Accessing an array variable in Ruby

Question

I'm currently teaching myself Ruby, via the 'Learn Ruby the Hard Way' course, and am having a problem referring to an array variable in a function.

On the section on while loops, it is required to turn a while-loop into a function, adjusting the while-loop parameters to be variables. My code:

#original while loop 
while i < 6 
  puts "At the top i is #{i}"
  numbers.push(i)
  i += 1
  puts "Numbers now: ", numbers
  puts "At the bottom i is #{i}"
end

#new function
number_array = [] 
def number_cruncher(number, cap)
  if number < cap
    puts "At the top number is #{number}"
    number_array.push(number)
    number += 1
    puts "Numbers now: ", number_array
    puts "At the bottom number is #{number}"
    number_cruncher(number, cap, number_array)
  end
end

number_cruncher(0,6)

Running this code gives a NameError, because number_array as cited in the function is an undefined local variable. I can understand that this is wrong, but I can't work out what variable scope or syntax would allow me to refer to a variable whose starting point is an empty array - except by specifying a variable 'number_array' as a parameter of the function, and then declaring number_array = [], which is functional, but seems overcomplicated.

I have researched variable scope in Ruby (here and elsewhere), but have nevertheless been unable to work this out.... Am 100% sure I am being really stupid, but help would nevertheless be appreciated!

Possible duplicate of Why can't I access a local variable inside a method in Ruby? — aristotll
– aristotll, Commented Aug 10, 2017 at 13:13

Tom Lord · Accepted Answer · 2017-08-10 13:14:05Z

1

The normal approach would just be to make this a parameter with a default value:

def number_cruncher(number, cap, number_array = [])

You're also missing the crucial return value for the recursive function. You need to do something like this:

def number_cruncher(number, cap, number_array = [])
  return number_array if number >= cap # <-- !!!!!

  puts "At the top number is #{number}"
  number_array.push(number)
  number += 1
  puts "Numbers now: ", number_array
  puts "At the bottom number is #{number}"
  number_cruncher(number, cap, number_array)
end

answered Aug 10, 2017 at 13:14

Tom Lord

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10 Comments

S Spooner Over a year ago

Thanks! Having looked at it again, I now see that it also works if I specify number_array as an instance variable in all cases (somehow couldn't get this to work before). Why is that return value crucial? The function seems to work in my terminal without it.

Tom Lord Over a year ago

Ah... It's actually working because you're mutating the variable! array.push changes the original variable's value, rather than just returning a new value. If you tried this approach using number_array += [number] for example, you'd see why the return value would be critical in order for this to work as a functional program.

Tom Lord Over a year ago

I'm not sure what you mean by "also works if I specify number_array as an instance variable", though... Your original code would never have worked since the method only took 2 arguments, but you were calling it with 3.

S Spooner Over a year ago

Yes that was a typo 3rd argument - I mean it works when I remove that third argument and use instance variables.

S Spooner Over a year ago

The method you give by specifying a default value doesn't work, though - gives an ArgumentError (given 3, expected 2) - is this because you can't change the default value, or is there something else I'm missing?

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