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

[Raimo Niskanen]

On Tue, Jul 03, 2018 at 04:08:18PM +0900, zxq9@REDACTED wrote:
> On 2018年7月3日火曜日 8時33分12秒 JST Raimo Niskanen wrote:
: :
> > 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)),
> > 
> > 
> 
> I think I'd rather have matrix operations in a matrix module.

This is not a matrix operation.

We have lists:seq/2,3 and lists:duplicate/2 but no such generating function
that takes a generator fun.

It could be named generate/3, gen/3 or build/3.  I suggested buildr/3
because it builds from the right end of the list analogous to foldr/3.

A buildl/3 would be more inefficient since it should return {List,Acc}
which does not allow for a recursive variant so it would become build3/r
followed by reverse/1.

So:     lists:buildr(N, GeneratorFun, Acc) -> {List, NewAcc}
with:       GeneratorFun(Acc) -> {Value, NewAcc}

Might be useful enough to add...


> 
> Every time I've needed to do matrix operations the need has either been:
> 
> 1)
> Trivial, flat matrix operations over regular matrices of regular
> dimensions populated by scalar values & a well-defined _|_.
> 
> OR
> 
> 2)
> Matrices that are specialized enough that they really deserve a custom
> set of handlers all their own.

In Wings3D I wrote a matrix library that was optimized for sparse matrixes,
for the automatic U-V mapping algorithm.

Yet another specialized matrix library I guess...

/ Raimo


> 
> In case 1 there are enough regular operations (especially against numeric
> values) that you could fill out a full-blown matrix_math module and still
> not cover everything. I wouldn't want that to clutter the lists module.
> 
> In case 2 trying to generalize is futile because remembering the details
> of the abstractions involved is more trouble than writing a special handler,
> hence things like this:
>    https://gitlab.com/zxq9/zxWidgets/blob/master/src/zxw.erl#L194
> 
> I wouldn't want to attempt to generalize a text grid metawidget generator
> because the assumptions embedded in it will break in any other situation,
> but nonetheless, it is a matrix operation to generate, populate, and extract
> or modify data in it.
> 
> I've got a naive matrix math module around here somewhere that I wrote as
> an example for a guy on SO a while back... but it only handles a few math
> operations, flips, inversions and rotations.
> 
> Would people be interested in a matrix_math library?
> 
> -Craig
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-- 

/ Raimo Niskanen, Erlang/OTP, Ericsson AB