A couple of quick questions

[Doug Hellmann]

cbarker at jps.net wrote:
> 
> Hi,
> 
> I've been using Python for a few months now, and I have a couple of
> things I havn't figured out how to do, that would be handy
> 
> 1) loop a known number of times, without creating a list of indexes:
> 
> for i in range(N):
>         # do something
> 
> works just fine, unless N is very large. if it's large, it wastes
> memory, if it's very large it can be impossible!

Check out xrange().  It creates a thing that behaves like a sequence,
but doesn't actually instantiate all of the members of the sequence like
range() does.

> 2) Is there a "case" construct?
> 
> I can use if...elif...elif...elif...else, but it's once, again, not
> very elegant.

If you have one variable which controls a switch, you could create a
dispatch table using a dictionary.

dispatch = {
 'one':func_one,
 'two':func_two,
}

func_to_call = dispatch[ switch_variable ]
func_to_call()

On the other hand, if your code doesn't break down neatly into separate
functions, that isn't always cleaner than a large if/elif/else.

> 3) lists of indexes:
> 
> let's say I have a list of the indexes of another list that I want. Is
> there a way to return the corresponding elements without a loop?
> 
> example:
> 
> list = [1,2,3,4,5,6,7,8,9,10,11]
> indexes = [3,5,9]
> new_list = []
> for i in indexes:
>         new_list.append(list[i])
> 
> This just isn't elegant (or fast!). What I want to be able to do is
> something like:
> 
> new_list = list[indexes]
> 
> Is there a way to do this?

Why not store references to the objects in the second list, instead of
the indexes?  Let the reference counter do its job.

Doug