# [erlang-questions] more random uniform

Bengt Kleberg bengt.kleberg@REDACTED
Sun Oct 27 20:36:01 CET 2013

```Thank you all.

By using X*1000, Y*1000 and the sum *1000 I got sufficiently random numbers.

Bengt

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Från: Tony Rogvall <tony@REDACTED>
Till: Bengt Kleberg <bengt.kleberg@REDACTED>
Kopia: "erlang-questions@REDACTED" <erlang-questions@REDACTED>
Skickat: 27-10-2013 6:14 em
Ämne: Re: [erlang-questions] more random uniform

No it was not necessary, just making sure it was fresh every time.

Any particular reason to keep the third argument to zero in the random:seed/3 call ?
This will, as you point out, generate an initial sequence of small random:uniform numbers.
Looking at random.erl the calculation is (modified in this context):

(((X+1)*171) rem 30269) / 30269  +
(((Y+1)*172) rem 30307) / 30307 +
(((0+1)*170) rem 30323) / 30323

Keeping the X and Y small enough (less than 176 ) we can rewrite this as:

R = ((X+1)*171) / 30269 + ((Y+1)*172) / 30307 + 170 / 30323

The value of this expression will be small given small values on X and Y,  and the expression
trunc(R*3) will be zero for X+Y < 56 (roughly)

In other words use larger numbers, and use them all, when seeding the random number generator.

/Tony

On 27 okt 2013, at 11:48, Bengt Kleberg <bengt.kleberg@REDACTED<mailto:bengt.kleberg@REDACTED>> wrote:

7> [erlang:spawn( fun() -> random:seed(X,Y,0), io:fwrite("~p ", [random:uniform(3)]) end ) || X<-lists:seq(1,10), Y<-lists:seq(1,10)].
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

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Skickat: 27-10-2013 11:33 fm
Ämne: Re: [erlang-questions] more random uniform

Thank you.

I have 15B.
Is the erase/1 necessary? I am using new processes each time.

Bengt

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----- Ursprungligt meddelande -----
Från: Tony Rogvall <tony@REDACTED<mailto:tony@REDACTED>>
Till: Bengt Kleberg <bengt.kleberg@REDACTED<mailto:bengt.kleberg@REDACTED>>
Kopia: "erlang-questions@REDACTED<mailto:erlang-questions@REDACTED>" <erlang-questions@REDACTED<mailto:erlang-questions@REDACTED>>
Skickat: 27-10-2013 11:13 fm
Ämne: Re: [erlang-questions] more random uniform

I can not replicate this (on R16B01).

> erase(random_seed), random:seed(erlang:now()), random:uniform(3).
1
> erase(random_seed), random:seed(erlang:now()), random:uniform(3).
2
...

?

/Tony

On 27 okt 2013, at 10:45, Bengt Kleberg <bengt.kleberg@REDACTED<mailto:bengt.kleberg@REDACTED>> wrote:

Greetings,

When doing random:uniform/1 for small values, ie 3, the result is 1 the first time. Changing the seed does not help.
What am I missing?

Bengt

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