Negative random numbers

Question

I know how to generate numbers with Rails but I don't know how to generate negative numbers?

prng.rand(1..6) for random of [1, 2, 3, 4, 5, 6]

Random doc says that you would get an ArgumentError.

You probably don't want to be calling `Random.new` unless you know the consequences of that. Instead just call `rand` by itself. — Andrew Marshall, Oct 28 '11 at 17:17
Random.rand(-5..5). Please check: http://stackoverflow.com/questions/198460/how-to-get-a-random-number-in-ruby/2773866#2773866 — fotanus, May 07 '12 at 04:04

Pepe · Accepted Answer · 2011-10-28T18:49:35.120

9

Let's say you want to generate a number between a and b you can always do that using this formula:

randomNum = (b-a)*prng.rand + a

So if you want a number between -8 and +7 for example then a=-8 and b=7 and your code would be

randomNum = (7-(-8))*prng.rand + (-8)

which is equivalent to

randomNum=15*prng.rand - 8

edited Oct 28 '11 at 18:49

answered Oct 28 '11 at 17:15

Pepe

randomNum = (7-(-8))*prng.rand(0..1) + (-8) Wouldn't this generate a with 50% chance ? (when you have 0 with prng.rand(0..1) – m_vdbeek Oct 28 '11 at 17:20
@Awake yes you are correct, use prng.rand instead to generate a random uniform number betwen 0 and 1 – Pepe Oct 28 '11 at 18:48

score 7 · Answer 2 · answered Nov 18 '13 at 14:23

7

Suppose you want to generate negative numbers between -1 to -5

You can simply do this:

rand(-5..-1)

It will generate random numbers between -5 to -1

answered Nov 18 '13 at 14:23

Radhika

The only rubyesque answer... Maybe add since which ruby version this is possible. – count0 Jan 29 '18 at 21:05

score 5 · Answer 3 · edited Apr 23 '17 at 11:17

5

How about you multiply the positive random number by -1

edited Apr 23 '17 at 11:17

answered Oct 28 '11 at 17:12

Mörre

yes but you will get 100% of negative numbers what i want to do is balance between positive / negative numbers if I do prng.rand(-10..10). But this shouldn't work because of the ArgumentError no ? – m_vdbeek Oct 28 '11 at 17:17
then go 0...10 and subtract 20! – Mörre Oct 29 '11 at 05:23

score 3 · Answer 4 · answered Oct 28 '11 at 17:25

3

def rand(a, b)
  return Random.rand(b - a + 1) + a 
end
rand(-3, 5)

answered Oct 28 '11 at 17:25

Steven Jackson

1

Would not this fail if both a and b are negative? – Andrey Oct 28 '11 at 17:37
Hmm, I just tested with rand(-9, -5) and it seems to work properly yet. Just have to make sure -9 is first as it's the lesser of the two. – Steven Jackson Oct 28 '11 at 17:39
If you do rand (-9, -5) you get Random.rand(-5-(-9)+1)+(-9) = Random.rand(5)-9 ... if rand returns 5 than 5-9=-4 and that is not in the range of (-9, -5) – Andrey Oct 28 '11 at 17:44
Ok, so is there some solution to this problem ? – m_vdbeek Oct 28 '11 at 17:59
Random.rand(5) returns something from 0-4, so it can't return 5 :) – Steven Jackson Oct 28 '11 at 18:08

score 0 · Answer 5 · answered Nov 27 '14 at 21:59

0

I needed to generate a random integer, either 1 or -1 and I used this:

prng = Random.new(1)    
prng.rand > 0.5 ? 1 : -1

answered Nov 27 '14 at 21:59

thomallen

5 Answers5