Re: Iterate from 2nd element of a huge list
Arnaud Delobelle wrote: Em 01-02-2012 01:39, Paulo da Silva escreveu: What is the best way to iterate thru a huge list having the 1st element a different process? I.e.: Nobody mentioned itertools.islice, which can be handy, especially if you weren't interested in the first element of the list: Also, skipping two or seven or ... items is just as easy. The example should be from itertools import islice: for el in islice(mylist, 1, None): process2(el) -- http://mail.python.org/mailman/listinfo/python-list
Re: Iterate from 2nd element of a huge list
Paulo da Silva p_s_d_a_s_i_l_...@netcabo.pt writes: process1(mylist[0]) for el in mylist[1:]: process2(el) This way mylist is almost duplicated, isn't it? I think it's cleanest to use itertools.islice to get the big sublist (not tested): from itertools import islice process1 (mylist[0]) for el in islice(mylist, 1, None): process2 (el) The islice has a small, constant amount of storage overhead instead of duplicating almost the whole list. -- http://mail.python.org/mailman/listinfo/python-list
Re: Iterate from 2nd element of a huge list
On 1 February 2012 08:11, Peter Otten __pete...@web.de wrote: Arnaud Delobelle wrote: The example should be from itertools import islice: for el in islice(mylist, 1, None): process2(el) Oops! -- Arnaud -- http://mail.python.org/mailman/listinfo/python-list
Re: Iterate from 2nd element of a huge list
Paul Rubin, 01.02.2012 10:25: Paulo da Silva writes: process1(mylist[0]) for el in mylist[1:]: process2(el) This way mylist is almost duplicated, isn't it? I think it's cleanest to use itertools.islice to get the big sublist (not tested): from itertools import islice process1 (mylist[0]) for el in islice(mylist, 1, None): process2 (el) The islice has a small, constant amount of storage overhead instead of duplicating almost the whole list. It also has a tiny runtime overhead, though. So, if your code is totally performance critical and you really just want to strip off the first element and then run through all the rest, it may still be better to go the iter() + next() route. python3.3 -m timeit -s 'l=list(range(10))' \ 'it = iter(l); next(it); all(it)' 1000 loops, best of 3: 935 usec per loop python3.3 -m timeit -s 'l=list(range(10))' \ -s 'from itertools import islice' \ 'all(islice(l, 1, None))' 1000 loops, best of 3: 1.63 msec per loop Stefan -- http://mail.python.org/mailman/listinfo/python-list
Re: Iterate from 2nd element of a huge list
Em 01-02-2012 04:55, Cameron Simpson escreveu: On 01Feb2012 03:34, Paulo da Silva p_s_d_a_s_i_l_...@netcabo.pt wrote: | BTW, iter seems faster than iterating thru mylist[1:]! I would hope the difference can be attributed to the cost of copying mylist[1:]. I don't think so. I tried several times and the differences were almost always consistent. I put mylist1=mylist[1:] outside the time control. iter still seems a little bit faster. Running both programs several times (1000 elements list) I only got iter being slower once! But, of course, most of the difference comes from the copy. -- http://mail.python.org/mailman/listinfo/python-list
Re: Iterate from 2nd element of a huge list
On Thu, 02 Feb 2012 07:23:04 +, Paulo da Silva wrote: Em 01-02-2012 04:55, Cameron Simpson escreveu: On 01Feb2012 03:34, Paulo da Silva p_s_d_a_s_i_l_...@netcabo.pt wrote: | BTW, iter seems faster than iterating thru mylist[1:]! I would hope the difference can be attributed to the cost of copying mylist[1:]. I don't think so. I tried several times and the differences were almost always consistent. Yes, actually iterating over a list-iterator appears to be trivially faster (although this may not apply to arbitrary iterators): steve@runes:~$ python -m timeit -s L=range(1) for x in L: pass 1000 loops, best of 3: 280 usec per loop steve@runes:~$ python -m timeit -s L=range(1) for x in iter(L): pass 1000 loops, best of 3: 274 usec per loop The difference of 6 microseconds would be lost in the noise if the loops actually did something useful. Also keep in mind that for tiny lists, the overhead of creating the iterator is probably much greater than the time of iterating over the list: steve@runes:~$ python -m timeit -s L=range(3) for x in L: pass 100 loops, best of 3: 0.238 usec per loop steve@runes:~$ python -m timeit -s L=range(3) for x in iter(L): pass 100 loops, best of 3: 0.393 usec per loop But of course the difference is only relatively significant, in absolute terms nobody is going to notice an extra 0.1 or 0.2 microseconds. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Iterate from 2nd element of a huge list
On 01Feb2012 01:39, Paulo da Silva p_s_d_a_s_i_l_...@netcabo.pt wrote: | What is the best way to iterate thru a huge list having the 1st element | a different process? I.e.: | | process1(mylist[0]) | for el in mylist[1:]: | process2(el) | | This way mylist is almost duplicated, isn't it? Yep. What about (untested): process1(mylist[0]) for i in xrange(1,len(mylist)): process2(mylist[i]) Cheers, -- Cameron Simpson c...@zip.com.au DoD#743 http://www.cskk.ezoshosting.com/cs/ The truth is, I will not give myself the trouble to write sense long, for I would as soon please fools as wise men; because fools are more numerous, and every prudent man will go with the majority.- Hugh Henry Brackenridge -- http://mail.python.org/mailman/listinfo/python-list
Re: Iterate from 2nd element of a huge list
On 1 February 2012 12:39, Paulo da Silva p_s_d_a_s_i_l_...@netcabo.ptwrote: Hi! What is the best way to iterate thru a huge list having the 1st element a different process? I.e.: process1(mylist[0]) for el in mylist[1:]: process2(el) This way mylist is almost duplicated, isn't it? If you are sure that mylist contains at least one element: mylist = [1, 2, 3] i = iter(mylist) print next(i) 1 for el in i: ... print el ... 2 3 Note: for older pythons, you may need i.next() instead of next(i). If mylist may be empty, you will need some error handling. Tim Delaney -- http://mail.python.org/mailman/listinfo/python-list
Re: Iterate from 2nd element of a huge list
On 01/02/12 01:39, Paulo da Silva wrote: Hi! What is the best way to iterate thru a huge list having the 1st element a different process? I.e.: process1(mylist[0]) for el in mylist[1:]: process2(el) This way mylist is almost duplicated, isn't it? Thanks. Maybe (untested), it = iter(mylist) process1(it.next()) for el in it: process2(el) Duncan -- http://mail.python.org/mailman/listinfo/python-list
Re: Iterate from 2nd element of a huge list
Em 01-02-2012 01:39, Paulo da Silva escreveu: Hi! What is the best way to iterate thru a huge list having the 1st element a different process? I.e.: process1(mylist[0]) for el in mylist[1:]: process2(el) This way mylist is almost duplicated, isn't it? Thanks. I think iter is nice for what I need. Thank you very much to all who responded. -- http://mail.python.org/mailman/listinfo/python-list
Re: Iterate from 2nd element of a huge list
Em 01-02-2012 03:16, Paulo da Silva escreveu: Em 01-02-2012 01:39, Paulo da Silva escreveu: Hi! What is the best way to iterate thru a huge list having the 1st element a different process? I.e.: process1(mylist[0]) for el in mylist[1:]: process2(el) This way mylist is almost duplicated, isn't it? Thanks. I think iter is nice for what I need. Thank you very much to all who responded. BTW, iter seems faster than iterating thru mylist[1:]! -- http://mail.python.org/mailman/listinfo/python-list
Re: Iterate from 2nd element of a huge list
On 01Feb2012 03:34, Paulo da Silva p_s_d_a_s_i_l_...@netcabo.pt wrote: | Em 01-02-2012 03:16, Paulo da Silva escreveu: | I think iter is nice for what I need. | Thank you very much to all who responded. | | BTW, iter seems faster than iterating thru mylist[1:]! I would hope the difference can be attributed to the cost of copying mylist[1:]. Do your timings suggest this? (Remembering also that for most benchmarking you need to run things many times unless the effect is quite large). Cheers, -- Cameron Simpson c...@zip.com.au DoD#743 http://www.cskk.ezoshosting.com/cs/ Any company large enough to have a research lab is large enough not to listen to it. - Alan Kay -- http://mail.python.org/mailman/listinfo/python-list
Re: Iterate from 2nd element of a huge list
On 1 February 2012 03:16, Paulo da Silva p_s_d_a_s_i_l_...@netcabo.pt wrote: Em 01-02-2012 01:39, Paulo da Silva escreveu: Hi! What is the best way to iterate thru a huge list having the 1st element a different process? I.e.: process1(mylist[0]) for el in mylist[1:]: process2(el) This way mylist is almost duplicated, isn't it? Thanks. I think iter is nice for what I need. Thank you very much to all who responded. Nobody mentioned itertools.islice, which can be handy, especially if you weren't interested in the first element of the list: from itertools import islice: for el in islice(mylist, 1): process2(el) -- Arnaud -- http://mail.python.org/mailman/listinfo/python-list
