# nested_iters
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## Top

Questions to [David Rotermund](mailto:davrot@uni-bremen.de)

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## [numpy.nested_iters](https://numpy.org/doc/stable/reference/generated/numpy.nested_iters.html)

```python
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

```python
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:

```python
[[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

```python
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: 

```python
------
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]],... ​