[Tutor] looping problem

[Kent Johnson]

John Fouhy wrote:
> On 24/09/06, Python <python at venix.com> wrote:
>> slices may be the best way to go
>> listA = biglist[0::3]   # start from index 0 taking every third element
>> listB = biglist[2::3]   # start from index 2 taking every third element
> 
> I'm not certain they would be.. If you do that, you will:
> 
> 1. Create a really big list.
> 2. Go through the list, taking every third element.
> 3. Go through the list again, taking every third+2 element.
> 
> If the list is really big, step 1. might take some time and/or space,
> and you would like to avoid it.

That's a good point, though the OP didn't seem to have a problem with 
memory.
> 
> If we have:
> 
>     f2= open('myfile','r')
>     listA = []
>     listB = []
> 
> then we can iterate through f2 as follows:
> 
>     for i, line in enumerate(f2):
>         if i % 3 == 0 then
>             listA.append(line)
>         elif i % 3 == 2 then
>             listB.append(line)
> 
> This may be faster..
> (although I should like to see evidence before committing to that
> statement :-) )

Since the end goal seems to be to create a dictionary, there is really 
no need to create the intermediate lists at all. You could do something 
like this (following your file example):

d = {}
while 1:
   try:
     key, _, value = f2.next(), f2.next(), f2.next()
     d[key] = value
   except StopIteration:
     pass

To do this with a list instead of a file use f2=iter(reallyBigList).

Kent