Undefined method


#1

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:inblkcheck’
from ./creat.rb:24:in times' from ./creat.rb:24:inblkcheck’
from ./creat.rb:33:in blkscheck' from ./creat.rb:32:ineach’
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)

#2

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’ :slight_smile:

David


David A. Black
removed_email_address@domain.invalid

“Ruby for Rails”, forthcoming from Manning Publications, April 2006!


#3

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 :slight_smile:


#4

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


#5

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.


#6

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