[Tutor] Function returns 'None'
Hello As en exercise I wrote the following function: def recursfac(x,carryover=1): print 'x:',x,'carryover:', carryover if x > 1: carryover *= x recursfac(x-1, carryover) else: return carryover print recursfac(3) Very much to my surprise I get the following output: x: 3 carryover: 1 x: 2 carryover: 3 x: 1 carryover: 6 None Where did I go wrong? Kind regards Dominik Danter ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Function returns 'None'
On 11/07/10 14:59, Dominik Danter wrote: Hello As en exercise I wrote the following function: def recursfac(x,carryover=1): print 'x:',x,'carryover:', carryover if x > 1: carryover *= x recursfac(x-1, carryover) else: return carryover print recursfac(3) Very much to my surprise I get the following output: x: 3 carryover: 1 x: 2 carryover: 3 x: 1 carryover: 6 None Where did I go wrong? Kind regards Dominik Danter I made a diagram to try to explain recursfac(3) recursfac(2, 3) <| recursfac(1, 6) _| As you can see recursfac(1,6) returns it's value (carryover) to recursfac(2, 3) which ignores it and completes it's execution ie returning None to your original call which then prints out that return value. I hope that's clear. Adam. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Function returns 'None'
"Dominik Danter" wrote def recursfac(x,carryover=1): print 'x:',x,'carryover:', carryover if x > 1: carryover *= x recursfac(x-1, carryover) No return value here so when the reursed function returns carryover we have nowhere to go in the calling function so we return None. else: return carryover This returns a value to the calling function, but in the recursive case that returned vaklue uis thrown away as the comment above shows. you need to add a return statement where you call recursively. HTH, -- Alan Gauld Author of the Learn to Program web site http://www.alan-g.me.uk/ ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Function returns 'None'
On 07/11/2010 04:59 PM, Dominik Danter wrote: Hello As en exercise I wrote the following function: def recursfac(x,carryover=1): print 'x:',x,'carryover:', carryover if x > 1: carryover *= x recursfac(x-1, carryover) else: return carryover print recursfac(3) Very much to my surprise I get the following output: x: 3 carryover: 1 x: 2 carryover: 3 x: 1 carryover: 6 None Where did I go wrong? Your problem is that you expect the "return" to exit the recursion altogether. Instead, carryover is passed to the previous level of the recursion, which has nothing more to execute and returns None. So the first step to fix this would be to make sure that your function returns carryover no matter what: def recursfac(x,carryover=1): print 'x:',x,'carryover:', carryover if x > 1: carryover *= x recursfac(x-1, carryover) return carryover else: return carryover Or simply (to remove the code duplication): def recursfac(x,carryover=1): print 'x:',x,'carryover:', carryover if x > 1: carryover *= x recursfac(x-1, carryover) return carryover Now there's still one more problem. The output is this: x: 3 carryover: 1 x: 2 carryover: 3 x: 1 carryover: 6 3 Why is it returning 3 istead of 6? Well, the function didn't catch the returned carryover value on the way up, so it's just returns what it knows: the value of carryover that it self has computed. Instead, you have to do this: def recursfac(x,carryover=1): print 'x:',x,'carryover:', carryover if x > 1: carryover *= x carryover = recursfac(x-1, carryover) return carryover And this returns x: 3 carryover: 1 x: 2 carryover: 3 x: 1 carryover: 6 6 Done! What you should learn from this is that, when doing recursion, figuring out what your function should do on the way up is as crucial as what you want it to do on the way down. Nick ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Function returns 'None'
On 07/11/2010 06:28 PM, Nick Raptis wrote: def recursfac(x,carryover=1): print 'x:',x,'carryover:', carryover if x > 1: carryover *= x carryover = recursfac(x-1, carryover) return carryover And this returns x: 3 carryover: 1 x: 2 carryover: 3 x: 1 carryover: 6 6 Done! Also, I realized that my final code may be tough to decipher now.. A nicer way to write it would be (the functionality is still exactly the same): def recursfac(x,carryover=1): print 'x:',x,'carryover:', carryover if x > 1: carryover *= x result = recursfac(x-1, carryover) else: # done with recursion, start our way up result = carryover return result ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Function returns 'None'
I think the new version is harder to understand. Sent from my iPhone On Jul 11, 2010, at 10:43 AM, Nick Raptis wrote: > On 07/11/2010 06:28 PM, Nick Raptis wrote: >> >> def recursfac(x,carryover=1): >>print 'x:',x,'carryover:', carryover >>if x > 1: >>carryover *= x >>carryover = recursfac(x-1, carryover) >>return carryover >> >> And this returns >> x: 3 carryover: 1 >> x: 2 carryover: 3 >> x: 1 carryover: 6 >> 6 >> >> Done! >> > > Also, I realized that my final code may be tough to decipher now.. A nicer > way to write it would be (the functionality is still exactly the same): > > def recursfac(x,carryover=1): >print 'x:',x,'carryover:', carryover >if x > 1: >carryover *= x >result = recursfac(x-1, carryover) >else: ># done with recursion, start our way up >result = carryover >return result > > > ___ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > http://mail.python.org/mailman/listinfo/tutor ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Function returns 'None'
On 07/11/2010 06:50 PM, Luke Paireepinart wrote:
I think the new version is harder to understand.
Sent from my iPhone
On Jul 11, 2010, at 10:43 AM, Nick Raptis wrote:
Aww! A critic! You humble me (really, I'm not being sarcastic here, I
welcome it gladly)
I won't argue about it, though. If you prefer it, the last version is
still there :)
My reasoning is that, with the new variable name ('result') in place,
now it is evident (to me at least) that 'passover' is passed on the way
down, while 'result' is passed on the way up. Harder it not, it seems
more ""clean""
Also, another preference of mine, would you be kind enough to answer to
the list and cc the original poster if you can? Doing it the other way
around breaks my (quite stupid I admit) filters, and perhaps others' too.
Thanks for the feedback.
Nick
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Re: [Tutor] Function returns 'None'
On Jul 11, 2010, at 11:14 AM, Nick Raptis wrote: >> >> >> > > Also, another preference of mine, would you be kind enough to answer to the > list and cc the original poster if you can? Doing it the other way around > breaks my (quite stupid I admit) filters, and perhaps others' too. That's the default email setup for the iPhone. I agree it's silly but it seems quite difficult to modify the fields. I see your point about the variable names but if it's at least fairly obvious what you're intending to do it seems like a bit of a waste to have an extra variable and else block. But you know, potato potato. It's negligible from a performance standpoint, it's really just a question of readability. Do what makes you ( and the people who will read your code) happy. -luke ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
[Tutor] Path?
I am running Ubuntu. I downloaded the source code examples for a book I purchased. Some of the examples load image files located in the same directory as the program. If I go to the current directory in the terminal the program can use the image files. However, if I use a launcher or the filemanager it pops up an error dialog saying the file does not exist even though it is in the same directory. The program simply uses the files name. Is there a way without editing the source and inserting the full path to run the program from a launcher or the filemanager and allow it to see files in the current directory? Thanks, Jim ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] feedback on permutations script
> On Sun, 11 Jul 2010 02:48:38 am Isaac wrote: > >* This is my interpretation of an algorithm to generate all *> >* permutations of items in a list. I would appreciate any feedback *> >* regarding advice for improvements. **> Steven D'Aprano* wrote: *Sun Jul 11 05:29:58 CEST 2010* > Some of this is going to just be personal style, some of this is going > to be slightly stronger semi-official recommended style (PEP 8 if you > want to google further), and right at the end some hints regarding the > algorithm. Thank you for your vaulable insight and time. I'll make some changes. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Path?
On 11/07/10 18:42, Jim Byrnes wrote: I am running Ubuntu. I downloaded the source code examples for a book I purchased. Some of the examples load image files located in the same directory as the program. If I go to the current directory in the terminal the program can use the image files. However, if I use a launcher or the filemanager it pops up an error dialog saying the file does not exist even though it is in the same directory. The program simply uses the files name. Is there a way without editing the source and inserting the full path to run the program from a launcher or the filemanager and allow it to see files in the current directory? Thanks, Jim Maybe create a bash script to call the python code something like: #!/bin/bash cd /directory/the/scripts/are/in python script_name HTH, Adam. PS if you want to use the same script for any python script you could change the last line to: python $1 and call the bash script with the python script as the first argument ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
[Tutor] extract a submatrix
Friends, Excuse me if this question is not appropriate for this forum. I have a matrix of size 550,550. I want to extract only part of this matrix say first 330 elements, i dnt need the last 220 elements in the matrix. is there any function in numpy that can do this kind of extraction. I am quite new to numpy. How can do the same ? Thank you, Bala ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Path?
On Mon, 12 Jul 2010 03:42:28 am Jim Byrnes wrote: > I am running Ubuntu. I downloaded the source code examples for a > book I purchased. Some of the examples load image files located in > the same directory as the program. If I go to the current directory > in the terminal the program can use the image files. However, if I > use a launcher or the filemanager it pops up an error dialog saying > the file does not exist even though it is in the same directory. > > The program simply uses the files name. Is there a way without > editing the source and inserting the full path to run the program > from a launcher or the filemanager and allow it to see files in the > current directory? What file manager are you using? Nautilus? Konqueror? Something else? What do you mean, "use a launcher"? Use a launcher to do what? What sort of launcher? What pops up an error dialog? The launcher? Which file does it claim doesn't exist? Python? The Python script? The image file? What is the exact error message it gives? There's probably a way to tell the launcher which working directory to use, but of course that depends on the answers to the above questions. -- Steven D'Aprano ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
