Rich Harkins wrote: > [EMAIL PROTECTED] wrote: >> Given a bunch of arrays, if I want to create tuples, there is >> zip(arrays). What if I want to do the opposite: break a tuple up and >> append the values to given arrays: >> map(append, arrays, tupl) >> except there is no unbound append() (List.append() does not exist, >> right?). >> > > list.append does exist (try the lower-case flavor). > >> Without append(), I am forced to write a (slow) explicit loop: >> for (a, v) in zip(arrays, tupl): >> a.append(v) >> > > Except that isn't technically the opposite of zip. The opposite would > be a tuple of single-dimensional tuples: > > def unzip(zipped): > """ > Given a sequence of size-sized sequences, produce a tuple of tuples > that represent each index within the zipped object. > > Example: > >>> zipped = zip((1, 2, 3), (4, 5, 6)) > >>> zipped > [(1, 4), (2, 5), (3, 6)] > >>> unzip(zipped) > ((1, 2, 3), (4, 5, 6)) > """ > if len(zipped) < 1: > raise ValueError, 'At least one item is required for unzip.' > indices = range(len(zipped[0])) > return tuple(tuple(pair[index] for pair in zipped) > for index in indices) > > This is probably not the most efficient hunk of code for this but this > would seem to be the correct behavior for the opposite of zip and it > should scale well. > > Modifying the above with list.extend would produce a variant closer to > what I think you're asking for: > > def unzip_extend(dests, zipped): > """ > Appends the unzip versions of zipped into dests. This avoids an > unnecessary allocation. > > Example: > >>> zipped = zip((1, 2, 3), (4, 5, 6)) > >>> zipped > [(1, 4), (2, 5), (3, 6)] > >>> dests = [[], []] > >>> unzip_extend(dests, zipped) > >>> dests > [[1, 2, 3], [4, 5, 6]] > """ > if len(zipped) < 1: > raise ValueError, 'At least one item is required for unzip.' > for index in range(len(zipped[0])): > dests[index].extend(pair[index] for pair in zipped) > > This should perform pretty well, as extend with a comprehension is > pretty fast. Not that it's truly meaningful, here's timeit on my 2GHz > laptop: > > bash-3.1$ python -m timeit -s 'import unzip; zipped=zip(range(1024), > range(1024))' 'unzip.unzip_extend([[], []], zipped)' > 1000 loops, best of 3: 510 usec per loop > > By comparison, here's the unzip() version above: > > bash-3.1$ python -m timeit -s 'import unzip; zipped=zip(range(1024), > range(1024))' 'unzip.unzip(zipped)' > 1000 loops, best of 3: 504 usec per loop > > Rich
As Paddy wrote, zip is its own unzip: >>> zipped = zip((1, 2, 3), (4, 5, 6)) >>> zipped [(1, 4), (2, 5), (3, 6)] >>> unzipped = zip(*zipped) >>> unzipped [(1, 2, 3), (4, 5, 6)] Neat and completely confusing, huh? :-) <http://paddy3118.blogspot.com/2007/02/unzip-un-needed-in-python.html> -- -- http://mail.python.org/mailman/listinfo/python-list
