Elixir templates - looping through a list with iterator value

Question

Coming from other programming languages, many like me will be in for a surprise. I have a simple problem. I have a list - say, users. I want to iterate through the users and display some information. Pretty simple until I got stumped with this:

Using an eex template, I was trying to do this:

<%= for i <- 0..length(@users) do %>
  <% user = Enum.at(@users) %>

  <!-- every third user, I need to display on a new "row" -->
  <%= if rem(i,3) == 0 do %>
    <div class="row">
  <% end %>

  <!-- display user information - like user name -->
  <%= user.name %>

  <!-- then close out the "row" tag if we are due for starting another on the next iteration -->

   <%= if rem(i+1,3) == 0 do %>
    <div class="row">
  <% end %> 

<% end %>

Firstly, the user value is turning out to be nil. Not sure why. Secondly, I don't have an explanation for this.

What is the difference between "user = Enum.at(@users, i)" and <%= for user <- @users %>? In the latter case, the user is not nil.

Finally, what is the best way to keep track of the times through the list and take a slightly different action (like starting the user on a new row as in the example above)?

To keep things simple, I have also ignored the fact that we may not have a final closing "/div" tag for the row (which depends on the number of users in the list). I've ignored putting any logic around that just to focus on the issue at hand.

Answer 1

What is the difference between user = Enum.at(@users, i) and <%= for user <- @users %>? In the latter case, the user is not nil.

You should be getting user = nil for the last iteration. That's because you're looping from 0 to length(@users), and the last iteration value will be length(@users) which is 1 more than the last valid index in the list as indexing is 0 based. You should loop from 0 to length(@users) - 1. Note that this will cause problems if @users is empty as Enum.to_list(0..-1) #=> [0, -1].

Finally, what is the best way to keep track of the times through the list and take a slightly different action (like starting the user on a new row as in the example above)?

I would use Enum.with_index and if. Also, length in Elixir is a slow (O(n)) operation, so it's best to cache its value outside the loop instead of recalculating it on every iteration. I also believe you meant to print "</div>" in the second if. Here's the final code:

<% users_length = length(@users) %>
<%= for {user, i} <- Enum.with_index(@users) %>
  <%= if rem(i, 3) == 0, do: ~s|<div class="row"| %>
  <%= user.name %>
  <%= if rem(i + 1, 3) == 0 || i + 1 == users_length, do: "</div>" %>
<% end %>

Answer 2

I think one problem is that you needed to loop from 0 to length - 1, instead of 0 to length. Because you looped till length the last recursion, Enum.at(@users, length) was giving nil