maps:put(key,value,Map) not working
Map = #{"a" => 1},
maps:put("a", 42, Map),
maps:put("b", 1337, Map),
io:fwrite("~p~n",[Map]).
output #
{"a" => 1}
Why I am not able to get output #{"a" => 42,"b" => 1337}?
maps:put(Key,Value,Map) not working in erlang
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1 Answers
The maps:put/3 function returns a new map. You can see that it doesn't alter Map if you look at the variable again after the first maps:put/3 call:
1> Map = #{"a" => 1}.
#{"a" => 1}.
2> maps:put("a", 42, Map).
#{"a" => 42}
3> Map.
#{"a" => 1}
As you can see, Map retains its initial value.
To fix the problem, you need to capture the maps:put/3 return value into a new variable:
4> Map1 = maps:put("a", 42, Map).
#{"a" => 42}
5> Map2 = maps:put("b", 1337, Map1).
#{"a" => 42,"b" => 1337}
Now the variable Map2 has the value you're looking for.
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