maps

Maps

Reading Material

• Learn You Some Erlang: Maps

Exercises

Sum of Values

Write a function maps_exercises:sum_of_values/1 that takes a map a returns a sum of all values in the map.

Note: the function maps:fold/3 can be useful.

Example:

1> maps_exercises:sum_of_values(#{a => 1, b =>3, c =>4}).
%%8

2> maps_exercises:sum_of_values(#{one => 1, three=> 3, seven => 7}).
%% 11

Min Value

Write a function maps_exercises:min_value(Map) that returns the minimum value of that Map.

Example:

1>maps_exercises:min_value(#{a => 1, b =>3, c =>4}).
%%1

2>maps_exercises:min_value(#{five => 5, three => 3, seven => 7, ten =>10}).
%%3

Sort keys

Write a function maps_exercises:sort_by_keys/1 that takes a map and returns a new map with the keys ordered from min to max.

Note: Remember you can use maps:keys/1 that returns a list with all the keys in a map.

Example:

1>maps_exercises:sort_by_keys(#{1=> one, 2 => two, 5 => five, 10 => ten, 3 => three, 15 => fifteen}).
%% #{1 => one,2 => two,3 => three,5 => five,10 => ten, 15 => fifteen}

2> maps:exercises:sort_by_keys(#{"one" => 1,"two" => 2, "three" => 3, "five" => 5, "four"=>4}).
#{"five" => 5,"four" => 4,"one" => 1,"three" => 3,"two" => 2}

Return values

Write a function maps_exercises:return_values/1 that takes a map a returns a list with all the values in that map

Example:

1> maps_exercises:return_values(#{"one" => 1,"two" => 2, "three" =>3 , "five" => 5, "four" => 4}).
%% [1,2,3,5,4]

2> maps_exercises:return_values(#{1 => one, 2 => two, 5 => five, 10 => ten, 3 => three, 15 => fifteen})
%%[one,two,three,five,ten,fifteen]

Merge Map

Write a function maps_exercises:merge/2 that merges 2 maps, if they have a key in common, keep the value from the second map.

Note: the function maps:fold/3 can be useful.

Example:

1> maps_exercises:merge(#{}, #{a => 1, b => 2}).
%% #{a => 1, b => 2}

2> maps_exercises:merge(#{a => 1, b => 2}, #{a => 5, c => 3}).
%% #{a => 5, b => 2, c => 3}

solution.

Mapping a Map

Write a function maps_exercises:map/2 for mapping a function over the values of a Map without using maps:map/2.

Example:

1> maps_exercises:map(fun(X) -> X + 1 end, #{a => 4, b => 2}).
%% #{a => 5, b => 3}

2> maps_exercises:map(#{}).
%% #{}

solution.

List to Map

Create a function maps_exercises:to_map/1 that converts a list to a Map without using maps:from_list.

Example:

1> maps_exercises:to_map([2, 1, 6, 4]).
%% #{1 => 2, 2 => 1, 3 => 6, 4 => 4}

2> maps_exercises:to_map([]).
%% #{}

solution.

Records to Maps

Create a record named person that has the attributes name and age. Then write a function maps_exercises:records_to_maps(Records) that converts a list of records (in this case people) into a list of maps with the attributes. For this you should use lists:map.

Example:

1> maps_exercises:records_to_maps([]).
%% []

2> maps_exercises:records_to_maps([#person{name="Pepe", age=28}, #person{name="Luis", age=77}]).
%% [#{age => 28,name => "Pepe"},#{age => 77,name => "Luis"}]

solution.

Maps to Records

Create a record named person that has the attributes name and age. Then write a function maps_exercises:maps_to_records(Records) that converts a list of maps into a list of records (in this case people). For this you should only use recursion.

Example:

1> maps_exercises:maps_to_records([]).
%% []

2> maps_exercises:maps_to_records([#{age => 28,name => "Pepe"},#{age => 77,name => "Luis"}]).
%% [#person{name = "Luis",age = 77}, #person{name = "Pepe",age = 28}]

solution.

Proplist to Map

DISCLAIMER Erlang provides functions for the following task: maps:from_list and maps:to_list, but we want you to implement it by hand.

Write a recursive function proplist_to_map/1 that takes a proplist (a list of tuples) and builds a map from it. Use the first component of each tuple as the key and the second component as the value.

Example:

1> maps_exercises:proplist_to_map([]).
%% #{}

2> maps_exercises:proplist_to_map([{firstname, "Pedro"}, {lastname, "Sanches"}, {age, 11}]).
%% #{age => 11,firstname => "Pedro",lastname => "Sanches"}

solution.