Michael - 1 month ago 11
Python Question

# Numpy create index/slicing programmatically from array

I can use

`numpy.mgrid`
as follows:

``````a = numpy.mgrid[x0:x1, y0:y1] # 2 dimensional
b = numpy.mgrid[x0:x1, y0:y1, z0:z1] # 3 dimensional
``````

Now, I'd like to create the expression in brackets programmatically, because I do not know whether I have 1, 2, 3 or more dimensions. I'm looking for something like:

``````shape = np.array([[x0, x1], [y0, y1], ... maybe more dimensions ...])
idx = (s[0]:s[1] for s in shape)
a = numpy.mgrid[idx]
``````

That gives at least a syntax error in the second line. How can I properly generate those indices/slices programmatically? (The mgrid here is rather an example/use case, the question is really about indexing in general.)

Use the `slice` object. For example:

``````shape = np.array([[0, 10], [0, 10]])
idx = tuple(slice(s[0],s[1], 1) for s in shape)
#yields the following
#(slice(0, 10, 1), slice(0, 10, 1))
np.mgrid[idx]
``````

yields

``````array([[[0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
[1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
[2, 2, 2, 2, 2, 2, 2, 2, 2, 2],
[3, 3, 3, 3, 3, 3, 3, 3, 3, 3],
[4, 4, 4, 4, 4, 4, 4, 4, 4, 4],
[5, 5, 5, 5, 5, 5, 5, 5, 5, 5],
[6, 6, 6, 6, 6, 6, 6, 6, 6, 6],
[7, 7, 7, 7, 7, 7, 7, 7, 7, 7],
[8, 8, 8, 8, 8, 8, 8, 8, 8, 8],
[9, 9, 9, 9, 9, 9, 9, 9, 9, 9]],

[[0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]]])
``````

Alternatively, you could use the Numpy shorthand `np.s_`, e.g. `np.s_[0:10:1]`, instead of `slice(1, 10, 1)`, but they are equivalent objects.