Declare list of large size

[David Eppstein]

In article <mailman.1016495070.23614.python-list at python.org>,
 "Delaney, Timothy" <tdelaney at avaya.com> wrote:

> > list = [0]*100
> 
> But note that this is only safe for immutable elements. You will obtain a
> reference to the *same* object as each element.
> 
> This normally bites people when they try to make multi-dimensional arrays
> ...
> 
> list = [[0] * 2] * 2
> print list
> list[0][0] = 1
> print list
> 
> [[0, 0], [0, 0]]
> [[1, 0], [1, 0]]
> 
> As you can see, the first element of each sub-list has been changed - this
> is because each sub-list is in fact the same list!

I got bitten by that, once.  It took very little time to learn not to do 
that.

So, is it considered more Pythonic to create a multidimensional array by
    list = [[0]*100 for i in range(100)]
and then access it by
    ...list[i][j]...

or is it more typical to just use dictionaries?
    list = {}
    ...list[i,j]...
?

I can see advantages either way -- the dictionary method is simpler, 
more space-efficient if you don't intend to fill the whole thing, and 
makes it easy to pass around and use tuples representing indexes into 
the array.  But the list-of-lists more explicitly states what you're 
doing, may be slightly faster (I haven't checked), gives you bounds 
checking,and more generally doesn't let you index by anything but a 
tuple of integers.

-- 
David Eppstein       UC Irvine Dept. of Information & Computer Science
eppstein at ics.uci.edu http://www.ics.uci.edu/~eppstein/