From: dblack@...
Date: 2005-12-14T12:51:08+09:00
Subject: Re: Undefined method

Hi --

On Wed, 14 Dec 2005, Mark Liang wrote:

> 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

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.

>     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)

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.

>     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)

Discover the joys of 'puts' :-)


David

-- 
David A. Black
dblack@wobblini.net

"Ruby for Rails", forthcoming from Manning Publications, April 2006!