Hey Denis, that was a nice explanation of recursion, thanks.
On Wed, Dec 11, 2013 at 8:37 AM, <tutor-requ...@python.org> wrote: > Send Tutor mailing list submissions to > tutor@python.org > > To subscribe or unsubscribe via the World Wide Web, visit > https://mail.python.org/mailman/listinfo/tutor > or, via email, send a message with subject or body 'help' to > tutor-requ...@python.org > > You can reach the person managing the list at > tutor-ow...@python.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Tutor digest..." > > > Today's Topics: > > 1. Re: formatting datetime.timedelta to "HH:MM:SS" (David Robinow) > 2. Re: recursive function example (spir) > 3. Re: formatting datetime.timedelta to "HH:MM:SS" (Jignesh Sutar) > 4. Re: recursive function example (spir) > 5. Re: formatting datetime.timedelta to "HH:MM:SS" (Mark Lawrence) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Wed, 11 Dec 2013 07:43:18 -0500 > From: David Robinow <drobi...@gmail.com> > Cc: tutor@python.org > Subject: Re: [Tutor] formatting datetime.timedelta to "HH:MM:SS" > Message-ID: > <CAF3XjbzRQ5+drp86= > scv9pnuqtget+qkbgj25zrouocanh7...@mail.gmail.com> > Content-Type: text/plain; charset=UTF-8 > > On Wed, Dec 11, 2013 at 5:55 AM, Jignesh Sutar <jsu...@gmail.com> wrote: > > Hi, > > > > I've googled around extensively to try figure this out assuming it > should be > > straight forward (and it probably is) but I'm clearly missing something. > > > > I'm trying to get the total run time of the program but have the final > time > > being displayed in a particular format. I.e. without the seconds in > > milliseconds decimal points and just to customize it a bit more. > > > > import time > > from datetime import datetime > > startTime = datetime.now() > > time.sleep(5.1564651443644) > > endTime = datetime.now() > > exe_time = endTime-startTime > > > > print type(startTime) > > print type(endTime) > > print type(exe_time) > > > > print "startTime: ", startTime > > print "endTime:", endTime > > print "exe_time: ", exe_time #how to format this to "D Days, HH: MM: SS" > ? > > #exe_time: 0:00:05.156000 > > #desired 0 Days, 0h: 00:m: 05s > > > print str(exe_time).split('.')[0] > > > ------------------------------ > > Message: 2 > Date: Wed, 11 Dec 2013 14:10:26 +0100 > From: spir <denis.s...@gmail.com> > To: tutor@python.org > Subject: Re: [Tutor] recursive function example > Message-ID: <52a86442.4030...@gmail.com> > Content-Type: text/plain; charset=UTF-8; format=flowed > > On 12/10/2013 03:48 PM, uga...@talktalk.net wrote: > > [...] > > Recursivity is hard to get really, meaning intuitively with your guts > so-to-say. > Maybe using another example may help. Lets us say you want a function that > sums > numbers from 1 up to n, the only input variable. (The result should thus be > n(n+1)/2.) > > def sum_up_to (n): > # 'res' for result because 'sum' is a Python builtin symbol > res = 0 > for i in range(1, n+1): > res += i > return res > print(sum_up_to(9)) > > Now, how to make this recursive? There are multiple ways, all based on: > # intuitive def: > sum_up_to(n) = 1 + 2 + ... + n > # recurrence def: > sum_up_to(n) = sum_up_to(n-1) + n > sum_up_to(1) = 1 > We want to just reproduce this second def stupidly in code: > > def sum_up_to_rec1 (n): > if n == 1: > return 1 > else: > return n + sum_up_to_rec1(n-1) > print(sum_up_to_rec1(9)) > > This does not help much and understand the recursion procedure yet, si let > use > add some debug output rightly placed: > > def sum_up_to_rec1 (n): > print("start of func ; n : %d" % n, end=" | ") > if n == 1: > return 1 > else: > sub_res = sum_up_to_rec1(n-1) > print("n : %d ; sub_res : %d" % (n, sub_res)) > return n + sub_res > print(sum_up_to_rec1(9)) > > Run it. > > The surprising point, maybe, is that all "start of func..." outputs get > written > in a row with all "sub_res..." outputs coming on lines one under the > other. This > is the core of the recurrent logic: starting from 9, we call the func for > 8; > nothing is yet computed, no result (thus nothing to print as sub_res). > From 8, > we call the func for 7; nothing is yet computed, still. Etc... until 1, > our stop > value, which finally returns a result. This result for 1 permits the call > for 2 > to (write the sub_res for 1 *at the end of the big row* and) complete, > which > permits the call for 3 to (write the sub_res for 2 and) complete... etc > until 9. > The call for 9 is out initial run, when it returns with its product, this > is the > final result. > > Thus, the logic is such: the execution cascade falls here top-down; but the > computation cascade inverts gravity and climbs down-up. > > Does this help? > > Now, why do we execute top-down? After all, the definition by recurrence > allows > us very well to start from 1 and go up to n, it would work as well, so why > not? > The reason is that we would need to keep n aside, since in this case it > becomes > our stop value; but we still need some index, here going up from 1 to n. > Thus, > we need another parameter to the recurrent func, say like in the easy func > above. And in fact we need yet a third parameter, namely the partial > result: how > else would the next call know the partial result? This gives something > like: > > def sum_up_to_rec2 (n, i=1, sub_res=0): > # 'res' for result because 'sum' is a Python builtin symbol > print("start of func ; i : %d ; sub_res : %d" % (i, sub_res), end=" | > ") > if i == n: > return i + sub_res > else: > sub_res += i > print("sub_call ; sub_res : %d" % sub_res) > return sum_up_to_rec2(n, i+1, sub_res) > print(sum_up_to_rec2(9)) > > Needless to say, this is more complicated (but strangely far easier to > understand for me). Another solution is to _define_ a new function, > recursive, > inside the outer one which thus keeps a simple interface (no 'i' or > 'sub_res'). > This is very often needed in recursion in functional programming, because > we > need to pass partial data anyway (state of partial execution). In fact, > your > case and mine are not typical. > > Thus, if we must define a new function anyway, we have the choice of > running > upwards or downwards; downwards is the normal case for a weird reason, > namely > that the typical data structure there is a linked list, to which one can > only > (efficiently) put new items at start; thus, if we run forwards, the > results are > backwards. > > Advice: use recursion whenever it corresponds to the application; meaning > the > idea you are expressing itself is recursive. This is the case in problems > which > decompose into sub-problems *of the same form*. Otherwise, avoid forcing > recursion whenever it drives to a distortion of ideas. > > There are situations where, like in our 2 cases, a simple conception is a > linear > recursion (recursion just mean repetition, literally re-running), while an > alternative view is of self-similar recursion, like fractals (what we call > recursion in programming, see self-similarity in wikimedia). I used to > program > both views to get used to recursive thinking. > > Denis > > > ------------------------------ > > Message: 3 > Date: Wed, 11 Dec 2013 13:12:18 +0000 > From: Jignesh Sutar <jsu...@gmail.com> > To: tutor@python.org > Subject: Re: [Tutor] formatting datetime.timedelta to "HH:MM:SS" > Message-ID: > < > cacvw2fxl4-ms8mc-jkcspq3zn+h0nty73wcjj_8nucoecvh...@mail.gmail.com> > Content-Type: text/plain; charset="iso-8859-1" > > > > > print str(exe_time).split('.')[0] > > > Sorry, I guess my question was why I can't use something similar to below > on exe_time (of type datetime.timedelta)? Rather than doing string > manipulation on decimals or colons to extract the same. > > now = datetime.now() > print now.hour > print now.minute > print now.year > > > On 11 December 2013 12:43, David Robinow <drobi...@gmail.com> wrote: > > > On Wed, Dec 11, 2013 at 5:55 AM, Jignesh Sutar <jsu...@gmail.com> wrote: > > > Hi, > > > > > > I've googled around extensively to try figure this out assuming it > > should be > > > straight forward (and it probably is) but I'm clearly missing > something. > > > > > > I'm trying to get the total run time of the program but have the final > > time > > > being displayed in a particular format. I.e. without the seconds in > > > milliseconds decimal points and just to customize it a bit more. > > > > > > import time > > > from datetime import datetime > > > startTime = datetime.now() > > > time.sleep(5.1564651443644) > > > endTime = datetime.now() > > > exe_time = endTime-startTime > > > > > > print type(startTime) > > > print type(endTime) > > > print type(exe_time) > > > > > > print "startTime: ", startTime > > > print "endTime:", endTime > > > print "exe_time: ", exe_time #how to format this to "D Days, HH: MM: > SS" > > ? > > > #exe_time: 0:00:05.156000 > > > #desired 0 Days, 0h: 00:m: 05s > > > > > print str(exe_time).split('.')[0] > > _______________________________________________ > > Tutor maillist - Tutor@python.org > > To unsubscribe or change subscription options: > > https://mail.python.org/mailman/listinfo/tutor > > > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: < > http://mail.python.org/pipermail/tutor/attachments/20131211/143a5ed5/attachment-0001.html > > > > ------------------------------ > > Message: 4 > Date: Wed, 11 Dec 2013 14:12:52 +0100 > From: spir <denis.s...@gmail.com> > To: tutor@python.org > Subject: Re: [Tutor] recursive function example > Message-ID: <52a864d4.9060...@gmail.com> > Content-Type: text/plain; charset=UTF-8; format=flowed > > On 12/11/2013 09:50 AM, Alan Gauld wrote: > > Remember that each time mult() is called it creates > > its own mini-world of variables independent of the > > previous calls. > > That, is a key point. > > Denis > > > ------------------------------ > > Message: 5 > Date: Wed, 11 Dec 2013 13:37:11 +0000 > From: Mark Lawrence <breamore...@yahoo.co.uk> > To: tutor@python.org > Subject: Re: [Tutor] formatting datetime.timedelta to "HH:MM:SS" > Message-ID: <l89ppq$obf$1...@ger.gmane.org> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > On 11/12/2013 13:12, Jignesh Sutar wrote: > > print str(exe_time).split('.')[0] > > Sorry, I guess my question was why I can't use something similar to > > below on exe_time (of type datetime.timedelta)? Rather than doing string > > manipulation on decimals or colons to extract the same. > > > > now = datetime.now() > > print now.hour > > print now.minute > > print now.year > > > > Old style > > print('%02d:%02d:%04d' % (now.hour, now.minute, now.year)) > > New style > > print('{}:{}:{}'.format(now.hour, now.minute, now.year)) > > Sorry I can never remember the formatting types to go between {} so look > for them around here > http://docs.python.org/3/library/string.html#formatstrings > > -- > My fellow Pythonistas, ask not what our language can do for you, ask > what you can do for our language. > > Mark Lawrence > > > > ------------------------------ > > Subject: Digest Footer > > _______________________________________________ > Tutor maillist - Tutor@python.org > https://mail.python.org/mailman/listinfo/tutor > > > ------------------------------ > > End of Tutor Digest, Vol 118, Issue 53 > ************************************** > -- Keith Winston Director, Earth Sun Energy Systems 301-980-6325
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