Update all values in a map using values from another map (Elixir)

user_15370402 0
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How can I update all values in a map, using the values from corresponding keys of another map? For example I have the two maps bellow:

map = %{"December 2021" => 0, "November 2021" => 0, "October 2021" => 0}
map_2 = %{"December 2021" => 7, "November 2021" => 6}

And I want to update all values from map with the corresponding values from map_2, so in the end map = %{"December 2021" => 7, "November 2021" => 6, "October 2021" => 0}

I have tried:

Enum.map(map_2, fn {key, value} -> %{map | k => v} end)

I have also tried the code above with other functions like Map.update!/3 and similar ones, but they all return a list with maps for each iteration of Enum.map.

Does someone have any idea on how to do it?

Thanks in advance

phoenix-framework elixir hashmap
2 Answers

I think you are looking for Enum.reduce/3. Reduce here means to reduce a collection to a single value. The collection to reduce here is map_2. The initial/single value you start with is the original collection map. The output is a modification of the initial value.

In this example, every key/value of map_2 is passed to a function that also expects an accumulator acc. acc is initialized with the initial value map (the second argument of Enum.reduce). In this case, every value of map_2 is added to map:

map = %{"December 2021" => 0, "November 2021" => 0, "October 2021" => 0}
map_2 = %{"December 2021" => 7, "November 2021" => 6}

Enum.reduce(map_2, map, fn {key, value}, acc ->
  Map.put(acc, key, value)
end)

#> %{"December 2021" => 7, "November 2021" => 6, "October 2021" => 0}

If you want different behavior, for example don't add values from map_2 with keys that are non-existent in map: try to change the reducer function.

user_4097920

The most idiomatic way to accomplish this is to use the Map.merge/2 function, which does exactly this:

Map.merge(map, map_2)

This will merge all key-value pairs from map_2 into map.

Of course, you could write a custom version of it using Enum.reduce/3 as well as suggested by @zwippie, but this would be less efficient and more verbose.

user_139795180
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