Re: Need help porting Perl function
2008/6/7, kj <[EMAIL PROTECTED]>: > > > > ... The original Perl function takes a reference to an array, removes > from this array all the elements that satisfy a particular criterion, > > and returns the list consisting of the removed elements. ... > > > > Just out of curiosity, as I didn't find any mention of filter() in the suggestions in this thread, I'd like to ask, whether there are some reasons, why not to use it (or the itertools equivalent); actually it seems to me to be the simplest way to acomplish the mentioned task (as the solutions not modifying the original list were proposed too). e.g.: >>> lst=range(14) >>> odd = filter(lambda x: x % 2, lst) >>> even = filter(lambda x: not x % 2, lst) >>> lst, odd, even (or a version using itertools.ifilter, ifilterfalse) Are there maybe some significant performance disadvantages in the two subsequent implied loops over the input list? Regards, Vlastimil Brom -- http://mail.python.org/mailman/listinfo/python-list
Re: Need help porting Perl function
In <[EMAIL PROTECTED]> kj <[EMAIL PROTECTED]> writes: >In <[EMAIL PROTECTED]> John Machin <[EMAIL PROTECTED]> writes: >>It's nothing to do with list comprehensions, which are syntactical >>sugar for traditional loops. You could rewrite your list comprehension >>in the traditional manner... >>and it would still fail for the same reason: mutating the list over >>which you are iterating. >I normally deal with this problem by iterating backwards over the >indices. Here's how I coded the function (in "Python-greenhorn >style"): ***BLUSH*** Somehow I missed that John Machin had posted almost the exact same implementation that I posted in my reply to him. Reading too fast. Reading too fast. Kynn -- NOTE: In my address everything before the first period is backwards; and the last period, and everything after it, should be discarded. -- http://mail.python.org/mailman/listinfo/python-list
Re: Need help porting Perl function
In <[EMAIL PROTECTED]> John Machin <[EMAIL PROTECTED]> writes: >It's nothing to do with list comprehensions, which are syntactical >sugar for traditional loops. You could rewrite your list comprehension >in the traditional manner... >and it would still fail for the same reason: mutating the list over >which you are iterating. I normally deal with this problem by iterating backwards over the indices. Here's how I coded the function (in "Python-greenhorn style"): def cull(list): culled = [] for i in range(len(list) - 1, -1, -1): if not_wanted(list[i]): culled.append(list.pop(i)) return culled ...where not_wanted() is defined elsewhere. (For my purposes at the moment, the greater generality provided by making not_wanted() a parameter to cull() was not necessary.) The specification of the indices at the beginning of the for-loop looks pretty ignorant, but aside from that I'm happy with it. Kynn -- NOTE: In my address everything before the first period is backwards; and the last period, and everything after it, should be discarded. -- http://mail.python.org/mailman/listinfo/python-list
Re: Need help porting Perl function
On Jun 8, 8:17 am, [EMAIL PROTECTED] wrote: > On Jun 7, 5:56 pm, John Machin <[EMAIL PROTECTED]> wrote: > > > > > On Jun 8, 6:05 am, [EMAIL PROTECTED] wrote: > > > > On Jun 7, 2:42 pm, "Daniel Fetchinson" <[EMAIL PROTECTED]> > > > wrote: > > > > > > Hi. I'd like to port a Perl function that does something I don't > > > > > know how to do in Python. (In fact, it may even be something that > > > > > is distinctly un-Pythonic!) > > > > > > The original Perl function takes a reference to an array, removes > > > > > from this array all the elements that satisfy a particular criterion, > > > > > and returns the list consisting of the removed elements. Hence > > > > > this function returns a value *and* has a major side effect, namely > > > > > the target array of the original argument will be modified (this > > > > > is the part I suspect may be un-Pythonic). > > > > > > Can a Python function achieve the same effect? If not, how would > > > > > one code a similar functionality in Python? Basically the problem > > > > > is to split one list into two according to some criterion. > > > > > This function will take a list of integers and modify it in place such > > > > that it removes even integers. The removed integers are returned as a > > > > new list (disclaimer: I'm 100% sure it can be done better, more > > > > optimized, etc, etc): > > > > > def mod( alist ): > > > > old = alist[:] > > > > ret = [ ] > > > > for i in old: > > > > if i % 2 == 0: > > > > ret.append( alist.pop( alist.index( i ) ) ) > > > > > return ret > > > > > x = range(10,20) > > > > > print x > > > > r = mod( x ) > > > > print r > > > > print x > > > > > HTH, > > > > Daniel > > > > -- > > > > Psss, psss, put it down! -http://www.cafepress.com/putitdown > > > > def mod( alist ): > > > return [ alist.pop( alist.index( x ) ) for x in alist if x % 2 == > > > 0 ] > > > > alist = range(10,20) > > > blist = mod( alist ) > > > > print alist > > > print blist > > > > The same thing with list comprehensions. > > > Not the same. The original responder was careful not to iterate over > > the list which he was mutating. > > > >>> def mod(alist): > > > ...return [alist.pop(alist.index(x)) for x in alist if x % 2 == 0] > > ...>>> a = range(10) > > >>> print mod(a), a > > > [0, 2, 4, 6, 8] [1, 3, 5, 7, 9]>>> a = [2,2,2,2,2,2,2,2] > > >>> print mod(a), a > > > [2, 2, 2, 2] [2, 2, 2, 2] > > # should be [2, 2, 2, 2, 2, 2, 2, 2] [] > > Alas, it appears my understanding of list comprehensions is > significantly less comprehensive than I thought =) It's nothing to do with list comprehensions, which are syntactical sugar for traditional loops. You could rewrite your list comprehension in the traditional manner: def mod(alist): ret = [] for x in alist: if x % 2 == 0: ret.append(alist.pop(alist.index(x)) return ret and it would still fail for the same reason: mutating the list over which you are iterating. At the expense of even more time and memory you can do an easy fix: change 'for x in alist' to 'for x in alist[:]' so that you are iterating over a copy. Alternatively, go back to basics: >>> def modr(alist): ...ret = [] ...for i in xrange(len(alist) - 1, -1, -1): ... if alist[i] % 2 == 0: ... ret.append(alist[i]) ... del alist[i] ...return ret ... >>> a = [2,2,2,2,2,2,2,2,2] >>> print modr(a), a [2, 2, 2, 2, 2, 2, 2, 2, 2] [] >>> HTH, John -- http://mail.python.org/mailman/listinfo/python-list
Re: Need help porting Perl function
On Jun 7, 5:56 pm, John Machin <[EMAIL PROTECTED]> wrote: > On Jun 8, 6:05 am, [EMAIL PROTECTED] wrote: > > > > > On Jun 7, 2:42 pm, "Daniel Fetchinson" <[EMAIL PROTECTED]> > > wrote: > > > > > Hi. I'd like to port a Perl function that does something I don't > > > > know how to do in Python. (In fact, it may even be something that > > > > is distinctly un-Pythonic!) > > > > > The original Perl function takes a reference to an array, removes > > > > from this array all the elements that satisfy a particular criterion, > > > > and returns the list consisting of the removed elements. Hence > > > > this function returns a value *and* has a major side effect, namely > > > > the target array of the original argument will be modified (this > > > > is the part I suspect may be un-Pythonic). > > > > > Can a Python function achieve the same effect? If not, how would > > > > one code a similar functionality in Python? Basically the problem > > > > is to split one list into two according to some criterion. > > > > This function will take a list of integers and modify it in place such > > > that it removes even integers. The removed integers are returned as a > > > new list (disclaimer: I'm 100% sure it can be done better, more > > > optimized, etc, etc): > > > > def mod( alist ): > > > old = alist[:] > > > ret = [ ] > > > for i in old: > > > if i % 2 == 0: > > > ret.append( alist.pop( alist.index( i ) ) ) > > > > return ret > > > > x = range(10,20) > > > > print x > > > r = mod( x ) > > > print r > > > print x > > > > HTH, > > > Daniel > > > -- > > > Psss, psss, put it down! -http://www.cafepress.com/putitdown > > > def mod( alist ): > > return [ alist.pop( alist.index( x ) ) for x in alist if x % 2 == > > 0 ] > > > alist = range(10,20) > > blist = mod( alist ) > > > print alist > > print blist > > > The same thing with list comprehensions. > > Not the same. The original responder was careful not to iterate over > the list which he was mutating. > > >>> def mod(alist): > > ... return [alist.pop(alist.index(x)) for x in alist if x % 2 == 0] > ...>>> a = range(10) > >>> print mod(a), a > > [0, 2, 4, 6, 8] [1, 3, 5, 7, 9]>>> a = [2,2,2,2,2,2,2,2] > >>> print mod(a), a > > [2, 2, 2, 2] [2, 2, 2, 2] > # should be [2, 2, 2, 2, 2, 2, 2, 2] [] Alas, it appears my understanding of list comprehensions is significantly less comprehensive than I thought =) -- http://mail.python.org/mailman/listinfo/python-list
Re: Need help porting Perl function
On Jun 8, 6:05 am, [EMAIL PROTECTED] wrote: > On Jun 7, 2:42 pm, "Daniel Fetchinson" <[EMAIL PROTECTED]> > wrote: > > > > > > Hi. I'd like to port a Perl function that does something I don't > > > know how to do in Python. (In fact, it may even be something that > > > is distinctly un-Pythonic!) > > > > The original Perl function takes a reference to an array, removes > > > from this array all the elements that satisfy a particular criterion, > > > and returns the list consisting of the removed elements. Hence > > > this function returns a value *and* has a major side effect, namely > > > the target array of the original argument will be modified (this > > > is the part I suspect may be un-Pythonic). > > > > Can a Python function achieve the same effect? If not, how would > > > one code a similar functionality in Python? Basically the problem > > > is to split one list into two according to some criterion. > > > This function will take a list of integers and modify it in place such > > that it removes even integers. The removed integers are returned as a > > new list (disclaimer: I'm 100% sure it can be done better, more > > optimized, etc, etc): > > > def mod( alist ): > > old = alist[:] > > ret = [ ] > > for i in old: > > if i % 2 == 0: > > ret.append( alist.pop( alist.index( i ) ) ) > > > return ret > > > x = range(10,20) > > > print x > > r = mod( x ) > > print r > > print x > > > HTH, > > Daniel > > -- > > Psss, psss, put it down! -http://www.cafepress.com/putitdown > > def mod( alist ): > return [ alist.pop( alist.index( x ) ) for x in alist if x % 2 == > 0 ] > > alist = range(10,20) > blist = mod( alist ) > > print alist > print blist > > The same thing with list comprehensions. Not the same. The original responder was careful not to iterate over the list which he was mutating. >>> def mod(alist): ...return [alist.pop(alist.index(x)) for x in alist if x % 2 == 0] ... >>> a = range(10) >>> print mod(a), a [0, 2, 4, 6, 8] [1, 3, 5, 7, 9] >>> a = [2,2,2,2,2,2,2,2] >>> print mod(a), a [2, 2, 2, 2] [2, 2, 2, 2] # should be [2, 2, 2, 2, 2, 2, 2, 2] [] -- http://mail.python.org/mailman/listinfo/python-list
Re: Need help porting Perl function
On Jun 7, 1:24 pm, kj <[EMAIL PROTECTED]> wrote:
> The original Perl function takes a reference to an array, removes
> from this array all the elements that satisfy a particular criterion,
> and returns the list consisting of the removed elements. Hence
> this function returns a value *and* has a major side effect, namely
> the target array of the original argument will be modified (this
> is the part I suspect may be un-Pythonic).
>
> Can a Python function achieve the same effect? If not, how would
> one code a similar functionality in Python? Basically the problem
> is to split one list into two according to some criterion.
>
If you want to avoid side-effects completely, return two lists, the
list of matches and the list of non-matches. In this example,
partition creates two lists, and stores all matches in the first list,
and mismatches in the second. (partition assigns to the associated
element of retlists based on False evaluating to 0 and True evaluating
to 1.)
def partition(lst, ifcond):
retlists = ([],[])
for i in lst:
retlists[ifcond(i)].append(i)
return retlists[True],retlists[False]
hasLeadingVowel = lambda x: x[0].upper() in "AEIOU"
matched,unmatched = partition("The quick brown fox jumps over the lazy
indolent dog".split(),
hasLeadingVowel)
print matched
print unmatched
prints:
['over', 'indolent']
['The', 'quick', 'brown', 'fox', 'jumps', 'the', 'lazy', 'dog']
-- Paul
--
http://mail.python.org/mailman/listinfo/python-list
Re: Need help porting Perl function
kj wrote: Hi. I'd like to port a Perl function that does something I don't know how to do in Python. (In fact, it may even be something that is distinctly un-Pythonic!) The original Perl function takes a reference to an array, removes from this array all the elements that satisfy a particular criterion, and returns the list consisting of the removed elements. Hence this function returns a value *and* has a major side effect, namely the target array of the original argument will be modified (this is the part I suspect may be un-Pythonic). The two solutions thus far use the .index() method, which itself runs in O(n) time. This means that the provided functions run in O(n^2) time, which can be a problem if the list is big. I'd go with this: def partition(alist, criteria): list1, list2 = [], [] for item in alist: if criteria(item): list1.append(item) else: list2.append(item) return (list1, list2) def mod(alist, criteria=lambda x: x % 2 == 0): alist[:], blist = partition(alist, criteria) return blist >>> partition(range(10), lambda x: x % 2 == 0) ([0, 2, 4, 6, 8], [1, 3, 5, 7, 9]) >>> l=range(10) >>> mod(l) [1, 3, 5, 7, 9] >>> l [0, 2, 4, 6, 8] -- http://mail.python.org/mailman/listinfo/python-list
Re: Need help porting Perl function
On Jun 7, 2:42 pm, "Daniel Fetchinson" <[EMAIL PROTECTED]> wrote: > > Hi. I'd like to port a Perl function that does something I don't > > know how to do in Python. (In fact, it may even be something that > > is distinctly un-Pythonic!) > > > The original Perl function takes a reference to an array, removes > > from this array all the elements that satisfy a particular criterion, > > and returns the list consisting of the removed elements. Hence > > this function returns a value *and* has a major side effect, namely > > the target array of the original argument will be modified (this > > is the part I suspect may be un-Pythonic). > > > Can a Python function achieve the same effect? If not, how would > > one code a similar functionality in Python? Basically the problem > > is to split one list into two according to some criterion. > > This function will take a list of integers and modify it in place such > that it removes even integers. The removed integers are returned as a > new list (disclaimer: I'm 100% sure it can be done better, more > optimized, etc, etc): > > def mod( alist ): > old = alist[:] > ret = [ ] > for i in old: > if i % 2 == 0: > ret.append( alist.pop( alist.index( i ) ) ) > > return ret > > x = range(10,20) > > print x > r = mod( x ) > print r > print x > > HTH, > Daniel > -- > Psss, psss, put it down! -http://www.cafepress.com/putitdown def mod( alist ): return [ alist.pop( alist.index( x ) ) for x in alist if x % 2 == 0 ] alist = range(10,20) blist = mod( alist ) print alist print blist The same thing with list comprehensions. -- http://mail.python.org/mailman/listinfo/python-list
Re: Need help porting Perl function
>This function will take a list of integers and modify it in place such >that it removes even integers. The removed integers are returned as a >new list Great! Thanks! kynn -- NOTE: In my address everything before the first period is backwards; and the last period, and everything after it, should be discarded. -- http://mail.python.org/mailman/listinfo/python-list
Re: Need help porting Perl function
> Hi. I'd like to port a Perl function that does something I don't > know how to do in Python. (In fact, it may even be something that > is distinctly un-Pythonic!) > > The original Perl function takes a reference to an array, removes > from this array all the elements that satisfy a particular criterion, > and returns the list consisting of the removed elements. Hence > this function returns a value *and* has a major side effect, namely > the target array of the original argument will be modified (this > is the part I suspect may be un-Pythonic). > > Can a Python function achieve the same effect? If not, how would > one code a similar functionality in Python? Basically the problem > is to split one list into two according to some criterion. This function will take a list of integers and modify it in place such that it removes even integers. The removed integers are returned as a new list (disclaimer: I'm 100% sure it can be done better, more optimized, etc, etc): def mod( alist ): old = alist[:] ret = [ ] for i in old: if i % 2 == 0: ret.append( alist.pop( alist.index( i ) ) ) return ret x = range(10,20) print x r = mod( x ) print r print x HTH, Daniel -- Psss, psss, put it down! - http://www.cafepress.com/putitdown -- http://mail.python.org/mailman/listinfo/python-list
