[erlang-questions] Filtering hierarchical data structure

Stanislav Ledenev <>
Fri Jul 13 15:25:56 CEST 2018


Hello,

I am new to Erlang and functional programming (1-1.5 months) and have task:

1. Suppose we have hierarchical tree-like data structure:
L = [
{section, "id1", [
{section, "id1.1", [
{policy, "p1", "v1"},
{policy, "p2", "v2"}
]},
{section, "id1.2", [
]}
]},
{section, "id2", [
{policy, "p3", "v3"}
] },
{section, "id3", [
] }
]

2. List of some "section"'s with children elements any of which could be
another "section" with children or "policy" with name and value.

3. Also we have another list which contains identifiers of policies:
F = ["p1", "p3"].
4. Now I need to write a function which takes lists L, F and returns list L2
such as L2 contains only those "section"'s which has "policies" wich Id
equals to those in F. Empty sections must be removed.

For lists L and F result should be:
L2 = [
{section, "id1", [
{section, "id1.1", [
{policy, "p1", "v1"},
]},
]},
{section, "id2", [
{policy, "p3", "v3"}
] },
]

5. I have solution (please see below) but it seems to me not as good as
it can be and even little weird. I am sure that it could be improved
and done in proper way.
If anyone could give me any glimpse or advice I would appreciate.

Source:
---------------------------------------------------------------------------
main() ->
    L = [
            {section, "id1", [
                {section, "id1.1", [
                    {policy, "p1", "v1"},
                    {policy, "p2", "v2"}
                ]},
                {section, "id1.2", [
                ]}
            ]},
            {section, "id2", [
                {policy, "p3", "v3"}
            ] },
            {section, "id3", [
            ] }
        ],
    %filter(L, ["p1", "p3"]).
    filter(L, ["p3"]).

filter([H | T], Search) ->
    lists:flatten([filter(H, Search, []) | filter(T, Search, [])]).

filter({section, I, C}, Search, _Acc) ->
    NewC = filter(C, Search, []),
    case NewC of
        [] -> [];
        _ -> {section, I, NewC}
    end;

filter([{section, I, C} | T], Search, Acc) ->
    NewC = filter(C, Search, []),
    case NewC of
        [] -> filter(T, Search, Acc);
        _ -> filter(T, Search, [{section, I, NewC} | Acc])
    end;

filter([{policy, I, V} | T], Search, Acc) ->
    case lists:member(I, Search) of
        true -> filter(T, Search, [{policy, I, V} | Acc]);
        false -> filter(T, Search, Acc)
    end;
filter([], _S, Acc) -> Acc.

-- Stanislav Ledenev
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