Question

I’m trying to write a map phase for riak in erlang and I’ve hit a problem. I need to concatenate a description and a code, which are separate items and combine them in the form “code-description”, which then needs to be given a value of 1 so “code-description”=1 to allow me to count the same occurrences in my reduce phase (The descriptions are from a static pool, not free text). The dash is also important. Using proplists I can navigate to the values I want and using {struct, values} = …etc I get something along the lines:

[{"desc":"Stuff Needs Doing","code":"SND"},{"desc":"Done","code":"DNE"}]

When I write the following snippet I get just the desc returning for each kv in my filtered list:

Desc = proplists:get_value(<<"desc">>, Data),
[Desc].

Seeing as how they are both character data I thought that [DescVar ++ CodeVar]. would work but when I ran it against my cluster I actually got an error message and it complains about the ++. + doesn’t work either so how do I do what I’m after in erlang?

Answer 1

I haven’t played with Riak, but I am assuming you are working with typical erlang decoded json (in [{struct,Values}] format).

[{"desc":"Stuff Needs Doing","code":"SND"},{"desc":"Done","code":"DNE"}]

The above json string would decode to erlang as:

[{struct,[{<<"desc">>,<<"Stuff Needs Doing">>},
      {<<"code">>,<<"SND">>}]},
{struct,[{<<"desc">>,<<"Done">>},{<<"code">>,<<"DNE">>}]}]

Then this List comprehension would give you the results you are looking for (and maintain the correct result format):

[{struct,[{<<X/binary,"-",Y/binary>>,1}]}||{struct,[{<<"desc">>,Y},{<<"code">>,X}]}<-Data].

With “Data” being the erlang decoded JSON.

This produces:

[{struct,[{<<"SND-Stuff Needs Doing">>,1}]},{struct,[{<<"DNE-Done">>,1}]}]

Which encodes to a JSON string as :

[{"SND-Stuff Needs Doing":1},{"DNE-Done":1}]

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