesting they describe it as pseudorandom instead. Jan 22, 2018 at 15:40
• "This is a bit long and unwieldy and I wonder if there is a shorter version to express this" is the requirement I'm referring to. Jan 22, 2018 at 16:07

All software-generated "random" numbers are pseduorandom, so that's not sufficient.

What you have is a repeatable random sequence, or repeatable psedorandom sequence, to be strictly correct. That captures that you can repeat the sequences at will.

Matlab's random number documentation has for commentary to this effect, and for instance, in an article entitled Generate Random Numbers That Are Repeatable.

These sequences are usually called pseudo random.

A pseudorandom process is a process that appears to be random but is not. Pseudorandom sequences typically exhibit statistical randomness while being generated by an entirely deterministic causal process. Such a process is easier to produce than a genuinely random one, and has the benefit that it can be used again and again to produce exactly the same numbers, which is useful for testing and fixing software. - Wikipedia

• the list[‘]s order is essentially random, but will stay the same over multiple runs

A list that is pseudorandom is “essentially random” in the sense that it is statistically random. Because the sequence is deterministic, the same values will be produced across multiple runs, where the software is made to repeat identically across runs.

This contrasts with unrepeatable sequences such as those based on temperature sampling.