Canonical way of deleting elements from lists
Hello,
From a list of strings I want to delete all empty ones. This works:
while '' in keywords: keywords.remove('')
However, to a long-term C programmer this looks like an awkward way of
accomplishing a simple goal, because the list will have to be re-evaluated
in each iteration. Is there a way to just walk the list once and throw out
unwanted elements as one goes along?
I started programming back when such little things were real performance
issues, so I have some sort of cringe reflex when something looks
inefficient.
robert
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Re: Canonical way of deleting elements from lists
Robert Latest wrote:
From a list of strings I want to delete all empty ones. This works:
while '' in keywords: keywords.remove('')
However, to a long-term C programmer this looks like an awkward way of
accomplishing a simple goal, because the list will have to be re-evaluated
in each iteration.
you're using a quadratic algorihm (in is a linear search, and remove
has to move everything around for each call), and you're worried about
the time it takes Python to fetch a variable?
Is there a way to just walk the list once and throw out unwanted
elements as one goes along?
creating a new list is always almost the right way to do things like
this. in this specific case, filter() or list comprehensions are good
choices:
keywords = filter(None, keywords) # get true items only
keywords = [k for k in keywords if k]
also see:
http://effbot.org/zone/python-list.htm#modifying
/F
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Re: Canonical way of deleting elements from lists
Fredrik Lundh wrote: creating a new list is always almost the right way to do things like message = message.replace(always almost, almost always) -- http://mail.python.org/mailman/listinfo/python-list
Re: Canonical way of deleting elements from lists
Robert Latest [EMAIL PROTECTED] writes:
From a list of strings I want to delete all empty ones. This works:
while '' in keywords: keywords.remove('')
If you're looking for a quick (no quadratic behavior) and convenient
way to do it, you can do it like this:
keywords = [s for s in keywords if s != '']
But that creates a new list, which might not be wanted for long lists
with few empty elements (or for shared lists). It also iterates over
every list element in a Python loop, which can take some time for long
lists.
Is there a way to just walk the list once and throw out unwanted
elements as one goes along?
I'd do it like this:
i = 0
while 1:
try:
i = keywords.index('', i)
except ValueError:
break
del keywords[i]
Or even:
try:
i = 0
while 1:
i = keywords.index('', i) # throws ValueError and stops the loop
del keywords[i]
except ValueError:
pass
In both cases the search for the empty string is done in efficient C
code, and you only loop in Python over the actual matches.
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Re: Canonical way of deleting elements from lists
Hrvoje Niksic [EMAIL PROTECTED] writes: If you're looking for a quick (no quadratic behavior) and convenient way to do it, you can do it like this: keywords = [s for s in keywords if s != ''] It now occurred to me that a good compromise between convenience and efficiency that retains the same list is: keywords[:] = (s for s in keywords if s) -- http://mail.python.org/mailman/listinfo/python-list
Re: Canonical way of deleting elements from lists
Fredrik Lundh wrote: keywords = filter(None, keywords) # get true items only Makes seinse. BTW, where can I find all methods of the built-in types? Section 3.6 only talks about strings and mentions the list append() method only in an example. Am I too stupid to read the manual, or is this an omission? robert -- http://mail.python.org/mailman/listinfo/python-list
Re: Canonical way of deleting elements from lists
Hrvoje Niksic wrote: keywords[:] = (s for s in keywords if s) Looks good but is so far beyond my own comprehension that I don't dare include it in my code ;-) robert -- http://mail.python.org/mailman/listinfo/python-list
Re: Canonical way of deleting elements from lists
Robert Latest: Fredrik Lundh wrote: keywords = filter(None, keywords) # get true items only Makes seinse. BTW, where can I find all methods of the built-in types? Section 3.6 only talks about strings and mentions the list append() method only in an example. Am I too stupid to read the manual, or is this an omission? filter isn't a method of list, it's a free function. For most situations that solution with filter(None,keywords) is good enough (but it filters away all false items, so it filters away None, 0, empty things, etc, too). The following one is an in-place version (it may be faster with Psyco), but you usually don't need it: def inplacefilter(pred, alist): inplacefilter(pred, alist): filters the given list like filter(), but works inplace, minimizing the used memory. It returns None. pr = lambda x: x 2 l = [] inplacefilter(pr, l) l [] l = [1,2,2] inplacefilter(pr, l) l [] l = [3] inplacefilter(pr, l) l [3] l = [1,2,3,1,5,1,6,0] r = filter(pr, l) # normal filter r [3, 5, 6] inplacefilter(pr, l) r == l True slow = 0 for fast, item in enumerate(alist): if pred(item): if slow != fast: alist[slow] = alist[fast] slow += 1 del alist[slow:] You may want to avoid the enumerate() too if you use Psyco and you need max speed. Bye, bearophile -- http://mail.python.org/mailman/listinfo/python-list
Re: Canonical way of deleting elements from lists
Robert Latest [EMAIL PROTECTED] wrote: Hrvoje Niksic wrote: keywords[:] = (s for s in keywords if s) Looks good but is so far beyond my own comprehension that I don't dare include it in my code ;-) :-) Worth understanding thought I think - here are some hints keywords[:] = (s for s in keywords if s) is equivalent to this (but without creating a temporary list) keywords[:] = list(s for s in keywords if s) which is equivalent to keywords[:] = [s for s in keywords if s] This keywords[:] = Replaces the contents of the keywords list rather than making a new list. Here is a demonstration of the fundamental difference a=[1,2,3,4] b=a a=[5,6,7] print a, b [5, 6, 7] [1, 2, 3, 4] a=[1,2,3,4] b=a a[:]=[5,6,7] print a, b [5, 6, 7] [5, 6, 7] Using keywords[:] stops the creation of another temporary list. The other behaviour may or may not be what you want! -- Nick Craig-Wood [EMAIL PROTECTED] -- http://www.craig-wood.com/nick -- http://mail.python.org/mailman/listinfo/python-list
Re: Canonical way of deleting elements from lists
Nick Craig-Wood wrote: Using keywords[:] stops the creation of another temporary list. in CPython, list[:] = iter actually creates a temporary list object on the inside, in case iter isn't already a list or a tuple. (see the implementation of PySequence_Fast() for details). /F -- http://mail.python.org/mailman/listinfo/python-list
Re: Canonical way of deleting elements from lists
Robert Latest [EMAIL PROTECTED] wrote: BTW, where can I find all methods of the built-in types? Section 3.6 only talks about strings and mentions the list append() method only in an example. Am I too stupid to read the manual, or is this an omission? 3.6 talks about features common to all sequence types. Strings are discussed specifically in 3.6.1 (String Methods). Lists are similarly discussed in 3.6.4 (Mutable Sequence Types). They are certainly not omitted, although maybe the title of 3.6.4 could be take a leaf from the Zen and be more explicit. -- \S -- [EMAIL PROTECTED] -- http://www.chaos.org.uk/~sion/ Frankly I have no feelings towards penguins one way or the other -- Arthur C. Clarke her nu becomeþ se bera eadward ofdun hlæddre heafdes bæce bump bump bump -- http://mail.python.org/mailman/listinfo/python-list
Re: Canonical way of deleting elements from lists
Fredrik Lundh [EMAIL PROTECTED] wrote: Nick Craig-Wood wrote: Using keywords[:] stops the creation of another temporary list. in CPython, list[:] = iter actually creates a temporary list object on the inside, in case iter isn't already a list or a tuple. (see the implementation of PySequence_Fast() for details). Ah, OK, thanks for the correction! -- Nick Craig-Wood [EMAIL PROTECTED] -- http://www.craig-wood.com/nick -- http://mail.python.org/mailman/listinfo/python-list
Re: Canonical way of deleting elements from lists
Sion Arrowsmith wrote: Robert Latest [EMAIL PROTECTED] wrote: BTW, where can I find all methods of the built-in types? Section 3.6 only talks about strings and mentions the list append() method only in an example. Am I too stupid to read the manual, or is this an omission? 3.6 talks about features common to all sequence types. Strings are discussed specifically in 3.6.1 (String Methods). Lists are similarly discussed in 3.6.4 (Mutable Sequence Types). OK, the latter then. Too stupid. Thanks ;-) robert -- http://mail.python.org/mailman/listinfo/python-list
