# Random Numbers

## 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 function from the standard Erlang distribution. It has two forms: (1) a no-argument version that returns a random float value in the range of 0.0 to 1.0, and (2) an version that takes a single integer (N) and returns a random integer in the range of 1 to N.

1> random:uniform(). 9.23009e-2 2> random:uniform(150). 67

This code generates and prints a random integer between 25 and 75, inclusive:

3> io:fwrite("Random = ~B\n", [random:uniform(51) + 24]). Random = 74 ok

In the example, we want to limit the range to 25 to 75, so we add 24 to the result of the random:uniform function. This means we really want a random number between 1 and 51, so we give the random:uniform function the argument 51 (since the generated number is always from 1 to N).

The canonical application for this kind of number generation is the random selection of an element from a vector:

4> Dict = dict:from_list([{1,$a}, {2,$b}, {3,$c}, {4,$d}, {5,$1}, 4> {6,$2}, {7,$3}]). {dict,7, 16, 16, 8, 80, 48, {[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]}, {{[], [[3|99]], [[6|50]], [], [], [], [[2|98]], [[5|49]], [], [], [], [[1|97]], [[4|100]], [[7|51]], [], []}}} 5> dict:fetch(random:uniform(7), Dict). 100 6> io:fwrite("~c\n", [100]). d ok

Another common example is generating a random password:

generate_8_password() -> lists:flatten(lists:foldl(fun(X,AccIn) -> [random:uniform(90) + 32|AccIn] end, [], lists:seq(1,8))). 7> generate_8_password(). "]VD'hEa;" 8> generate_8_password(). "Zju!W9'Z" 9> generate_8_password(). "uw6uX\"JF" 10> generate_8_password(). "-)3Ue=8n"

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.