How To equal <<"xxxasdew">> , and '<<"xxxasdew">>' in erlang

Question

I am having Data like the below:

Data = [{<<"status">>,<<"success">>},
       {<<"META">>,
       {struct,[{<<"createdat">>,1406895903.0},
       {<<"user_email">>,<<"gopikrishnajonnada@gmail.com">>},
       {<<"campaign">>,<<"5IVUPHE42HP1NEYvKb7qSvpX2Cm">>}]}},
       {<<"mode">>,1}]

And Now i am having a

FieldList = ['<<"5IVUPHE42HP1NEYvKb7qSvpX2Cm">>']

Now:

I am trying like the below but i am getting empty instead of the value

90> [L || L <- FieldList,proplists:get_value(<<"campaign">>,element(2,proplists:get_value(<<"META">>,Data,{[],[]}))) == L].
[]

so how to get the both values are equal and get the final value.

score 2 · Answer 1 · edited May 23 '17 at 10:33

You can parse the atom as if it were an Erlang term:

atom_to_binary(Atom) ->
    L = atom_to_list(Atom),
    {ok, Tokens, _} = erl_scan:string(L ++ "."),
    {ok, Result} = erl_parse:parse_term(Tokens),
    Result.

You can then do

[L ||
   L <- FieldList,
   proplists:get_value(<<"campaign">>,
       element(2,
           proplists:get_value(<<"META">>,Data,{[],[]})))
   == atom_to_binary(L)
].

You can also do it the other way round, (trying to) convert the binary to an atom using this function:

binary_literal_to_atom(Binary) ->
    Literal = lists:flatten(io_lib:format("~p", [Binary])),
    try
        list_to_existing_atom(Literal)
    catch
        error:badarg -> undefined
    end.

This function will return undefined if the atom is not known yet (s. Erlang: binary_to_atom filling up atom table space security issue for more information on this). This is fine here, since the match can only work if the atom was known before, in this case by being defined in the FieldList variable.

How did you get those values in the first place?

is there any other way round to do it as it was a line used in mqpreduce function on riak so it runs on number of keys , so this will take time i think so — Krish gopi, Aug 12 '14 at 15:36
I understand neither your question nor the problem. In your original question you asked why your code did not work. The problem there was that you were trying to match an atom (`'<<"asdf">>'`, indicated by the single quotation marks) on a binary (`<<"asdf">>`). The solution given here is to convert the atom to a binary and compare those. You could have also done it the other way round but that is a bit more difficult to get right. — filmor, Aug 13 '14 at 08:03

score 0 · Answer 2 · answered Aug 12 '14 at 19:24

Data = [{<<"status">>,<<"success">>},
        {<<"META">>,
            {struct,[{<<"createdat">>,1406895903.0},
                     {<<"user_email">>,<<"gopikrishnajonnada@gmail.com">>},
                     {<<"campaign">>,<<"5IVUPHE42HP1NEYvKb7qSvpX2Cm">>}]
            }
         },
         {<<"mode">>,1}].

[_,{_,{struct,InData}}|_] = Data.

[X || {<<"campaign">>,X} <- InData].

it gives you the result in the form : [<<"5IVUPHE42HP1NEYvKb7qSvpX2Cm">>]

of course you can use the same kind of code if the tuple {struct,InData} may be in a different place in the Data variable.

score 0 · Answer 3 · answered Aug 13 '14 at 01:12

-module(wy).
-compile(export_all).

main() ->
    Data = [{<<"status">>,<<"success">>},
        {<<"META">>,
         {struct,[{<<"createdat">>,1406895903.0},
              {<<"user_email">>,<<"gopikrishnajonnada@gmail.com">>},
              {<<"campaign">>,<<"5IVUPHE42HP1NEYvKb7qSvpX2Cm">>}]
         }
        },
        {<<"mode">>,1}],
    Fun = fun({<<"META">>, {struct, InData}}, Acc) ->
          Value =  proplists:get_value(<<"campaign">>, InData, []),
          [Value | Acc];
         (_Other, Acc)->
          Acc
      end,
    lists:foldl(Fun, [], Data).

I think you can use this code.

How To equal <<"xxxasdew">> , and '<<"xxxasdew">>' in erlang

3 Answers3