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Help, multidimensional list

Terry Reedy tjreedy at udel.edu
Sun Dec 28 17:50:07 EST 2003 
"Christopher Koppler" <klapotec at chello.at> wrote in message
news:jbrtuvg9fh5e0hdj0d6vseo7287r94sa7d at 4ax.com...
> On Sun, 28 Dec 2003 14:27:41 +0000, Crawley
> <crawley.storm at IGetTooMuchSpamAsItIs.com> wrote:
>
> >Im trying to create a list of lists and for some reason its not working:
> >
> >class Tile:
> >    _next = { 'n':None, 'ne':None, 'e':None, 'se':None, 's':None,
> >'sw':None, 'w':None, 'nw':None }
> >
> >
> >def blankGridCanvas( maxx, maxy ):
> >    grid = []
> >    for y in xrange( 0, maxy ):
> >        yline = []
> >        for x in xrange( 0, maxx ):
> >            t = Tile()
> >            if y != 0:
> >                t._next[ 'n' ] = grid[ y - 1 ][ x ]
> >                t._next[ 'n' ]._next[ 's' ] = t
> >
> >            yline.append( t )
> >        grid.append( yline )
> >
> >    for y in xrange( 0, maxy ):
> >        for x in xrange( 0, maxx ):
> >            print grid[ x ][ y ], grid[ x ][ y ]._next
> >    return grid
> >
> >now by my reconing this should be a list of lists with each element
being
> >an instance of Tile,
>
> You only create a *class* Tile, and provide no way to create
> *instances* of it

No, the line 't=Tile()' does create a separate Tile for each grid position.
However, there is only one class attribute shared by all instances.

> - so much like with static variables in other
> languages, you only have one 'instance' here. To be able to create
> separate instances, you need a constructor in your class to
> instantiate every instance, like so:
>
> class Tile:
>     def __init__(self):
>         self._next = { 'n':None, 'ne':None, 'e':None, 'se':None,
>                        's':None, 'sw':None, 'w':None, 'nw':None }

You do not need __init__ for separate instances, but do need it to give
each instance  its own map.

>  and some references being manipulated to point to each
> >other.  But strangly no, its actually a list of lists where each element
is
> >the SAME instance of Tile, so I change one, i change them ALL!

As stated above, you do have separate instances but only one map attached
to the class instead of a separate map for each instance. Koppler's
__init__ will fix this.

Terry J. Reedy
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