How do I generate a list of n unique random numbers in Ruby?

Question

This is what I have so far:

myArray.map!{ rand(max) }

Obviously, however, sometimes the numbers in the list are not unique. How can I mak

Answer 1

Method 1

Using Kent's approach, it is possible to generate an array of arbitrary length keeping all values in a limited range:

# Generates a random array of length n.
#
# @param n     length of the desired array
# @param lower minimum number in the array
# @param upper maximum number in the array
def ary_rand(n, lower, upper)
    values_set = (lower..upper).to_a
    repetition = n/(upper-lower+1) + 1
    (values_set*repetition).sample n
end

Method 2

Another, possibly more efficient, method modified from same Kent's another answer:

def ary_rand2(n, lower, upper)
    v = (lower..upper).to_a
    (0...n).map{ v[rand(v.length)] }
end

Output

puts (ary_rand 5, 0, 9).to_s # [0, 8, 2, 5, 6] expected
puts (ary_rand 5, 0, 9).to_s # [7, 8, 2, 4, 3] different result for same params
puts (ary_rand 5, 0, 1).to_s # [0, 0, 1, 0, 1] repeated values from limited range
puts (ary_rand 5, 9, 0).to_s # []              no such range :)

Answer 2

No loops with this method

Array.new(size) { rand(max) }

require 'benchmark'
max = 1000000
size = 5
Benchmark.realtime do
  Array.new(size) { rand(max) }
end

=> 1.9114e-05 

Answer 3

You could use a hash to track the random numbers you've used so far:

seen = {}
max = 100
(1..10).map { |n|
  x = rand(max)
  while (seen[x]) 
    x = rand(max)
  end
  x
}