On Fri, Aug 14, 2009 at 14:20, Keith Goodman<[email protected]> wrote: > On Fri, Aug 14, 2009 at 11:52 AM, Robert Kern<[email protected]> wrote: >> On Fri, Aug 14, 2009 at 13:05, John Hunter<[email protected]> wrote: >>> I just tracked down a subtle bug in my code, which is equivalent to >>> >>> >>> In [64]: x, y = np.random.rand(2, n) >>> >>> In [65]: z = np.zeros_like(x) >>> >>> In [66]: mask = x>0.5 >>> >>> In [67]: z[mask] = x/y >>> >>> >>> >>> I meant to write >>> >>> z[mask] = x[mask]/y[mask] >>> >>> so I can fix my code, but why is line 67 allowed >>> >>> In [68]: z[mask].shape >>> Out[68]: (54,) >>> >>> In [69]: (x/y).shape >>> Out[69]: (100,) >>> >>> it seems like broadcasting would fail >> >> Broadcasting doesn't take place with boolean masks. Instead, the >> values repeat if there are too few and extra values are ignored. >> Boolean indexing derives from Numeric's putmask() implementation, >> which had these semantics, rather than other forms of indexing. >> >> You may consider this a wart or a bad design decision (and I would >> probably agree), but it is not a bug. > > Are the last two, x[[1]] and x[np.array([1])], broadcasting? > >>> x = np.array([1,2,3]) >>> x[1] = np.array([4,5,6]) > ValueError: setting an array element with a sequence. >>> x[(1,)] = np.array([4,5,6]) > ValueError: array dimensions are not compatible for copy >>> x[[1]] = np.array([4,5,6]) >>> x > array([1, 4, 3]) >>> x[np.array([1])] = np.array([4,5,6]) >>> x > array([1, 4, 3])
I guess I'm just makin' stuff up again. kern_is_right() == False. All forms repeat, not broadcast, since they derive from put() and putmask() which both have the repeating/ignoring semantics. -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco _______________________________________________ NumPy-Discussion mailing list [email protected] http://mail.scipy.org/mailman/listinfo/numpy-discussion
