How do I truncate a list?

If I have a list and want to truncate it so it is no more than 100 items, ho开发者_开发问答w do I do this?

To modify the list in place (rather than make a shorter copy of the list), use:

del l[100:]

You can use list slicing:

a = a[0:100]

The items[:100] other mentioned gives you a new list which contains the first 100 items of items. If you want to modify the list in-place, either use items[:] = items[:100] (slice assignment) or while len(items) > 100: items.pop()use del items[100:] as proposed by Ned Batchelder.

The correct answer is, of course:

>>> x = range(10)
>>> x
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> x = x[:5]
>>> x
[0, 1, 2, 3, 4]

But, importantly, if you're interested in the values above 100 and want to pull them off one by one for whatever reason, you can also use POP()

>>> x = range(10)
>>> x
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> x.pop()
9
>>> x
[0, 1, 2, 3, 4, 5, 6, 7, 8]

You can even specify which element to pull out:

>>> x= range(10,20)
>>> x
[10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
>>> x.pop(3)
13
>>> x.pop()
19
>>> x
[10, 11, 12, 14, 15, 16, 17, 18]
>>> x.pop(-1)
18
[10, 11, 12, 14, 15, 16, 17]

This removes individual elements, shortening the list, without copying.

So, an obtuse and yucky answer (but also correct) would be to iterate down. I'm only going down from 12 to 8 for ease of reading here:

>>> x=range(12)
>>> for i in range(len(x), 8, -1):
...     y = x.pop()
...     print "popping x: ", y, ", x=", x
...
popping x:  11 , x= [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
popping x:  10 , x= [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
popping x:  9 , x= [0, 1, 2, 3, 4, 5, 6, 7, 8]
popping x:  8 , x= [0, 1, 2, 3, 4, 5, 6, 7]

Sure, it's not optimal, but I just ran into a situation where I needed this so I thought I'd share it here (I'm truncating a list when I see the first not-None value).

You can do something like:

truncated = list[:100]

You can use slicing if you don't mind just simply creating a new copy of the list that contains only the elements you want... however this leaves the original list unmodified.

>>> a = [0,1,2,3,4,5,6,7,8,9]
>>> b = a[0:5]
>>> a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> b
[0, 1, 2, 3, 4]

If you really want to truncate the original list, just delete the elements you don't want by using slicing with del

>>> del a[5:]
>>> a
[0, 1, 2, 3, 4]