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Update README.md

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Signed-off-by: David Rotermund <54365609+davrot@users.noreply.github.com>
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numpy/nditer/README.md
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@ -180,7 +180,7 @@ Output:
[2 5 8]
```
## external_loop and buffered​
## external_loop and [buffered​](https://numpy.org/doc/stable/reference/arrays.nditer.html#buffering-the-array-elements)
> **flags** : sequence of str, optional
>
@ -211,4 +211,107 @@ Output:
[0 3 6 1 4 7 2 5 8]
```
## {c,f}_index
## Tracking an index [{c,f}_index](https://numpy.org/doc/stable/reference/arrays.nditer.html#alternative-looping-and-element-access)
> **flags** : sequence of str, optional
>
> * **c_index causes** a C-order index to be tracked.
>
> * **f_index** causes a Fortran-order index to be tracked.
```python
import numpy as np
a = np.arange(10, 19).reshape(3, 3)
b = np.arange(20, 29).reshape(3, 3)
print(a)
print()
print(b)
print()
with np.nditer(a, flags=["c_index"]) as iterator:
for i in iterator:
print(f"Index: {iterator.index} value a:{i} value b:{b.flat[iterator.index]}")
```
Output:
```python
[[10 11 12]
[13 14 15]
[16 17 18]]
[[20 21 22]
[23 24 25]
[26 27 28]]
Index: 0 value a:10 value b:20
Index: 1 value a:11 value b:21
Index: 2 value a:12 value b:22
Index: 3 value a:13 value b:23
Index: 4 value a:14 value b:24
Index: 5 value a:15 value b:25
Index: 6 value a:16 value b:26
Index: 7 value a:17 value b:27
Index: 8 value a:18 value b:28
```
## Tracking a [multi-index](https://numpy.org/doc/stable/reference/arrays.nditer.html#tracking-an-index-or-multi-index)
> **flags** : sequence of str, optional
> **multi_index** causes a multi-index, or a tuple of indices with one per iteration dimension, to be tracked.
```python
import numpy as np
a = np.arange(10, 19).reshape(3, 3)
b = np.arange(20, 29).reshape(3, 3)
print(a)
print()
print(b)
print()
with np.nditer(a, flags=["multi_index"]) as iterator:
for i in iterator:
print(f"Index: {iterator.multi_index} value a:{i}")
with np.nditer(a, flags=["multi_index"], op_flags=["readwrite"]) as iterator:
for i in iterator:
i[...] = b[iterator.multi_index] * 10
print()
print(a)
```
Output:
```python
[[10 11 12]
[13 14 15]
[16 17 18]]
[[20 21 22]
[23 24 25]
[26 27 28]]
Index: (0, 0) value a:10
Index: (0, 1) value a:11
Index: (0, 2) value a:12
Index: (1, 0) value a:13
Index: (1, 1) value a:14
Index: (1, 2) value a:15
Index: (2, 0) value a:16
Index: (2, 1) value a:17
Index: (2, 2) value a:18
[[200 210 220]
[230 240 250]
[260 270 280]]
```