Nested iters

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Questions to David Rotermund

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numpy.nested_iters

numpy.nested_iters(op, axes, flags=None, op_flags=None, op_dtypes=None, order='K', casting='safe', buffersize=0)

Create nditers for use in nested loops Create a tuple of nditer objects which iterate in nested loops over different axes of the op argument. The first iterator is used in the outermost loop, the last in the innermost loop. Advancing one will change the subsequent iterators to point at its new element.

2d

import numpy as np


a = np.arange(1, 7).reshape((2, 3))
print(a)
print()

iter_a, iter_b = np.nested_iters(a, [[0], [1]], flags=["multi_index"])

for x in iter_a:
    for y in iter_b:
        print(
            f"iter_a.multi_index: {iter_a.multi_index}, iter_b.multi_index: {iter_b.multi_index}"
        )

Output:

[[1 2 3]
 [4 5 6]]

iter_a.multi_index: (0,), iter_b.multi_index: (0,)
iter_a.multi_index: (0,), iter_b.multi_index: (1,)
iter_a.multi_index: (0,), iter_b.multi_index: (2,)
iter_a.multi_index: (1,), iter_b.multi_index: (0,)
iter_a.multi_index: (1,), iter_b.multi_index: (1,)
iter_a.multi_index: (1,), iter_b.multi_index: (2,)

3d

import numpy as np


a = np.zeros((2, 3, 4))
print(a.shape) # -> (2, 3, 4)

print("------")
iter_a, iter_b = np.nested_iters(a, [[0], [1, 2]], flags=["multi_index"])

for x in iter_a:
    for y in iter_b:
        print(
            f"iter_a.multi_index: {iter_a.multi_index}, iter_b.multi_index: {iter_b.multi_index}"
        )
print("------")


print("++++++")
iter_a, iter_b = np.nested_iters(a, [[0, 1], [2]], flags=["multi_index"])

for x in iter_a:
    for y in iter_b:
        print(
            f"iter_a.multi_index: {iter_a.multi_index}, iter_b.multi_index: {iter_b.multi_index}"
        )
print("++++++")

Output:

------
iter_a.multi_index: (0,), iter_b.multi_index: (0, 0)
iter_a.multi_index: (0,), iter_b.multi_index: (0, 1)
iter_a.multi_index: (0,), iter_b.multi_index: (0, 2)
iter_a.multi_index: (0,), iter_b.multi_index: (0, 3)
iter_a.multi_index: (0,), iter_b.multi_index: (1, 0)
iter_a.multi_index: (0,), iter_b.multi_index: (1, 1)
iter_a.multi_index: (0,), iter_b.multi_index: (1, 2)
iter_a.multi_index: (0,), iter_b.multi_index: (1, 3)
iter_a.multi_index: (0,), iter_b.multi_index: (2, 0)
iter_a.multi_index: (0,), iter_b.multi_index: (2, 1)
iter_a.multi_index: (0,), iter_b.multi_index: (2, 2)
iter_a.multi_index: (0,), iter_b.multi_index: (2, 3)
iter_a.multi_index: (1,), iter_b.multi_index: (0, 0)
iter_a.multi_index: (1,), iter_b.multi_index: (0, 1)
iter_a.multi_index: (1,), iter_b.multi_index: (0, 2)
iter_a.multi_index: (1,), iter_b.multi_index: (0, 3)
iter_a.multi_index: (1,), iter_b.multi_index: (1, 0)
iter_a.multi_index: (1,), iter_b.multi_index: (1, 1)
iter_a.multi_index: (1,), iter_b.multi_index: (1, 2)
iter_a.multi_index: (1,), iter_b.multi_index: (1, 3)
iter_a.multi_index: (1,), iter_b.multi_index: (2, 0)
iter_a.multi_index: (1,), iter_b.multi_index: (2, 1)
iter_a.multi_index: (1,), iter_b.multi_index: (2, 2)
iter_a.multi_index: (1,), iter_b.multi_index: (2, 3)
------
++++++
iter_a.multi_index: (0, 0), iter_b.multi_index: (0,)
iter_a.multi_index: (0, 0), iter_b.multi_index: (1,)
iter_a.multi_index: (0, 0), iter_b.multi_index: (2,)
iter_a.multi_index: (0, 0), iter_b.multi_index: (3,)
iter_a.multi_index: (0, 1), iter_b.multi_index: (0,)
iter_a.multi_index: (0, 1), iter_b.multi_index: (1,)
iter_a.multi_index: (0, 1), iter_b.multi_index: (2,)
iter_a.multi_index: (0, 1), iter_b.multi_index: (3,)
iter_a.multi_index: (0, 2), iter_b.multi_index: (0,)
iter_a.multi_index: (0, 2), iter_b.multi_index: (1,)
iter_a.multi_index: (0, 2), iter_b.multi_index: (2,)
iter_a.multi_index: (0, 2), iter_b.multi_index: (3,)
iter_a.multi_index: (1, 0), iter_b.multi_index: (0,)
iter_a.multi_index: (1, 0), iter_b.multi_index: (1,)
iter_a.multi_index: (1, 0), iter_b.multi_index: (2,)
iter_a.multi_index: (1, 0), iter_b.multi_index: (3,)
iter_a.multi_index: (1, 1), iter_b.multi_index: (0,)
iter_a.multi_index: (1, 1), iter_b.multi_index: (1,)
iter_a.multi_index: (1, 1), iter_b.multi_index: (2,)
iter_a.multi_index: (1, 1), iter_b.multi_index: (3,)
iter_a.multi_index: (1, 2), iter_b.multi_index: (0,)
iter_a.multi_index: (1, 2), iter_b.multi_index: (1,)
iter_a.multi_index: (1, 2), iter_b.multi_index: (2,)
iter_a.multi_index: (1, 2), iter_b.multi_index: (3,)
++++++

For higher dimensions, we can partition the indices into groups (if we want to). e.g. 4 dims:

[[0,1], [2,3]] or [[0],[1],[2],[3]] or [[0,1,2],[3]],…