Efficiently working with arrays and hashes? - ruby-on-rails

Question

I have these arrays:
#users = [[1,'Mark'],[2,'Bill'],[3,'John']]
#projects = [[1,'Change the world'],[2,'Build a computer'],[3,'Run in a circle']]
#points_due = [[1,1,"40"],[1,3,"80"],[2,1,"20"]]
I have a few solutions but I am looking for the most performance efficient way of iterating all these items as if the results were given to me via ActiveRecord.
I want to display a <UL> with a Project, and a <LI> with the user name and the points due to that particular user. I came up with some solutions converting the arrays into hashes using inject and map, however, I feel there is a better way to do such a thing.
I would like to do something like:
#projects.each do |project|
<%= project.name %>
<ul>
project.each do |user|
<li><%= user.name %> | <%= user.points_due %></li>
end
</ul>
end

Answer 1

To do things like what you're talking about (iterating over model X and then showing something for each model Y), a nested .each loop (more or less like the one you've sort-of included in your question) is a pretty standard method. If the display starts getting complicated, you might extract it into one or more layers of partials.
Worrying about the performance of this or other methods is likely premature at best. The performance of different ways of manipulating data structures is not going to be a significant factor in the overall performance of your page. Once you have something working, see if it's actually slow, then worry about how to make it faster.
Edit, based on your comments: An Array of Hashes (with symbols for keys) would probably be better than an Array of Arrays for this data. Not because they're more performant, but because it will make your code more understandable (to yourself, even if no one else ever works on it).
Consider:
Using AR objects, you would do the following to print the names of each Project:
#projects.each do |project|
puts project.name
end
If you simulate this behavior using a Hash:
#projects.each do |project|
puts project[:name]
end
Whereas using Arrays:
#projects.each do |project|
puts project[1]
end
Much harder to remember, and you have to make sure your code puts attributes in the proper order, or that might be some other aspect of the project, instead of the name.
Assuming you've agreed with me and are using this structure (#projects = [{:title => 'foo', 'id' => 1}, {:title => 'bar', :id => 2}] etc.), here's a way to perform the iteration you mention, simply printing out (what I believe to be) the desired data - the number of points listed in #points_due for each user that has some in that project:
#projects.each do |project|
puts project[:name]
#points_due.select{|pd| pd[:project_id] == project[:id] }.each do |project_points|
user = #users.select{|u| u[:id] == project_points[:user_id]}.first
puts "#{user[:name]} has #{project_points[:points]} points due on this project."
end
end
I hope this approximates the output you want, but if not, it should be fairly easy to use it as an example - judicious use of .select will probably get it done.
Edit the second: Converting Arrays to Hashes:
#projects_as_arrays = [[1,'Change the world'],[2,'Build a computer'],[3,'Run in a circle']]
#projects_as_hashes = #projects_as_array.map do |p_arr|
{:id => p_arr[0], :name => p_arr[1]}
end
You should be able to do something along these general lines for each of your arrays.