Detailed introduction to map group map in Erlang

% No matter how you sort it, the final result is in dictionary order of the keys
#{ name => "wittyfox", age => 19 }.
% => #{age => 20,name => "wittyfox"}

% You can also create an empty map group
#{}.
% => #{}

% Now I
Me = #{ name => "wittyfox", age => 19 }.
% => #{age => 19,name => "wittyfox"}

% It's the New Year, I'm one year older, I've become a brand new me
NewMe = Me#{ age => 20 }.
% => #{age => 20,name => "wittyfox"}

% can also be modified directly
#{ name => "wittyfox", age => 19 }#{ age => 20 }.
% => #{age => 20,name => "wittyfox"}

% I originally wanted to update age, but accidentally spelled it wrongly and created a new map
Me#{ aeg => 20 }.
% => #{aeg => 20,age => 19,name => "wittyfox"}.

% Aeg key does not exist, badarg exception is thrown
Me#{ aeg := 20 }.
% ** exception error: bad argument ... blabla

% Only existing maps can be updated
Me#{ age := 20 }.
% => #{age => 20,name => "wittyfox"}

#{ name := "wittyfox", age := 19 }.
% * 1: only association operators '=>' are allowed in map construction

% Partial match: We just want to remove age, so we only care about whether there is a map of age in the parameters
#{ age := Age } = Me.
% => #{age => 19,name => "wittyfox"}

% Age.
% => 19

% Illegal match
#{ age => Age } = Me.
% * 1: illegal pattern

% Here is to create a mapping group, only =>
 new() ->
     {ok, {?MODULE, #{name => "wittyfox", age => 19}}}.

% Here is a match, only :=
 show({?MODULE, #{name := Name, age := Age}}) ->
     io:format("Name: ~p, Age: ~p~n", [Name, Age]).

% Create a map group
maps:new().
% => #{}

% Return all keys
maps:keys(Me).
% => [age,name]

% Determine whether there is a key
maps:is_key(age, Me).
% => true
maps:is_key(aeg, Me).
% => false

% key order returns all values
maps:values(Me).
% =>[19,"wittyfox"]

% Map number
maps:size(Me).
% => 2

% You can also use erlang:map_size/1
% This function can be used in Guard, and this function is also used in the maps module.
map_size(Me).
% => 2

% maps:get/2 Exception will be thrown when the key does not exist
maps:get(age, Me).
% => 19

% maps:get/3 will return the value of the third parameter when the key does not exist.
maps:get(aeg, Me, 20).
% => 20

% is used to update or create a map, similar to =>
% The so-called update is just to return the updated new mapping group, and the original mapping group will not change.
maps:put(gender, male, Me).
% => #{age => 19,gender => male,name => "wittyfox"}

% is used to update the map, similar to :=, badarg exception will be thrown when the key does not exist
maps:update(age, 20, Me).
% => #{age => 20,name => "wittyfox"}

% Delete a map. When the key does not exist, it is equivalent to doing nothing and no exception is thrown.
maps:remove(age, Me).
% => #{name => "wittyfox"}

% Find the value of the key, return error when the key does not exist
maps:find(age, Me).
% => {ok, 19}

maps:find(aeg, Me).
% => error

% merges two mapping groups. Note that the second parameter is to create a new mapping group, so you can only use =>
maps:merge(Me, #{ age => 10 }).  
% => #{age => 10,name => "wittyfox"}

% is equivalent to
Me#{ age => 10 }.

% Returns a list of mapped tuple pairs
maps:to_list(Me).
% => [{age,19},{name,"wittyfox"}]

%Build a mapping group from a list
maps:from_list([]).
% => #{}

maps:from_list([{name, "wittyfox"}, {age, 19}]).
% => #{age => 19,name => "wittyfox"}

% Perform an operation on each pair of maps in the map group
%X and Y are the keys and values ​​of a pair of maps, respectively
maps:map(fun (X, Y) -> io:format("~p => ~p~n", [X, Y]) end, Me). 
% age => 19                                                           �
% name => "wittyfox"                                                         �
% => #{age => ok,name => ok}                                                                                                                      �

%X and Y are the keys and values ​​of a pair of maps, V is the result of the last iteration, and 0 is the initial value of the iteration
% is simply used to add 1 to each iteration, and the result is the number of maps of the map group
maps:fold(fun (X, Y, V) -> V + 1 end, 0, Me).
% => 2

maps:with([], Me).
% => #{}

maps:with([age], Me).
% => #{age => 19}

The % key may not exist
maps:with([aeg], Me).
% => #{}

maps:without([], Me).
% => #{age => 19,name => "wittyfox"}

maps:without([age], Me).
% => #{name => "wittyfox"}

The % key may not exist
maps:without([age, neme], Me).
% => #{name => "wittyfox"}