[SciPy-User] creating sparse indicator arrays

Is there a simple or fast way to create a sparse indicator array, `a` below, without going through the dense matrix first?

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from scipy import sparse g = np.array([0, 0, 1, 1]) #categories, integers, u = np.arange(2) #unique's, range(number_categories)

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g[:,None] == u array([[ True, False], [ True, False], [False, True], [False, True]], dtype=bool)

this is the one I want:

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a = sparse.csc_matrix((g[:,None] == u)) a <4x2 sparse matrix of type '<type 'numpy.int8'>' with 4 stored elements in Compressed Sparse Column format> a.todense() matrix([[1, 0], [1, 0], [0, 1], [0, 1]], dtype=int8)

Thanks, Josef