How to Print a Multi-Dimensional Array in Ruby

How do I print a multi-dimensional array in ruby?

If you're just looking for debugging output that is easy to read, "p" is useful (it calls inspect() on the array)

p x
[[1, 2, 3], [4, 5, 6]]

Print out 2D array

Suppose:

arr = Array.new(10) { (0..20).to_a.sample(10) }

Then

puts arr.map { |x| x.join(' ') }
1 9 6 15 7 19 18 3 0 12
13 20 18 15 0 3 19 1 14 16
7 16 5 3 12 19 4 9 20 10
6 10 9 1 18 17 7 19 5 15
12 3 8 16 10 5 2 18 20 6
12 9 0 18 2 11 16 8 7 15
8 9 14 19 3 16 6 20 13 17
7 19 16 14 13 6 9 2 3 5
10 17 8 15 11 2 13 14 16 7
14 9 20 17 15 3 4 2 11 19

is not very, er, attractive. For something more pleasing, you could quite easily do something like this:

width = arr.flatten.max.to_s.size+2
  #=> 4
puts arr.map { |a| a.map { |i| i.to_s.rjust(width) }.join }
   1   9   6  15   7  19  18   3   0  12
  13  20  18  15   0   3  19   1  14  16
   7  16   5   3  12  19   4   9  20  10
   6  10   9   1  18  17   7  19   5  15
  12   3   8  16  10   5   2  18  20   6
  12   9   0  18   2  11  16   8   7  15
   8   9  14  19   3  16   6  20  13  17
   7  19  16  14  13   6   9   2   3   5
  10  17   8  15  11   2  13  14  16   7
  14   9  20  17  15   3   4   2  11  19

If you have too many columns to display on the screen you can do this:

puts arr.map { |a| a.map { |i| i.to_s.rjust(width) }.join.tinyfy }

^{1   9   6  15   7  19  18   3   0  12
   13  20  18  15   0   3  19   1  14  16
    7  16   5   3  12  19   4   9  20  10
    6  10   9   1  18  17   7  19   5  15
   12   3   8  16  10   5   2  18  20   6
   12   9   0  18   2  11  16   8   7  15
    8   9  14  19   3  16   6  20  13  17
    7  19  16  14  13   6   9   2   3   5
   10  17   8  15  11   2  13  14  16   7
   14   9  20  17  15   3   4   2  11  19}

Ruby - How to access elements in a multi-dimensional array

source.map { |row| row.each_index.select { |i| row[i] == "a" } }
# => [[1, 2], [], [0]]

You can't do it because your logic is mistaken :) source.each {...} will return source, doesn't matter what you do inside the block. To return the result, use map. source.each.each_index calls each_index on the enumerator returned by each without block, and that's not a method available on enumerators (you want each_index on arrays).
Indeed. each with block and each without block will do very different things.

Specifically, in my code above:

Start with source array. map with block will process the block for each element (called row), and return an array of the results. For each row, row.each_index without block will return an iterator of all indices of the row array, select with block on the iterator will return an array that will contain only some of those elements.

Print multidimensional array in rails views

Fix is :

<% @allposts.each do |posts, slots| %>
  <% slots.each do |post| %>  # <--- see I removed @ symbol
    <%= post.Title %>
  <% end %>
<% end %>

Your block variable is slots, and you attempted as @slots. Which I think is a typo. As there is no such @slots variable defined, you got nil, when you wanted to use it. There is a sweet difference between local variable and instance variable. If you don't defined anywhere a local variable say foo, you will be ended up with undefined local variable or method if you want to use foo anywhere else.But for instance variable, no error, you will silently get nil.

How do you loop through and print a multidimensional array in a Rails view?

There's no reason you can't use a loop within your view, and that's exactly the way you'd want to do it:

<% @groups.each do |group| %>
  <!-- Render stuff -->
  <% group.groups.each do |child| %>
    <!-- Render child stuff -->
  <% end %>
<% end %>

It may be a lot easier to manage if you do this using partials instead:

# view.html.erb
<% render partial: 'group', collection: @groups, as: :group %>

# _group.html.erb
<!-- Render stuff -->
<% render partial: 'first_child', collection: group.groups, as: :group %>

Check out the Rendering Collections section of the Rails guide on Layouts and Rendering for some more info.

EDIT: If you need to handle an unknown level of nesting, partials would work here too. You can even track the depth if you want. Say you wanted to display them using nested ul elements:

# _group.html.erb
<% content_tag :li, class: "depth-#{depth}" do %>
  <!-- Any output needed for the group itself -->
  <% unless group.group.empty? %>
    <ul>
      <%= render partial "group", collection: group.groups, as: :group,
              locals: { depth: depth+1 } %>
    </ul>
  <% end %>
<% end %>

# In your view file
<ul>
  <% render partial "group", collection: @groups, as: :group,
         locals: { depth: 0 } %>
</ul>

Ruby multidimensional array

Strictly speaking it is not possible to create multi dimensional arrays in Ruby. But it is possible to put an array in another array, which is almost the same as a multi dimensional array.

This is how you could create a 2D array in Ruby:

a = [[1,2,3], [4,5,6], [7,8,9]]

As stated in the comments, you could also use NArray which is a Ruby numerical array library:

require 'narray'
b = NArray[ [1,2,3], [4,5,6], [7,8,9] ]

Use a[i][j] to access the elements of the array. Basically a[i] returns the 'sub array' stored on position i of a and thus a[i][j] returns element number j from the array that is stored on position i.

Ruby: how does one print a 2D array?

One could do something like this:

WINNING_ROWS = [[1,2,3],[4,5,6]]
WINNING_ROWS.map { |x| x.inspect }.join("")

Which will get you a string formatted as you requested

Is there a simple way to duplicate a multi-dimensional array in Ruby?

Here's the "Ruby-esque" way to handle it:

temp_array = Marshal.load(Marshal.dump(your_array_to_be_cloned))