Why is this local variable labeled as undefined? - Ruby

I've got a function that goes through an array of objects, and creates a new array of objects based on certain attributes from the original array. When I run this code I get the error

in 'nonstop': undefined local variable or method `sort_list' for main:Object (NameError)

I made sure the sort_list array was initialized outside of the loop, and I've tried initializing it with a certain size too, but I keep getting this error. I'm pretty new to ruby, so what am I doing incorrectly?

    开发者_Go百科def nonstop(flight_list)
      index = 0
      sort_list[] = nil
      flight_list.each do |curr|
        if (curr.depapt == ARGV[2] && curr.arrapt == ARGV[3])
          sort_list[index] = curr
          index += 1
        end
      end
      sort_list.sort! { |a,b|  a.deptime <=> b.deptime}
      sort_list.each do |curr|
         puts "#{curr.flightnum}\t#{curr.deptime}\t#{curr.arrtime}"
      end
      if (sort_list.empty?)
        puts "none"
      end
    end

I think you need to initialize it like this:

sort_list = []

This doesn't throw an error in irb for me.

You have already got right answer.
I'll make a little refactoring, which can be useful for novice.

def nonstop(flight_list)
  sort_list = flight_list.select { |curr| curr.depapt == ARGV[2] && curr.arrapt == ARGV[3] }
  sort_list.sort_by! &:deptime
  sort_list.each do |curr|
    puts "#{curr.flightnum}\t#{curr.deptime}\t#{curr.arrtime}"
  end
  puts "none" if sort_list.empty?
end

Here is why sort_list[] = nil does not work in ruby:

[] On its own is a shortcut for Array.new

[] after a variable name is calling [] on the variable. So in that context [] is a method.

sort_list is not set to anything, so it is undefined. So sort_list[] is calling [] on an undefined value

Setting any variable to nil makes it of nil class.