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3057 :: nim-random not that random at all?

• Bernd (orginal) [2017-07-19T16:08:18+02:00] view original

  Hey there!

  I have been playing with nim for a few days. Now I stumbled upon a behavior that I do not understand. :P

  The random generator does not behave like I expect.

  
  import random
  randomize()
  echo random(1280)
  Run

  This should echo a "random" number between 0 and 1280 (-1) right? Well, it does, kind of.

  ---

  
  nim c -r rnd.nim
  1118
  Run

  
  nim c -r rnd.nim
  1117
  Run

  
  nim c -r rnd.nim
  1116
  Run

  
  nim c -r rnd.nim
  1116
  Run

  
  nim c -r rnd.nim
  1117
  Run

  ---

  This one behaves more random:

  
  import random
  randomize()
  
  for i in 1..10
     echo random(1200)
  Run

  
  nim c -r rnd.nim
  841
  1014
  609
  635
  547
  574
  1171
  233
  280
  595
  Run

  
  nim c -r rnd.nim
  780
  275
  1029
  985
  1143
  865
  686
  97
  945
  858
  Run

  
  nim c -r rnd.nim
  793
  166
  604
  244
  338
  868
  482
  281
  173
  7
  Run

  ---

  Is there a smarter way to create a random number with min/max value?

---

• Krux02 (orginal) [2017-07-19T22:09:22+02:00] view original

  The random module is not very good at all, sorry for that. But there is a mersenne twister in the standard library.

  This is what I use. Upper bound is exclusive.

  proc rand*(maxval: uint32): uint32 =
    let limit = uint32(high(uint32)) - uint32(high(uint32)) mod maxval
    var bits = rand_u32()
    while bits > limit:
      bits = rand_u32()
    result = bits mod maxval
  Run

---

• bluenote (orginal) [2017-07-19T22:12:59+02:00] view original

  Are you on Windows? My guess is that this is mainly an issue of randomize and the low resolution of the timer used for initialization here.

  Update: Actually I just remembered that I changed this recently. This is on devel. Are you using the Nim devel branch?

---

• Bernd (orginal) [2017-07-20T07:16:10+02:00] view original

  Thanks for the replies! :)

  No I have been using nim-0.17.0 so far. But nice to see that this is a fixable thing. I'll look into that later today. :)

  Edit// @bluenote I added your fix now, and the result looks much more random. Thanks for that!

---

• Araq (orginal) [2017-07-20T08:32:36+02:00] view original

  proc rand*(maxval: uint32): uint32 =
    let limit = uint32(high(uint32)) - uint32(high(uint32)) mod maxval
    var bits = rand_u32()
    while bits > limit:
      bits = rand_u32()
    result = bits mod maxval
  Run

  Looks suspicious. high - high is 0, isn't it? And there are MUCH more effective ways than while bits > limit (super slow for small numbers?).

---

• euant (orginal) [2017-07-20T22:01:01+02:00] view original

  Depending on what you're using the random numbers for, I also have a library that makes use of system sources of random (arc4random on OpenBSD, RtlGenRandom on Windows and /dev/urandom elsewhere) that I've been working on: https://github.com/euantorano/sysrandom.nim

  It could do with some more heavy testing, as I haven't used it outside of testing during development yet.

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