On Dec 11, 8:29 pm, "Terry Reedy" <[EMAIL PROTECTED]> wrote: > "sofeng" <[EMAIL PROTECTED]> wrote in message > > news:[EMAIL PROTECTED] > |I would like to use the following recipe to transpose a list of lists > | with different > lengths.http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/410687 > | > | Here is an example of what I would like to do: > | > | Python 2.5.1 (r251:54863, May 18 2007, 16:56:43) > | [GCC 3.4.4 (cygming special, gdc 0.12, using dmd 0.125)] on cygwin > | Type "help", "copyright", "credits" or "license" for more information. > | >>> a = [[1,2,3],[4,5,6,7],[8,9]] > | >>> print map(lambda *row: list(row), *a) > | [[1, 4, 8], [2, 5, 9], [3, 6, None], [None, 7, None]] > | >>> > | > | However, in the Python 3000 FAQ (http://www.artima.com/weblogs/ > | viewpost.jsp?thread=211200), Guido says not to use map with lambda > | because a list comprehension is clearer and faster. > | > | How can I rewrite the above recipe using a list comprehension instead? > > Here is the sort of thing Guido is talking about: > > >>> a = [[1,2,3],[4,5,6,7],[8,9]] > >>> [list(r) for r in zip(*a)] > > [[1, 4, 8], [2, 5, 9]] > > Except in this case, zip does not pad, while map does, so the list comp > form has to be > > >>> [list(r) for r in map(None, *a)] > > [[1, 4, 8], [2, 5, 9], [3, 6, None], [None, 7, None]] > > but at this point, it is about as easy to put the lambda in place of None. > > But when one maps one sequence or equal-length sequences, Guido's point > applies. > > tjr
Thank you very much. -- http://mail.python.org/mailman/listinfo/python-list
