Randomizing array elements

Question

I have an array @number = [1,2,3,4,5,6,7,8,9]
Now, I want to randomize the array content... something like eg: [5,3,2,6,7,1,8]
Please guide me how to proceed with it.

Duplicate http://stackoverflow.com/questions/1816378/how-to-randomly-sort-scramble-an-array-in-ruby — Nakilon, Sep 29 '10 at 04:47
Your hypotetic function dropped 2 elements from array. Even `shuffle` can't do that, gg! — Nakilon, Sep 29 '10 at 05:00

score 13 · Accepted Answer · answered Sep 29 '10 at 04:42

13

Use the shuffle method ...

irb(main):001:0> [1,2,3,4,5].shuffle
=> [3, 4, 2, 5, 1]

answered Sep 29 '10 at 04:42

Dave Pirotte

3,806
21
14

score 8 · Answer 2 · answered Sep 29 '10 at 04:43

8

the shuffle command returns a randomized version of an array

eg:

[1,2,3].shuffle => [2,3,1]

answered Sep 29 '10 at 04:43

Nikolaus Gradwohl

19,708
3
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3

and if you want to randomize in place, you can just write `@number.shuffle!` – Peter Sep 29 '10 at 05:08

score 1 · Answer 3 · edited Jan 10 '11 at 11:41

1

[1,2,3,4,5,6,7,8,9].sort_by {rand}[0,9]  
=> [5, 7, 3, 8, 9, 4, 2, 1, 6]

edited Jan 10 '11 at 11:41

Nakilon

34,866
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answered Sep 29 '10 at 12:49

Dhanu

69
7

score -1 · Answer 4 · answered Sep 29 '10 at 05:01

-1

If you are using old version of ruby... this will work

def randomize(array)
b = []
array.length.downto(1) { |n|
    b.push array.delete_at(rand(n))
} 
b

end

a = [1,2,3,4,5] b=randomize(a) print b

answered Sep 29 '10 at 05:01

Avinasha Shastry

822
1
8
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At least, he is the only one here, who gave a working solution without built-in functions. – Nakilon Sep 30 '10 at 07:12

score -2 · Answer 5 · answered Sep 29 '10 at 04:41

-2

loop n times
   i = random array index
   j = random array index
   swap elements i and j
end

answered Sep 29 '10 at 04:41

RyanHennig

1,072
9
12

Random is not guaranteed to give properly distributed results. Therefore, swapping elements with two random indexes might give you array with blocks of unchanged sequences in the middle. You should increment i from 0 to n and take random j to ensure *all* elements get swapped at least once. – Dan Abramov Sep 29 '10 at 04:45
Awful. Even and odd `n` gives two different sets of permutations. It's so sad, but still some teachers teach students of this method... Never do that! – Nakilon Sep 29 '10 at 04:54
@Nakilon: What are you talking about? Why does it matter if n is even or odd? – RyanHennig Sep 29 '10 at 17:37
@RyanHennig. 1. If you give me original `array` and `n`, I should tell you, what half of permutations' set you can't get, and what you can. With `[1,2,3,4,5]` and `n%2 == 1` you'll never get `[5,4,3,2,1]`. 2. Also, in ideal random suffled array statistically the `1` element is in its _original_ (where it would be in oredered array) place. Your shuffling of array with `1mln` elements will take at least `7mln` swappings to make array with `2` element on their _original_ places. – Nakilon Sep 30 '10 at 06:59
@gaearon: There was no requirement to ensure that all elements get swapped at least once. – RyanHennig Oct 15 '10 at 03:51
@Nakilon: 1. Of course you can. Swap 1 and 5, 2 and 4, and then 3 with itself. 2. I don't know what you mean by this point – RyanHennig Oct 15 '10 at 03:54
@RyanHennig, hm, sorry, I didn't think about swapping with itself. – Nakilon Oct 15 '10 at 09:20

Randomizing array elements

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