Errors in my ruby script - ruby

Question

I am getting an error I don't understand. It runs how I intended but throws an error at the end. I am still new to ruby but I know it has something to do with my for loops.
This is my code
nums = Array.new(24){Array.new(80){'X'}}
system('cls')
for i in 0..24
for j in 0..79
print nums[i][j]
end
end
And this is the error messages
K:/Ruby 2/RubyInvaders.rb:5:in block (2 levels) in <main>': undefined method `[]' for nil:NilClass (NoMethodError)
from K:/Ruby 2/RubyInvaders.rb:4:in each'
from K:/Ruby 2/RubyInvaders.rb:4:in block in <main>'
from K:/Ruby 2/RubyInvaders.rb:3:in each'
from K:/Ruby 2/RubyInvaders.rb:3:in <main>'
Its ok to offer better ways to do this but I would also like to understand why I am getting this error

Answer 1

You are creating an array with 24 elements and then a loop with 25 iterations. When you try to print the 25th iteration of that loop, the array position doesn't exist. If you change for i in 0..24 to for i in 0..23, the error should be addressed:
nums = Array.new(24){Array.new(80){'X'}}
system('cls')
for i in 0..23
for j in 0..79
print nums[i][j]
end
end
To amplify on Jon's comment, ruby ranges created with the ... operator are exclusive and won't use the highest specified value (as opposed to the .. operator, which is inclusive).
And--while your approach to looping is valid--it's not idiomatic for ruby. Something like this would be more common:
nums = Array.new(24){Array.new(80){'X'}}
system('cls')
(0..23).each do |i|
(0..79).each do |j|
print nums[i][j]
end
end

Answer 2

When i is 24, nums[i] is nil, and you are calling [] on it.