CODEWITHSUNDEEP
geterlangkeytuples
Santiago Ignacio Poli
Santiago Ignacio Poli

Reputation: 1065

Erlang: Extracting values from a Key/Value tuple

I have a tuple list that look like this:

{[{<<"id">>,1},
  {<<"alerts_count">>,0},
  {<<"username">>,<<"santiagopoli">>},
  {<<"facebook_name">>,<<"Santiago Ignacio Poli">>},
  {<<"lives">>,{[{<<"quantity">>,8},
                 {<<"max">>,8},
                 {<<"unlimited">>,true}]}}]}

I want to know how to extract properties from that tuple. For example:

get_value("id",TupleList), %% should return 1.

get_value("facebook_name",TupleList), %% should return "Santiago Ignacio Poli".

get_value("lives"), %% should return another TupleList, so i can call      

get_value("quantity",get_value("lives",TupleList)).

I tried to match all the "properties" to a record called "User" but I don't know how to do it.

To be more specific: I used the Jiffy library (github.com/davisp/jiffy) to parse a JSON. Now i want to obtain a value from that JSON.

Thanks!

Upvotes: 1

Views: 2515

Answers (2)

zxq9
zxq9

Reputation: 13164

The first strange thing is that the tuple contains a single item list: where [{Key, Value}] is embedded in {} for no reason. So let's reference all that stuff you wrote as a variable called Stuff, and pull it out:

{KVList} = Stuff

Good start. Now we are dealing with a {Key, Value} type list. With that done, we can now do:

lists:keyfind(<<"id">>, 1, KVList)

or alternately:

proplists:get_value(<<"id">>, KVList)

...and we would get the first answer you asked about. (Note the difference in what the two might return if the Key isn't in the KVList before you copypasta some code from here...).

A further examination of this particular style of question gets into two distinctly different areas:

  • Erlang docs regarding data functions that have {Key, Value} functions (hint: the lists, proplists, orddict, and any other modules based on the same concept is a good candidate for research, all in the standard library), including basic filter and map.
  • The underlying concept of data structures as semantically meaningful constructs. Honestly, I don't see a lot of deliberate thought given to this in the functional programming world outside advanced type systems (like in Haskell, or what Dialyzer tries hard to give you). The best place to learn about this is relational database concepts -- once you know what "5NF" really means, then come back to the real world and you'll have a different, more insightful perspective, and problems like this won't just be trivial, they will beg for better foundations.

Upvotes: 2

mpm
mpm

Reputation: 3584

You should look into proplists module and their proplist:get_value/2 function.

You just need to think how it should behave when Key is not present in the list (or is the default proplists behavior satisfying).

And two notes:

  • since you keys are binnary, you should use <<"id">> in your function

  • proplists works on lists, but data you presented is list inside one element tuple. So you need to extract this you Data.

    {PropList} = Data, Id = proplists:get_value(<<"id">>, PropList),

Upvotes: 0

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