[Tutor] 2d list index inverting?

[Alex Hall]

On 5/25/10, Hugo Arts <hugo.yoshi at gmail.com> wrote:
> On Wed, May 26, 2010 at 3:13 AM, Alex Hall <mehgcap at gmail.com> wrote:
>> Hello all,
>> I have a 2d list being used for a battleship game. I have structured
>> the program so that it uses a grid class, which implements this array
>> along with a bunch of other methods and vars. For example, to get at
>> the top left square, you would say:
>> Grid.getSquareAt(0,0)
>> and inside getSquareAt is simply:
>>  def getSquareAt(self, x, y):
>>  return self.b[x][y] #b is the internal 2d list for the class
>>
>> However, I am getting very confused with indexing. I keep getting
>> errors about list index out of range and I am not sure why. I have a
>> feeling that using 2d lists is supposed to go like a matrix
>> (row,column) and not like a coordinate plane (column, row).
>
> A 2D list doesn't really exist. What you're using is just a list whose
> elements are also lists. A nested data structure. And whether those
> sub-lists should be the rows or the columns? It doesn't matter. A list
> is just a list. Sequential data elements. It doesn't care whether it
> represents a row or a column. What are 'row' and 'column' anyway? just
> words designating some arbitrary notion. Conventions. You can swap one
> for the other, and the data remains accessible. As long as you're
> consistent, there's no problem.
I thought so, but I was hoping you would not say that as this means a
logic bug deep in my code, and those are the hardest to track down...
>
> The real problem is something else entirely. Somewhere in your code,
> you are using an index that is greater than the size of the list.
Yes, and it looks like my coordinates are bing reversed somewhere, but
I cannot find anywhere where that is happening in the code.
> Perhaps you're not consistent, somewhere. Mixing up your row/column
> order. Perhaps something else is amiss. No way to tell from the
> snippet.
So, a lot of print() statements then...
>
> Hugo
>


-- 
Have a great day,
Alex (msg sent from GMail website)
mehgcap at gmail.com; http://www.facebook.com/mehgcap