Erlang Central

Random Numbers

Revision as of 07:32, 24 March 2009 by Wesley (Talk | contribs)

Problem

You want a random number from a given range. For example, you wish to randomly select one element from an array, simulate rolling a die in a game of chance, or generate a random password.

Solution

Use random:uniform/0 and random:uniform/1 ; the first returns a random float uniformly distributed between [0.0,1.1], and the second returns a random integer uniformly distributed between [1,N].

1> random:uniform().
0.159112
2> random:uniform(150).
67

To select random integers from [N,M], can use N+random:uniform(M-N).

A common example is generating a random password:

generate_password(N) ->
    lists:map(fun (_) -> random:uniform(90)+$\s+1 end, lists:seq(1,N)).

3> generate_password(8).
"l?8j#$&&"
4> generate_password(8).
"V<bFi(EW"
5> generate_password(8).
"irp^DXTz"

The Erlang random number generator is attributed to B.A. Wichmann and I.D.Hill, in 'An efficient and portable pseudo-random number generator', Journal of Applied Statistics. AS183. 1982. Also Byte March 1987.

It can support bigint values, so no special recipe is required for extremely large random numbers.