Hi Amir 2008/7/18 Amir <[EMAIL PROTECTED]>: > A very beginner question about indexing: let x be an array where n = > len(x). I would like to create a view y of x such that: > > y[i] = x[i:i+m,...] for each i and a fixed m << n > > so I can do things like numpy.cov(y). With n large, allocating y is a > problem for me. Currently, I either do for loops in cython or > translate operations into correlate() but am hoping there is an easier > way, maybe using fancy indexing or broadcasting. Memory usage is > secondary to speed, though.
Robert Kern's recently added numpy.lib.stride_tricks should help: In [84]: x = np.arange(100).reshape(10,-1) In [85]: x Out[85]: array([[ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9], [10, 11, 12, 13, 14, 15, 16, 17, 18, 19], [20, 21, 22, 23, 24, 25, 26, 27, 28, 29], [30, 31, 32, 33, 34, 35, 36, 37, 38, 39], [40, 41, 42, 43, 44, 45, 46, 47, 48, 49], [50, 51, 52, 53, 54, 55, 56, 57, 58, 59], [60, 61, 62, 63, 64, 65, 66, 67, 68, 69], [70, 71, 72, 73, 74, 75, 76, 77, 78, 79], [80, 81, 82, 83, 84, 85, 86, 87, 88, 89], [90, 91, 92, 93, 94, 95, 96, 97, 98, 99]]) In [86]: x.stridesOut[86]: (40, 4) In [87]: xx = np.lib.stride_tricks.as_strided(x, shape=(8, 3, 10), strides=(40, 40, 4)) In [88]: xx Out[88]: array([[[ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9], [10, 11, 12, 13, 14, 15, 16, 17, 18, 19], [20, 21, 22, 23, 24, 25, 26, 27, 28, 29]], [[10, 11, 12, 13, 14, 15, 16, 17, 18, 19], [20, 21, 22, 23, 24, 25, 26, 27, 28, 29], [30, 31, 32, 33, 34, 35, 36, 37, 38, 39]], [[20, 21, 22, 23, 24, 25, 26, 27, 28, 29], [30, 31, 32, 33, 34, 35, 36, 37, 38, 39], [40, 41, 42, 43, 44, 45, 46, 47, 48, 49]], [...] Cheers Stéfan _______________________________________________ Numpy-discussion mailing list Numpy-discussion@scipy.org http://projects.scipy.org/mailman/listinfo/numpy-discussion