Lists & two dimensions

[Gregor Lingl]

Pichai Asokan schrieb:
> 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 ...

if you got a list of 9 elements the expression
board[1][2] = 'x'
wouldn't make sense.

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

What you get is a list of three identical elements (i. e.
three pointers to the same three element list)

like:

 >>> l = ['a','b','c']
 >>> v = [l]*3
 >>> v[1][2]='x'
 >>> v
[['a', 'b', 'x'], ['a', 'b', 'x'], ['a', 'b', 'x']]
 >>>

a possible workaround:

 >>> v = ['a','b','c']
 >>> v2 = []
 >>> for i in range(3):
	v2.append(v[:])  # copy of v

	
 >>> v2
[['a', 'b', 'c'], ['a', 'b', 'c'], ['a', 'b', 'c']]
 >>> v2[2][0]='z'
 >>> v2
[['a', 'b', 'c'], ['a', 'b', 'c'], ['z', 'b', 'c']]
 >>>

I'm sure there ar more elegant ways ...

Gregor




> 
> P Asokan