On 9 January 2012 18:26, Steven D'Aprano <st...@pearwood.info> wrote:
> Sarma Tangirala wrote: > >> Hi list, >> >> I was banging my head about a pythonic way of doing the following, >> >> Given a nested list, how do I sort the uppermost list based on one key and >> when a special condition occurs a sort on another key should be performed? >> For example, [[1,2], [2, 2], [3, 2], [4, 0]] would be sorted, in my >> example >> as, [[4, 0], [3, 2], [2, 2], [1, 2]]. That is, sort on the second value >> and >> in case they are equal, reverse sort on the first value. >> > > That is not exactly a good example. There are at least two other ways to > get the result you show, both much more obvious: > > py> L = [[1,2], [2, 2], [3, 2], [4, 0]] > py> list(reversed(L)) > > [[4, 0], [3, 2], [2, 2], [1, 2]] > py> sorted(L, reverse=True) > > [[4, 0], [3, 2], [2, 2], [1, 2]] > > > If I ignore your example, and just use the description: > > "sort on the second value, and in case they are equal, reverse sort on the > first value" > > the way to do this is with a double sort. Note that this works because > Python's sort is stable: in the event of ties, the first item remains > first. In earlier versions of Python, this was not always the case. > > So, given this list: > > L = [[1,2], [4,0], [3,2], [2,2], [5,1], [1,1]] > > first sort in reverse by the first item, then by the second: > > > py> L.sort(key=lambda sublist: sublist[0], reverse=True) > py> L.sort(key=lambda sublist: sublist[1]) > py> print L > [[4, 0], [5, 1], [1, 1], [3, 2], [2, 2], [1, 2]] > > > > Note that using a key function is MUCH more efficient than a cmp function. > Comparison functions end up doing much more work, and hence are very much > slower, than a key function. > > Also note that in recent versions of Python, you can do without the lambda > function and use the special "itemgetter" function: > > > py> from operator import itemgetter > py> L = [[1,2], [4,0], [3,2], [2,2], [5,1], [1,1]] > py> L.sort(key=itemgetter(0), reverse=True) > py> L.sort(key=itemgetter(1)) > py> print L > [[4, 0], [5, 1], [1, 1], [3, 2], [2, 2], [1, 2]] > > I tried this a lot yesterday but seemed to get a wrong answer but now I realize its because of a bad test case. Thank you. > > Last but not least, I will show how to do it using a custom cmp function. > But I recommend that you don't use this! > > def my_cmp(list1, list2): > x = cmp(list1[1], list2[1]) > if x == 0: # a tie > x = cmp(list2[0], list1[0]) # swap the order for reverse sort > # or if you prefer, x = -cmp(list1[0], list2[0]) > return x > > sorted(L, my_cmp) > > > > > -- > Steven > ______________________________**_________________ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > http://mail.python.org/**mailman/listinfo/tutor<http://mail.python.org/mailman/listinfo/tutor> > -- Sarma Tangirala, Class of 2012, Department of Information Science and Technology, College of Engineering Guindy - Anna University
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