# `Enum.map_every/3` but with start offset

There is sometimes cases where you want to map every nth item starting on the nth item. Eg map every 2nd item starting on the 2nd one. Currently the `map_every/3` always gives back the first item in the enumerable and then every nth item. I propose to add an optional parameter to the function for the offset for the first item. Another option would be to add a parameter to start on the nth item instead but that’s probably less flexible than the offset. What’s your thoughts? Is there another way to do this with the existing APIs?

1 Like

@mackeyja92 I’d probably use `Enum.drop`.
eg: “Map every 4th thing starting on the 2nd thing”

``````list |> Enum.drop(2) |> Enum.map_every(4, fun)
``````
1 Like

The problem with drop is it removes the items from the list. If you only need to do a certain operation on nth items but need to do another operation on the whole list after you’re stuck with removing items and storing in a temporary list and then pushing them back on the front after the map_every.

A real world example is the Luhn checksum.

``````# Reverse them first
numbers = Enum.reverse(numbers)

# Pop the first one off because elixir always does the first one when using map_every
{first, numbers} = List.pop_at(numbers, 0)

numbers
|> Enum.map_every(2, fn num ->
num = num * 2
# If the number after doubling is greater than 9 then subtract 9
if num > 9 do
num - 9
else
num
end
end)
# Put the first one back
|> List.insert_at(0, first)
# Sum them all
|> Enum.sum()
# If the modulo of 10 is 0 then it is a valid checksum
|> Kernel.rem(10)
|> Kernel.==(0)
``````
1 Like

I believe that is a perfectly fine solution. If you need to perform different operations on even and odd elements you could also try chunking the list.

E.g.

``````[1,2,3,4,5,6]
|> Enum.chunk_every(2)
# [[1,2], [3,4], [5,6]]
|> Enum.map(fn [first, second] -> [first*2, second*3] end)
|> List.flatten
# [2, 6, 6, 12, 10, 18]
``````

That doubles every odd element and triples every even one.

2 Likes

I had fun turning the Luhn checksum into a simple Elixir pipeline a couple of years ago. The algorithm as stated is of a very imperative/mutable mindset, but you can do it with immutable pipelines.

1 Like

If you want to get “fancy”, then you could simply use `Enum.with_index` and `Enum.map`. Bonus points if you use the Stream module

``````[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
|> Enum.with_index()
|> Enum.map(fn {num, index} ->
cond do
index < 4 -> num
rem(num, 2) == 0 -> num * 2
true -> num * 3
end
end)

[1, 2, 3, 4, 15, 12, 21, 16, 27, 20]
``````
1 Like