For each key in map_A (we don’t care about the values) get another known map (for example sake: map_B ) which is referenced by the key at map_A (that we are looping) and test whether any of the keys in map_B matches some variable x . If match, update value at key in map_A to "yes" , else "no" .

    x = "something"
    map_A = %{"a" => "no", "b" => "no"}

    for f <- Map.keys(map_A) do
        if(Enum.member?(Map.keys(map_B), x) == true) do
            Map.replace(map_A, f, "yes")
        else
            Map.replace(map_A, f, "no")
        end
     end

The problem is that Map.replace(map_A, f, "yes | no") cannot affect map_A outside of the for iteration. The same would apply for Enum.each/2.