Thanks, everyone! While I do not understand the function provided a few messages back, it works perfectly. The rest of the responses were very interesting. I need to try, yet again, to really understand list comprehensions, possibly the most difficult topic I have yet come across, along with the colon when used in lists. Thanks again to all!
On 12/2/10, Steven D'Aprano <st...@pearwood.info> wrote: > Alan Gauld wrote: >> >> "Alex Hall" <mehg...@gmail.com> wrote >> >>> Alright, I have it working. Now the problem is that it does not throw >>> out reversals. I tried to do this myself with a couple loops, but I >>> get index errors. My total list of permutations is called l. >>> >>> for i in range(0, len(l)): >>> r=l[i]; r.reverse() >> >> You don''t need to specify the 0, its the default. >> But you don't need to use indexing either. >> >> for r,i in enumerate(l): r.reverse() >> >>> for j in range(0, len(l)): >>> print l[j], r, i, j >> >> Here you are comparing the reversed item to every item in the >> list, including the ones you have already reversed, including the >> item itself. So you will always find a match when i==j... >> >> Using the in operator and list slicing would be easier >> >> if r in l[i:]: l.remove(r) >> >> but... >> >>> if r==l[j]: l.remove(r) >> >> Its never a good idea to remove items from a list you are iterating >> over, its like the old cartoon of the guy cutting down the tree branch >> he is sitting on. > > You can cut off the branch you're sitting on so long as you remember to > cut on the correct side :) > > Back in Ancient Days when dinosaurs walked the earth, and I programmed > in Pascal, computers didn't have much memory, and were slow. > Consequently it wasn't practical to make a copy of a list if you wanted > to delete a few items. The only practical way to modify lists was to > modify them in place, and if you needed to delete items, you had to work > backwards. It took me a while to break myself of the habit of doing this: > > for i in range(len(mylist)-1, -1, -1): > if mylist[i] == "something": > del mylist[i] > > (Note that you only need to work backwards if you're *deleting* entries, > not if you replace them with something else.) > > This is still a useful technique to have in your toolbox, but generally > speaking the above is better written as: > > mylist = [x for x in mylist if x != "something"] > > > If you really need to modify the list in place, and not just re-bind the > name "mylist" to the new list, then one tiny change will do it: > > mylist[:] = [x for x in mylist if x != "something"] > > > > -- > Steven > > _______________________________________________ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > http://mail.python.org/mailman/listinfo/tutor > -- Have a great day, Alex (msg sent from GMail website) mehg...@gmail.com; http://www.facebook.com/mehgcap _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
