On Tue, Mar 8, 2011 at 3:03 PM, Jonathan Taylor <jonathan.tay...@utoronto.ca> wrote: > I am wanting to use an array b to index into an array x with dimension > bigger by 1 where the element of b indicates what value to extract > along a certain direction. For example, b = x.argmin(axis=1). > Perhaps I want to use b to create x.min(axis=1) but also to index > perhaps another array of the same size. > > I had a difficult time finding a way to do this with np.take easily > and even with fancy indexing the resulting line is very complicated: > > In [322]: x.shape > Out[322]: (2, 3, 4) > > In [323]: x.min(axis=1) > Out[323]: > array([[ 2, 1, 7, 4], > [ 8, 0, 15, 12]]) > > In [324]: x[np.arange(x.shape[0])[:,np.newaxis,np.newaxis], > idx[:,np.newaxis,:], np.arange(x.shape[2])] > Out[324]: > array([[[ 2, 1, 7, 4]], > > [[ 8, 0, 15, 12]]]) > > In any case I wrote myself my own function for doing this (below) and > am wondering if this is the best way to do this or if there is > something else in numpy that I should be using? -- I figure that this > is a relatively common usecase. > > Thanks, > Jon. > > def mytake(A, b, axis): > assert len(A.shape) == len(b.shape)+1 > > idx = [] > for i in range(len(A.shape)): > if i == axis: > temp = b.copy() > shapey = list(temp.shape) > shapey.insert(i,1) > else: > temp = np.arange(A.shape[i]) > shapey = [1]*len(b.shape) > shapey.insert(i,A.shape[i]) > shapey = tuple(shapey) > temp = temp.reshape(shapey) > idx += [temp] > > return A[tuple(idx)].squeeze() > > > In [319]: util.mytake(x,x.argmin(axis=1), 1) > Out[319]: > array([[ 2, 1, 7, 4], > [ 8, 0, 15, 12]]) > > In [320]: x.min(axis=1) > Out[320]: > array([[ 2, 1, 7, 4], > [ 8, 0, 15, 12]])
fewer lines but essentially the same thing and no shortcuts, I think >>> x= np.random.randint(5, size=(2, 3, 4)) >>> x array([[[3, 1, 0, 1], [4, 2, 2, 1], [2, 3, 2, 2]], [[2, 1, 1, 1], [0, 2, 0, 3], [2, 3, 3, 1]]]) >>> idx = [np.arange(i) for i in x.shape] >>> idx = list(np.ix_(*idx)) >>> idx[axis]=np.expand_dims(x.argmin(axis),axis) >>> x[idx] array([[[2, 1, 0, 1]], [[0, 1, 0, 1]]]) >>> np.squeeze(x[idx]) array([[2, 1, 0, 1], [0, 1, 0, 1]]) >>> mytake(x,x.argmin(axis=1), 1) array([[2, 1, 0, 1], [0, 1, 0, 1]]) Josef > _______________________________________________ > NumPy-Discussion mailing list > NumPy-Discussion@scipy.org > http://mail.scipy.org/mailman/listinfo/numpy-discussion > _______________________________________________ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion