[SciPy-user] array vs matrix, converting code from matlab
Travis Oliphant
oliphant at ee.byu.edu
Thu Apr 20 13:28:26 CDT 2006
Gennan Chen wrote:
>Hi! All,
>
>I am also in the same process. And I would like to add one more
>question:
>
>In Matlab, for a 3D array or matrix, the indexing is a(i,j,k). In
>numpy, it became a[k-1,i-1,j-1]. Is there any way to make it become
>a[i-1,j-1,k-1]? Or I am doing something wrong here??
>
>
>
In NumPy, arrays and matrices are by default in C-contiguous order so
that the last index varies the fastest. Matlab is based on Fortran
originally and defines arrays in Fortran-contiguous order (the first
index varies the fastest as you walk linearly throught memory). The
only time this really matters is if you are interfacing with some
compiled code. Otherwise, how you think about indexing is up to you
and how you define the array.
So, to make it a[i-1,j-1,k-1] you need to reshape the array from the
way you defined it in MATLAB. It really does just depend on how you
define things. Perhaps you could give a specific example so we could
be more specific on how you would write the same thing in NumPy.
-Travis
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