`Enum.map_every/3` but with start offset

@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)

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)   

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.

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.

https://github.com/gvaughn/elixir_kata/blob/master/luhn/luhn.exs

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]

found a simple solution:
basic concept is, adding a dummy value at the beginning, and choose tail later.

list=[1,2,3,4,5]
Enum.map_every(list, 2, & &1+10)
[11, 2, 13, 4, 15]
[99 | list] |> Enum.map_every(2, & &1+10) |> tl
[1, 12, 3, 14, 5]

for more offset, comprehension would work.

for i <- 1..5 do i end ++ list |> Enum.map_every(7, & &1+10) |> Enum.drop(5)
[1, 2, 13, 4, 5]

if it is just alternate pattern and need to choose offset 0 or 1, it might be useful