the_thing.ex

defmodule SomeModule do
  def func(map, key) do
    for {k, v} <- map, into: %{} do
      {k, v/map[key]}
    end
  end
end

defmodule MapDivisionTest do
  use ExUnit.Case
  doctest MapDivision

  test "the thing" do
    map =
      %{
        "cash" => 100,
        "a" => 1,
        "b" => 2
      }

    expected =
      %{
        "cash" => 1,
        "a" => 1/100,
        "b" => 2/100
      }

    assert SomeModule.func(map, "cash") == expected
  end
end