Hi, I thought it might be useful to summarize the different ways to use numpy's indexing, slice and fancy. The document so far is here:
http://www.scipy.org/Cookbook/Indexing While writing it I ran into some puzzling issues. The first of them is, how is Boolean indexing supposed to work when given more than one array? What I mean is, suppose we have the following array: In [3]: A = np.arange(9) In [4]: B = np.reshape(A,(3,3)) In [5]: B Out[5]: array([[0, 1, 2], [3, 4, 5], [6, 7, 8]]) Then we can use boolean indexing to select some rows or some columns: In [12]: B[np.array([True,False,True]),:] Out[12]: array([[0, 1, 2], [6, 7, 8]]) In [13]: B[:,np.array([True,False,True])] Out[13]: array([[0, 2], [3, 5], [6, 8]]) But if you supply Boolean arrays in both row and column places, rather than select the corresponding rows and columns, you get something basically mysterious: In [14]: B[np.array([True,False,True]),np.array([True,True,False])] Out[14]: array([0, 7]) In [15]: B[np.array([True,False,True]),np.array([True,False,False])] Out[15]: array([0, 6]) In [16]: B[np.array([True,False,True]),np.array([True,True,True])] --------------------------------------------------------------------------- <type 'exceptions.ValueError'> Traceback (most recent call last) /home/peridot/<ipython console> in <module>() <type 'exceptions.ValueError'>: shape mismatch: objects cannot be broadcast to a single shape What is going on here? Thanks, Anne _______________________________________________ Numpy-discussion mailing list Numpy-discussion@scipy.org http://projects.scipy.org/mailman/listinfo/numpy-discussion
