How to get unique random items from an array?

Question

Hey there,
I have an array of (unique) objects and would like to get 4 random objects of this array, but the same object shouldn't be fetched twice, so using a simple random function won't work.

I had two ideas:
1. Shuffle the array and get the first 4 items. -> Overhead, because the array can be very large.
2. Generate a random index. If that index was used before, try again, and so on... -> Somewhat unreliable...

Do you know a better, more efficient method?

Mark Byers · Accepted Answer · 2011-02-19 21:12:48Z

10

Shuffle the array and get the first 4 items. -> Overhead, because the array can be very large.

Use the modern version of the Fisher Yates shuffle. It can be performed in-place and if you stop after the first four swaps it will give you the result you need. There is no need to shuffle the entire array.

Compared to some other solutions, it also has the advantage that, if only part of the resulting permutation is needed, it can be stopped halfway through

edited Feb 19, 2011 at 21:12

answered Feb 19, 2011 at 20:59

Mark Byers

844k202 gold badges1.6k silver badges1.5k bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

user166390 Over a year ago

Is there ever a "get unique random items from an array" where Fisher-Yates is not applicable? :-/

klaussner Over a year ago

Thank you, that's perfect. And pretty simple, too. :)

ProEvilz Over a year ago

You link a Wikipedia page of a guy. How does this answer the question? You may as well link any person BIO who did a bit of math.

Collectives™ on Stack Overflow

How to get unique random items from an array?

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related