Forum: Ruby Undefined method

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Mark Liang (Guest)
on 2005-12-14 05:08
(Received via mailing list)
I've recently started learning Ruby, I cannot seem to locate the
cause of this error. Please pardon me if the answer is more
straightforward as it seems.

I've double-checked, blkcheck method works fine. I suspect the
problem lies with line 32, Blk.each ..., however, I don't see why
a problem should be there. Please help shed some light.

The error message:

../creat.rb:25:in `blkcheck': undefined method `[]' for nil:NilClass
(NoMethodError)
        from ./creat.rb:25:in `times'
        from ./creat.rb:25:in `blkcheck'
        from ./creat.rb:24:in `times'
        from ./creat.rb:24:in `blkcheck'
        from ./creat.rb:33:in `blkscheck'
        from ./creat.rb:32:in `each'
        from ./creat.rb:32:in `blkscheck'
        from ./creat.rb:45


The src:

      1 #!/usr/bin/ruby
      2
      3 Val = [1, 2, 3, 4, 5, 6, 7, 8, 9]
      4 Blk = [[0,0],[0,3],[0,6],[3,0],[3,3],[3,6],[6,0],[6,3],[6,6]]
      5
      6 def linescheck (tab)
      7    num = 0
      8    9.times { |i| num += 1 if tab[i].sort == Val }
      9    1 if num == 9
     10 end
     11
     12 def colscheck (tab)
     13    num = 0
     14    9.times do |i|
     15       tmp = []
     16       9.times { |j| tmp <<= tab[j][i] }
     17       num += 1 if tmp.sort == Val
     18    end
     19    1 if num == 9
     20 end
     21
     22 def blkcheck (tab, i, j)
     23    tmp = []
     24    3.times do |k|
     25       3.times { |l| tmp <<= tab[i+k+l][j+k] }
     26    end
     27    1 if tmp.sort == Val
     28 end
     29
     30 def blkscheck (tab)
     31    num = 0
     32    Blk.each do |i,j|
     33       num += 1 if blkcheck(tab,i,j)
     34    end
     35    1 if num == 9
     36 end
     37
     38 # Create the table of values
     39 val = []
     40 9.times do |i|
     41    val <<= Val
     42 end
     43
     44 #blkscheck(val)
     45 print "Hello world!\n" unless blkscheck(val)
unknown (Guest)
on 2005-12-14 05:52
(Received via mailing list)
Hi --

On Wed, 14 Dec 2005, Mark Liang wrote:

> ../creat.rb:25:in `blkcheck': undefined method `[]' for nil:NilClass
>
>      9    1 if num == 9
>     10 end
>     11
>     12 def colscheck (tab)
>     13    num = 0
>     14    9.times do |i|
>     15       tmp = []
>     16       9.times { |j| tmp <<= tab[j][i] }
>     17       num += 1 if tmp.sort == Val
>     18    end
>     19    1 if num == 9

I'm not sure why you would need 1 there.  If you're just testing the
truth value, then you'd want simply:

   num == 9

If you need to return an integer value then be warned that if the
condition fails, you'll be returning a non-integer, namely nil.

>     30 def blkscheck (tab)
>     31    num = 0
>     32    Blk.each do |i,j|
>     33       num += 1 if blkcheck(tab,i,j)

On the third iteration, j is 6.  That means that in blkcheck, i+k+j is
eventually going to add up to 9.  Since tab only has nine elements,
there's no tab[9].  tab[9] is nil, so when you do tab[9][j+k] you're
calling [] on nil.

>     44 #blkscheck(val)
>     45 print "Hello world!\n" unless blkscheck(val)

Discover the joys of 'puts' :-)


David

--
David A. Black
removed_email_address@domain.invalid

"Ruby for Rails", forthcoming from Manning Publications, April 2006!
Johannes F. (Guest)
on 2005-12-14 07:01
(Received via mailing list)
The simplest way to debug something like this is to add a few print
statements so you see which values are involved.

The error
>> /creat.rb:25:in `blkcheck': undefined method `[]' for nil:NilClass
> > (NoMethodError)
means that your line 25 has just called '[]' on nil.

So in line 25
>> 3.times { |l| tmp <<= tab[i+k+l][j+k] }
either "tab" or "tab[i+k+l]" is nil. But you don't know which, or why.

By adding a couple of print statements to your method, like this
--------
def blkcheck (tab, i, j)
   tmp = []
   printf("input: i=%i, j=%i, tab(length=%i)=%s\n", i, j, tab.length,
tab.inspect)
   3.times do |k|
      3.times { |l|
        e1=tab[i+k+l]
        e2=e1[j+k] if e1
        printf("tab[%i]=%s, tab[%i][%i]=%s\n", i+k+l, e1.inspect,
i+k+l, j+k, e2.inspect)
        tmp <<= tab[i+k+l][j+k]
      }
   end
   1 if tmp.sort == Val
end
--------

you'll get a printout like this
------
input: i=6, j=0, tab(length=9)=[[1, 2, 3, ...]....]
tab[6]=[1, 2, 3, 4, 5, 6, 7, 8, 9], tab[6][0]=1
tab[7]=[1, 2, 3, 4, 5, 6, 7, 8, 9], tab[7][0]=1
tab[8]=[1, 2, 3, 4, 5, 6, 7, 8, 9], tab[8][0]=1
tab[7]=[1, 2, 3, 4, 5, 6, 7, 8, 9], tab[7][1]=2
tab[8]=[1, 2, 3, 4, 5, 6, 7, 8, 9], tab[8][1]=2
tab[9]=nil, tab[9][1]=nil
NoMethodError: undefined method `[]' for nil:NilClass
-----

and it's easy to see that the nil error comes from calling tab[9] on
an array of length 9.

Good luck with your future debugging :)
Tom R. (Guest)
on 2005-12-14 17:30
(Received via mailing list)
You could also define [] for the nil class and have it return whatever
you thought appropriate.  For example:

class NilClass
    def  []
       nil  #or whatever you want returned
    end
end
Jacob F. (Guest)
on 2005-12-14 19:18
(Received via mailing list)
On 12/14/05, Tom R. <removed_email_address@domain.invalid> wrote:
> You could also define [] for the nil class and have it return whatever
> you thought appropriate.  For example:
>
> class NilClass
>     def  []
>        nil  #or whatever you want returned
>     end
> end

No, no, NO! See, this situation is exactly why I *wouldn't* want
Ruby's nil to follow the NilObject pattern (or whatever it's called).
If it did, Mark would be getting wrong results as opposed to the
undefined method exception. Which do you think is easier to debug?
I'll take the exception that tells me what line number, any day.

Jacob F.
Damphyr (Guest)
on 2005-12-19 02:56
(Received via mailing list)
Jacob F. wrote:
>
>
> No, no, NO! See, this situation is exactly why I *wouldn't* want
> Ruby's nil to follow the NilObject pattern (or whatever it's called).
> If it did, Mark would be getting wrong results as opposed to the
> undefined method exception. Which do you think is easier to debug?
> I'll take the exception that tells me what line number, any day.
>
I'll second that and quote Dave and Andy:
"Crash Early"
V.-

--
http://www.braveworld.net/riva
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