CODEWITHSUNDEEP
arrayslistelixir
posixpascal
posixpascal

Reputation: 3140

How to get list elements by index in elixir

{status, body} = File.read("/etc/hosts")
if status == :ok do
    hosts = String.split body, "\n"
    hosts = Enum.map(hosts, fn(host) -> line_to_host(host) end)
else
    IO.puts "error reading: /etc/hosts"
end

I have the following elixir function where I read the /etc/hosts file and try to split it line by line using String.split.

Then I map through the line list of hosts and call line_to_host(host) for each. The line_to_host method splits the line by " " and then I want to set the from and to variable:

def line_to_host(line) do
    data = String.split line, " "
    from = elem(data, 0) // doesn't work
    to = elem(data, 1) // doesn't work either
    %Host{from: from, to: to}
end

I looked through stackoverflow, the elixir docs and googled about how to get an list element at a specific index. I know there is head/tail but there has to be a better way of getting list elements.

elem(list, index) does exactly what I need but unfortunately it's not working with String.split.

How to get list/tuple elements by ID in elixir

Upvotes: 49

Views: 49313

Answers (2)

JuliSmz
JuliSmz

Reputation: 1146

You can consider implementing a recursive function instead Enum.at. It would provide more performance. It depends on if you need a high-performance code, of course.

Benchmarking:

Comparison: 
Recursive Enum At        9.41 M
Native Enum At           6.48 M - 1.45x slower +48.05 ns

Example code:

defmodule Test do
  def enum_at([h | t] = x, i) when i > 0  do
    enum_at(t, i-1)
  end
  def enum_at([h | t], i), do: h
end

#[1,2,3,4,5,6,7,8] |> Test.enum_at(4)
# 5

In your case:

data = String.split line, " "
%Host{from: enum_at(data, 0), to: enum_at(data, 1)}

Pattern Matching could be the best option, but it could "fail" if the split returns a 3 elements list. You should add validations on both cases.

Upvotes: 2

Patrick Oscity
Patrick Oscity

Reputation: 54674

You can use pattern matching for that:

[from, to] = String.split line, " "

Maybe you want to add parts: 2 option to ensure you will get only two parts in case there is more than one space in the line:

[from, to] = String.split line, " ", parts: 2

There is also Enum.at/3, which would work fine here but is unidiomatic. The problem with Enum.at is that due to the list implementation in Elixir, it needs to traverse the entire list up to the requested index so it can be very inefficient for large lists.

Edit: here's the requested example with Enum.at, but I would not use it in this case

parts = String.split line, " "
from = Enum.at(parts, 0)
to = Enum.at(parts, 1)

Upvotes: 53

Related Questions