Erlang's solution to reorder a List_PHP tutorial

I have been reading Erlang systematically recently, and I am currently using "Erlang Programming" as a learning material. After reading the first seven or eight chapters, I tried to write something, but found that I was at a loss, and there were very few helpful materials and documents.

Actually, I hope someone can study or discuss with me.

Today’s topic: Shuffle a List.

Method 1: A more sophisticated out-of-order solution.

[php]

-module(shuffle).

-export([do/1]).                                          

do(L) ->

Len = length(L),                                                                                  

NL = lists:map(fun(X) -> {random:uniform(Len), X} end, L),

NLL = lists:sort(NL),                                                      

[ V || {_,V} <- NLL].

Actually, this problem bothers me a lot. Once a variable is assigned a value, it cannot be modified. This feature makes the original idea and idea of ​​​​writing a program completely different. What I did above is to use random numbers to generate a list like [{Rand1,Elem1},...,{RandN,ElemN}].

Then use sort to sort, and then print out Elem again, thus achieving the out-of-order effect.

The results are as follows:

[php]

45> c(shuffle).                                      

{ok,shuffle}

46> shuffle:do([1,2,3,4,5]).

[4,5,1,2,3]

47> shuffle:do([1,2,3,4,5]).

[2,1,5,3,4]

48> shuffle:do([1,2,3,4,5]).

[3,1,2,4,5]

49> shuffle:do([1,2,3,4,5]).

[1,5,2,3,4]

50> shuffle:do([1,2,3,4,5]).

[5,1,4,3,2]

51>

Method 2: Universal Shuffling Algorithm

Continuing from the above, shuffling a List, I think the original general shuffling algorithm is feasible. Try writing another function. It has been tested and works, but I think there is definitely room for optimization. The style of the code still reveals strong procedural thinking.

% Scheme using shuffling algorithm                                                                                          

[php]

do2(L) ->

do2(L,[]).

do2([],L) ->

L;

do2(L1,L2) ->

%io:format("L1=~w L2=~w~n",[L1,L2]),

Len = length(L1),                                                                                          

if

Len > 1 ->

NL = lists:split(random:uniform(Len-1), L1),

                                                                                                                                                                       

        NL2 = lists:flatten([T1],[H1|T2]),                                      

L11 = lists:append(L2,[H2]),

do2(NL2, L11);

true ->

do2([],lists:append(L2,L1))

end.

The result is:

128> c(shuffle).

{ok,shuffle}

129> shuffle:do2(lists:seq(0,9)).

[9,2,5,1,8,0,7,3,6,4]

130> shuffle:do2(lists:seq(0,9)).

[8,3,6,5,7,4,9,0,2,1]

131> shuffle:do2(lists:seq(0,9)).

[5,3,7,8,1,9,0,6,2,4]

132> shuffle:do2(lists:seq(0,9)).

[3,0,5,2,1,6,8,4,9,7]

133>

There are a few points worth thinking about whether these two solutions are real shuffling algorithms. Is each number equally likely to appear in a certain position?

There are other methods, welcome to discuss.

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