On Tue, Oct 27, 2009 at 7:15 PM, Lambda <stephenh...@gmail.com> wrote: > I defined a function to raise a 2x2 matrix to nth power: > > def matrix_power(m, n): > result = m[:]
Note that this only copies the *outer* list. It does NOT copy any of the inner, nested lists, it just makes fresh *references* to them, which is what's causing your problem; the inner lists get shared, thus the 2D lists are almost the same list for practical purposes. You want: result = [row[:] for row in m] Which explicitly copies the inner lists. <snip> > I find the matrix and m lists are always equal. > But these two lists are separate objects, right? Right, but the *inner* list objects *are* the same in your code, and lists with the same contents (in this case, the same inner lists) compare as equal (==) but not identical (`is`). Remember that all lists in Python are 1-dimensional; "multidimensional" lists are merely by convention, hence you have to think a bit more carefully when working with them. Cheers, Chris -- http://blog.rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
