findIndexOf.ex

defmodule Sample1 do
  # combining Enum functions
  def find_indexes(collection, function) do
    Enum.filter_map(Enum.with_index(collection), fn({x, _y}) -> function.(x) end, elem(&1, 1))
  end
end

defmodule Sample2 do
  # implementing as similar way as Enum.find_index
  def find_indexes(collection, function) do
    do_find_indexes(collection, function, 0, [])
  end

  def do_find_indexes([], _function, _counter, acc) do
    Enum.reverse(acc)
  end

  def do_find_indexes([h|t], function, counter, acc) do
    if function.(h) do
      do_find_indexes(t, function, counter + 1, [counter|acc])
    else
      do_find_indexes(t, function, counter + 1, acc)
    end
  end
end

IO.puts "Sample1"
IO.inspect Sample1.find_indexes(["a", "b", "c", "b", "b"], fn(x) -> x == "a" end)
IO.inspect Sample1.find_indexes(["a", "b", "c", "b", "b"], fn(x) -> x == "c" end)
IO.inspect Sample1.find_indexes(["a", "b", "c", "b", "b"], fn(x) -> x == "b" end)

IO.puts "Sample2"
IO.inspect Sample2.find_indexes(["a", "b", "c", "b", "b"], fn(x) -> x == "a" end)
IO.inspect Sample2.find_indexes(["a", "b", "c", "b", "b"], fn(x) -> x == "c" end)
IO.inspect Sample2.find_indexes(["a", "b", "c", "b", "b"], fn(x) -> x == "b" end)