[Tutor] removing items from a list

[alan.gauld@bt.com]

> This doesn't work, I'm not sure why.  Mabye item isn't a 
> refence to the list item, but just has the same value.
> 
> for item in list:
> 	if item == data: del item

This deletes item but the list still has a reference to it.
ie item no longer points at the object but the list does!

> This works, its ok, but still seems like there should be something
> cleaner if not faster.

Actually it doesn't quite work!

> i=0
> for item in list:
> 	if item == data:
> 		del list[i]
> 	i=i+1

Two adjacent items that equal data will only result in 
one being deleted:

a
b <---- item
b
c

becomes

a
b
c <----- item after an iteration of the loop with item = b.

You really need to use a while loop to do this manually:

i = 0
while i < len(list):
   if list[i] == data: del list[i]
   else: i += 1


A better solution is to use the filter() function or a list 
comprehension

list = filter(lambda x: x != data, list)

list = [item for item in list if item != data]

filter is designed to filter out items that match the test 
function. comprehensions are designed to look a little 
like set notation in math....

Be aware that the performance may not be much better than 
the loops, but they look nicer...

HTH,

Alan G.