erlang JSON data - how to select only one value - arrays

Question

in erlang language I receive this kids of JSON data:
{"Time":"2020-08-16T15:28:55","BME680":{"Temperature":29.8,"Humidity":55.5,"Pressure":1003.5,"Gas":422.24},"PressureUnit":"hPa","TempUnit":"C"}
or
{"Time":"2020-08-16T11:39:49","SI7021":{"Temperature":32.4,"Humidity":99.9},"TempUnit":"C"}
I need to select only one value per each JSON data for example:
based on "BME680" value
{"Temperature":29.8,"Humidity":54.8,"Pressure":1005.0,"Gas":1513.60}
or
based on "SI7021" value
{"Temperature":32.4,"Humidity":99.9}
How can acive this task in erlang language ?
If it cold be simpler I need to extract the second value of each JSON data.

Answer 1

For decode JSON format into Erlang format data like proplists or into the maps, you can try to use 3rd party library jiffy, then you can use simple pattern matching, eg:
Maps:
1> JSON = "{\"Time\":\"2020-08-17T05:32:09\",\"BME680\":{\"Temperature\":29.6,\"Humidity\":54.6,\"Pressure\":1003.9,\"Gas\":1517.91},\"PressureUnit\":\"hPa\",\"TempUnit\":\"C\"}".
2> Map = jiffy:decode(JSON,[return_maps]).
3> #{<<"BME680">> := BME680} = Map.
4> BME680.
#{<<"Gas">> => 1517.91,<<"Humidity">> => 54.6, <<"Pressure">> => 1003.9,<<"Temperature">> => 29.6}
Proplists:
1> JSON = "{\"Time\":\"2020-08-17T05:32:09\",\"BME680\":{\"Temperature\":29.6,\"Humidity\":54.6,\"Pressure\":1003.9,\"Gas\":1517.91},\"PressureUnit\":\"hPa\",\"TempUnit\":\"C\"}".
2> {Proplists} = jiffy:decode(JSON).
3> [BME680] = [V || {K, V} <- Proplists, K == <<"BME680">>].
4> BME680.
{[{<<"Temperature">>,29.6},{<<"Humidity">>,54.6},{<<"Pressure">>,1003.9},{<<"Gas">>,1517.91}]}

Answer 2

You can use 2 modules from mochiweb project:
https://github.com/mochi/mochiweb/blob/master/src/mochijson2.erl
https://raw.githubusercontent.com/mochi/mochiweb/master/src/mochinum.erl
Put them in some folder (f.e. test) and run erl shell and compile this files:
alexei#MacBook-Pro test % erl
Erlang/OTP 23 [erts-11.0] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe] [dtrace]
Eshell V11.0 (abort with ^G)
1> c("mochijson2.erl").
{ok,mochijson2}
2> c("mochinum.erl").
{ok,mochinum}
Decode json-as-text to json-as-erlang-term:
3> Body = "{\"Time\":\"2020-08-16T15:28:55\",\"BME680\":{\"Temperature\":29.8,\"Humidity\":55.5,\"Pressure\":1003.5,\"Gas\":422.24},\"PressureUnit\":\"hPa\",\"TempUnit\":\"C\"}".
"{\"Time\":\"2020-08-16T15:28:55\",\"BME680\":{\"Temperature\":29.8,\"Humidity\":55.5,\"Pressure\":1003.5,\"Gas\":422.24},\"PressureUnit\":\"hPa\",\"TempUnit\":\"C\"}"
4> {struct, Map} = mochijson2:decode(Body).
{struct,[{<<"Time">>,<<"2020-08-16T15:28:55">>},
{<<"BME680">>,
{struct,[{<<"Temperature">>,29.8},
{<<"Humidity">>,55.5},
{<<"Pressure">>,1003.5},
{<<"Gas">>,422.24}]}},
{<<"PressureUnit">>,<<"hPa">>},
{<<"TempUnit">>,<<"C">>}]}
Retrive the items you need:
5> {struct, Obj1} = proplists:get_value(<<"BME680">>, Map).
{struct,[{<<"Temperature">>,29.8},
{<<"Humidity">>,55.5},
{<<"Pressure">>,1003.5},
{<<"Gas">>,422.24}]}
6> Item1 = proplists:lookup(<<"Temperature">>, Obj1).
{<<"Temperature">>,29.8}
7> Item2 = proplists:lookup(<<"Humidity">>, Obj1).
{<<"Humidity">>,55.5}
And encode erlang term to text:
8> List = [Item1, Item2].
[{<<"Temperature">>,29.8},{<<"Humidity">>,55.5}]
9> iolist_to_binary(mochijson2:encode({struct, List})).
<<"{\"Temperature\":29.8,\"Humidity\":55.5}">>
There is a very short documentation for mochijson2, but it is easy to learn just practice in shell.

Answer 3

You need to familiarize yourself with rebar3, which is erlang's package manager. Then, you can use a package like jsx, which will allow you to convert between binary types and maps. First, you will need to convert whatever it is that you call "JSON" to a binary type, perhaps using tuple_to_list() then list_to_binary(), then you can use the jsx package to convert the binary to a map, which will enable you to extract the target value from the map.
Here's an example:
my.erl:
json_to_map(Bin) ->
jsx:decode(
%%<<"{\"data\": [1, 2, 3]}">>,
Bin,
[return_maps]
).
In the shell:
~/erlang_programs/myapp$ rebar3 compile
===> Verifying dependencies...
===> Compiling myapp
src/my.erl:2: Warning: export_all flag enabled - all functions will be exported
~/erlang_programs/myapp$ rebar3 shell
===> Verifying dependencies...
===> Compiling myapp
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [hipe] [kernel-poll:false]
Eshell V9.3 (abort with ^G)
1> Bin1 = <<"{\"A\": 100, \"B\":{\"x\": 4,\"y\": [1,2,3]}}">>.
<<"{\"A\": 100, \"B\":{\"x\": 4,\"y\": [1,2,3]}}">>
2> Bin2 = <<"{\"A\":200, \"C\":{\"a\": 6, \"b\": [3,4,5]}}">>.
<<"{\"A\":200, \"C\":{\"a\": 6, \"b\": [3,4,5]}}">>
3> M1 = my:json_to_map(Bin1).
#{<<"A">> => 100,
<<"B">> => #{<<"x">> => 4,<<"y">> => [1,2,3]}}
4> M2 = my:json_to_map(Bin2).
#{<<"A">> => 200,
<<"C">> => #{<<"a">> => 6,<<"b">> => [3,4,5]}}
5> [V1] = maps:values( maps:without([<<"A">>], M1)).
[#{<<"x">> => 4,<<"y">> => [1,2,3]}]
6> V1.
#{<<"x">> => 4,<<"y">> => [1,2,3]}
7> [V2] = maps:values( maps:without([<<"A">>], M2)).
[#{<<"a">> => 6,<<"b">> => [3,4,5]}]
8> V2.
#{<<"a">> => 6,<<"b">> => [3,4,5]}
9>

Answer 4

Hi following your advice is it correct my below code if "Body" is equal to:
"{\"Time\":\"2020-08-17T05:32:09\",\"BME680\":{\"Temperature\":29.6,\"Humidity\":54.6,\"Pressure\":1003.9,\"Gas\":1517.91},\"PressureUnit\":\"hPa\",\"TempUnit\":\"C\"}"
my code
....
eBody = jiffy:decode(<<Body>>),
Map = #eBody,
#{"BME680" := BME680} = Map,
newbody = jiffy:encode(BME680),
HTTPOptions = [],
Options = [],
R = httpc:request(Method, {URL, Header, Type, newbody},HTTPOptions, Options),
....
I'm newbie in erlang.