[Tutor] Skipping elements from inside generator loops

[alan.gauld@bt.com]

> ... I find that you can not do this with "normal" for loops:
> 
> ===============================
> for a in range(len(procession)):
>     if procession[a] == 'ALARM!':
>         a = a+1
>     else:
>         print procession[a]
> ===============================

This has nothing to do with the processing of the 
list, just how the value of a is assigned.

for a in range(len...) creates a line like:

for a in [0,1,2,3,...,<lenth-1>]

your a = a+1 changes a inside the loop but when it 
comes round to the beginning a is set to the next 
value in the list regardless of what you have set 
it to.

Instead try deleting the a+1 element from the list,
that will be closer. The only snag there is that 
you will now run off the end of the list because it 
doesn't correspond to the original length! Bad idea.

Alternative technique is to use a while loop and 
manually maintain the list length. generators etc 
provide a nicer way of dealing with these kinds of 
issue.

Alan G