From: Axel Etzold <AEtzold@...>
Date: 2009-01-15T18:36:11+09:00
Subject: Re: undefined method `[]' for nil:NilClass - how to prevent?


-------- Original-Nachricht --------
> Datum: Thu, 15 Jan 2009 13:51:06 +0900
> Von: Mmcolli00 Mom <mmc_collins@yahoo.com>
> An: ruby-talk@ruby-lang.org
> Betreff: undefined method `[]\' for nil:NilClass - how to prevent?

> This returns correct value so long as I do not use array[0][i]. The
> error is:
> 
> >searchArray.rb:13: undefined method `[]' for nil:NilClass (NoMethodError)
> >  from shifting1.rb:7:in `each_line'
> >  from shifting1.rb:7
> >Exit code: 1
> >
> #**************************************
> #Searches through a text file and, at one row at a time, counts how many
> occurrences of the value exist for the column. For instance, if word,
> 'bird' exists in column1 row1 then count how many birds exist for the
> whole column. Next..count the occurrences of bird in the column 2. If
> the numbers do not match then outputs "Value does not contain a match:"
> +Value.
> 
> I am having this issue with NilClass a lot this week. What do you think
> I should change?
> 
> ##start code
> 
>  col1 = 0
>  col2 = 0
> 
>  File.open('temp.txt', 'r+').each_line do |temp|
>    array = []
>    i = 0
>   array = temp.chomp.split(",",0)
> 
>    arrayVal1 = array[0][i]
>    arrayVal2 = array[1][i]
> 
>      if array[0] == arrayVal1 then
>        col1 = col1 + 1
>       end
> 
>      if array[1] == arrayVal2 then
>        col2 = col2 + 1
>     end
>      i = i + 1
> 
>   if col1 != col2 then
>     puts "Value does not contain a match:"
>     puts arrayVal1
>  end
> 
> end
> 
> ##end code
> 
> Attachments:
> http://www.ruby-forum.com/attachment/3164/searchArray.rb
> 
> -- 
> Posted via http://www.ruby-forum.com/.

Hi ---
in some lines of your data, the split function doesn't find the appropriate pattern, so you have 
array==[], and thus, there isn't any array[0], i.e. array[0]==nil, and for nil, which is NilClass rather
than Array, there are no elements defined.
An alternative would be to do something like this:

begin 
  arrayVal1 = array[0][i]
rescue
  arrayVal1=["something_else_you'll_have_to_fill_in"]
end
begin 
  arrayVal2 = array[0][i]
rescue
  arrayVal2=["still_something_else_you'll_have_to_fill_in"]
end

which will try to execute the first statement after "begin", and if that fails, the statement after "rescue".
A conceptually cleaner way yet would be to assign a default value to the array variable,

array=Array.new(['nothing in here'])

Best regards,

Axel 

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