# [erlang-questions] Generating a list of lists containing random numbers

Raimo Niskanen <>
Tue Jul 3 08:33:12 CEST 2018

```On Tue, Jul 03, 2018 at 01:15:17PM +0900,  wrote:
> On 2018年7月2日月曜日 23時36分10秒 JST Awsaf Rahman wrote:
> > Hello!
> >
> > I am trying to determine the execution time for my matrix multiplication
> > program and for that I need to generate a large enough List of Lists of
> > random integers. Say I want to generate a matrix with a 1000 rows (that is
> > 1000 lists inside a list). Is there any way of doing it? I can generate a
> > list of random numbers using list comprehension but can't figure out how to
> > go about building a list of lists.
>
> Wheels within wheels, man...
>
>   -module(genset).
>   -export([generate/1]).
>
>   generate(Size) ->
>       Generate = fun() -> rand:uniform(1000) end,
>       MakeList = fun() -> populate(Generate, Size, []) end,
>       populate(MakeList, Size, []).
>
>
>   populate(_, Size, Acc) when Size < 0 ->
>       Acc;
>   populate(Generate, Size, Acc) ->
>       populate(Generate, Size - 1, [Generate() | Acc]).
>
>
> -Craig

This may be a missing function for the lists module:

buildr(N, Fun, Acc) ->
buildr(N, Fun, Acc, []).
%%
buildr(0, _Fun, Acc, R) ->
{R, Acc};
buildr(N, Fun, Acc, R) ->
{V, NewAcc} = Fun(Acc),
buildr(N - 1, Fun, NewAcc, [V|R]).

Example usage:

Base = 0,
Range = 1 bsl 32,
{ListOfLists, _NewSeed} =
buildr(
1000,
fun (Seed) ->
buildr(
1000,
%%fun rand:uniform_s/1,
fun (S) ->
{V, NewS} = rand:uniform_s(Range, S),
{Base + V - 1, NewS}
end,
Seed)
end,
rand:seed_s(exrop)),

--

/ Raimo Niskanen, Erlang/OTP, Ericsson AB
```