On Friday, March 29, 2013 9:15:36 AM UTC+10:30, Chris Angelico wrote: > On Fri, Mar 29, 2013 at 9:11 AM, Eric Parry <joan4e...@gmail.com> wrote: > > > Thank you for that explanation. > > > No, I do not understand recursion. It is missing from my Python manual. I > > would be pleased to receive further explanation from anyone. > > > > If you already know what recursion is, just remember the answer. > > Otherwise, find someone who is standing closer to Douglas Hofstadter > > than you are; then ask him or her what recursion is. > > > > :) > > > > Recursion is a form of self-referential code. Take this simple, and > > rather silly, means of calculating the sum of numbers in a list (like > > the sum() function): > > > > # The sum of numbers in an empty list is 0. > > # Otherwise it is the first number plus the sum of the rest of the list. > > def list_sum(lst): > > if not lst: return 0 > > return lst[0] + list_sum(lst[1:]) > > > > >>> list_sum([1,2,3,4,5,6]) > > 21 > > > > Note how the function calls itself - but not always. That's critical > > to recursion - a termination condition. In this case, it's quite > > obvious that the list will eventually have nothing left in it, so the > > function will terminate. Sometimes it's less obvious. Sometimes a bug > > results in infinite recursion... and: > > > > RuntimeError: maximum recursion depth exceeded in comparison > > > > Hope that helps! > > > > ChrisA
Thank you for that example Chris. That explains why the program keeps running after a solution is found. Eric -- http://mail.python.org/mailman/listinfo/python-list
