Lists & two dimensions

[Chris Gonnerman]

----- Original Message ----- 
From: "Pichai Asokan" <asokanp at virtusa.com>


> I am a trainer and in a training session a participant came up with a
> problem that stumped me:
> 
> Here is the code snippet that isolates what I want:
> --------------------
> board = [[-1] * 3 ]*3
> print board
> board[1][2] = 'x'
> print board
> --------------------
> Output
> -----------------------
> [ [-1, -1, -1 ], [-1, -1, -1 ], [-1, -1, -1 ] ]
> [ [-1, -1, 'x'], [-1, -1, 'x'], [-1, -1, 'x'] ]
> -----------------------
> 
> I thought 
> board = [[-1] * 3 ]*3
> should give me a list of 9 elements;
> but ...
> 
> What is going on?

Multiplication of a list by N results in N references to the
same list.  The inner list contains (references) scalars, so
each slot is effectively independent, but the outer list 
contains lists... so there you go.

> Where can we read more to understand what is goingg on?

This is a FAQ I think; go to www.python.org and take a look
there.  (Or someone else may post the pointer if you are
lucky).

Chris Gonnerman -- chris.gonnerman at newcenturycomputers.net
http://newcenturycomputers.net