Generate n unique random integers in ruby / rails

Question

Im trying to generate n unique random numbers between 1 and max

I tried the following code but doesn't work (returns repeated numbers)

r = [ ]
n.times { v = rand(max) while r.include? v ; r << v}

Whats wrong with it? Thanks

added:

max is thousands

n is 10

Answer 1

I think your while r.include logic is the wrong way around. Try this:

r = [ ]
while r.length < n 
  v = rand(max)
  r << v unless r.include? v
end

Note that this will go into an infinite loop if max < n.

Answer 2

See this question.

Answer 3

No, no, don't generate randomly and then check, generate the uniq numbers and then sort randomly!

(1..max).sort_by{rand}

Or, in 1.9:

(1..max).to_a.shuffle

Answer 4

Kernel#rand generates a pseudorandom number. This should cause concern if security is an issue.

In Rails use ActiveSupport::SecureRandom.random_number

Also both Kernel#rand and ActiveSupport::SecureRandom.random_number may return 0, you say the value must be between 1 and max.

Answer 5

I am not sure of the exact ruby syntax, but as an algorithm:

list = [1 .. 10000];

nums = [];

while (nums.length < needed) {
    nums.push( list.splice(rand() * list.length) )
}

should work fairly well, so long as your range isn't too big and won't need to waste time shuffling the entire list