Recursive method returning undefined method '[]'

Question

I'm not sure why this recursive method is returning NoMethodError: undefined method '[]' for nil:NilClass

def test_method(a, b)
    (a[0] == b[0] ? 0 : 1) + test_method(a[1..-1], b[1..-1])
end

edit: I am sending in strings for the arguments to compare them.

What arguments are you sending to this method to get that exception? — Alex Wayne
– Alex Wayne, Commented Sep 11, 2012 at 19:15

sawa · Accepted Answer · 2012-09-12 02:26:50Z

1

Actually, at one time, it will reduce to empty string and then to nil. And then causes NoMethodError: undefined method '[]' for nil:NilClass.

Something like this is happening:

'ss'[1..-1] => 's'
'ss'[1..-1][1..-1] => ""
'ss'[1..-1][1..-1][1..-1] => nil

edited Sep 12, 2012 at 2:26

sawa

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answered Sep 11, 2012 at 19:24

Eugene Dorian

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maximus ツ · Accepted Answer · 2012-09-11 19:20:53Z

1

This is because you are not checking whether a and b are empty arrays or not. At one time that will reduce to empty array

before calling you should write

 if not (a.empty? or b.empty?)
  (a[0] == b[0] ? 0 : 1) + test_method(a[1..-1], b[1..-1])
 end

answered Sep 11, 2012 at 19:20

maximus ツ

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5 Comments

Fralcon Over a year ago

I planned on adding empty string checking later on. Do I need to check it right off the bat? All the tests I have been doing have been with strings longer than 5 characters in length, so I assumed this wouldn't be a problem yet.

maximus ツ Over a year ago

but at one time like a[6..-6] will gives back nil. Check this out in console

Azolo Over a year ago

If it's recursive, at some point you you have to reach the end.

maximus ツ Over a year ago

but you have to stop some where explicitly. It will not happen automatically.

Fralcon Over a year ago

Ah, i get it. Can't believe I didn't see it before. Thanks.

sawa · Accepted Answer · 2012-09-12 02:28:34Z

1

Since you're iterating over an ever smaller array/string/whatever, you have to account for reaching the end of it. I have no idea what you're trying to accomplish, but this'll at least stop it from blowing up:

def test_method(a, b)
  return 0 if a.nil? || b.nil?
  (a[0] == b[0] ? 0 : 1) + test_method(a[1..-1], b[1..-1])
end

edited Sep 12, 2012 at 2:28

sawa

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answered Sep 11, 2012 at 19:26

Tom L

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