[Numpy-discussion] dumb question about creating a complex array

Christopher Barker Chris.Barker@noaa....
Thu Feb 22 17:28:22 CST 2007

Mathew Yeates wrote:
> given an array of floats, 2N columns and M rows, where the elements 
> A[r,2*j] and A[r,2*j+1] form the real and imaginary parts of a complex 
> number ....... What is the simplest way to create a complex array? It's 
> a fairly large array so I want to keep copying to a minimum.
> (Actually, it's not a float array, its elements are byte sized, in case 
> that matters)

It does. If it was a float array, you may even be able to do it without 
any copying at all. Anyway, this should work:

 >>> a = N.array([[1,2,3,4],[2,3,4,5],[4,5,6,7]], dtype=N.byte)
 >>> a
array([[1, 2, 3, 4],
        [2, 3, 4, 5],
        [4, 5, 6, 7]], dtype=int8)
# have I got that right?
 >>> b = N.empty((a.shape[0],a.shape[1]/2), dtype=N.complex)
 >>> b.real = a[:,range(0,a.shape[1],2)]
 >>> b.imag = a[:,range(1,a.shape[1],2)]
 >>> b
array([[ 1.+2.j,  3.+4.j],
        [ 2.+3.j,  4.+5.j],
        [ 4.+5.j,  6.+7.j]])

Is that what you wanted?

By the way, I think there is a trick for doing the every other column 
trick without using range(), but I can't find it at the moment.


Christopher Barker, Ph.D.

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