Hello there I am reading "How to think like a computer scientist" which is an introductory test in Python.
I wanna clarify the behaviour of multiply operator(*) when applied to list(data structure). Consider the function make_matrix def make_matrix(rows, columns): """ >>> make_matrix(4, 2) [[0, 0], [0, 0], [0, 0], [0, 0]] >>> m = make_matrix(4, 2) >>> m[1][1] = 7 >>> m [[0, 0], [0, 7], [0, 0], [0, 0]] """ return [[0] * columns] * rows The actual output is [[0, 7], [0, 7], [0, 7], [0, 7]] The correct version of make_matrix is : def make_matrix(rows, columns): """ >>> make_matrix(3, 5) [[0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]] >>> make_matrix(4, 2) [[0, 0], [0, 0], [0, 0], [0, 0]] >>> m = make_matrix(4, 2) >>> m[1][1] = 7 >>> m [[0, 0], [0, 7], [0, 0], [0, 0]] """ matrix = [] for row in range(rows): matrix += [[0] * columns] return matrix The reason why first version of make_matrix fails ( as explained in the book at 9.8 ) is that "...each row is an alias of the other rows..." I wonder why [[0] * columns] * rows causes "...each row is an alias of the other rows..." but not [[0] * columns] i.e. why each [0] in a row is not an alias of other row element. /* Everything worth doing is worth doing in excess */ _______________________________________________ BangPypers mailing list BangPypers@python.org http://mail.python.org/mailman/listinfo/bangpypers
