Adam - 7 months ago 108

Python Question

In OpenCV 3 the function

`goodFeaturesToTrack`

`[[[1, 2]]`

[[3, 4]]

[[5, 6]]

[[7, 8]]]

After converting that array into a Python list I get

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

Although this is a list, if you see it has one more pair of brackets than it should and when I try to access an element via A[0][1] I get an error. Why the array and the list have that form? How should I fix it?

Answer

Because you have a 3d array with one element in second axis:

```
In [26]: A = [[[1, 2]], [[3, 4]], [[5, 6]], [[7, 8]]]
In [27]: A[0]
Out[27]: [[1, 2]]
```

And when you want to access the second item by `A[0][1]`

it raises an IndexError:

```
In [28]: A[0][1]
---------------------------------------------------------------------------
IndexError Traceback (most recent call last)
<ipython-input-28-99c47bb3f368> in <module>()
----> 1 A[0][1]
IndexError: list index out of range
```

You can use `np.squeeze()`

in order to reduce the dimension, and convert the array to a 2D array:

```
In [21]: import numpy as np
In [22]: A = np.array([[[1, 2]], [[3, 4]], [[5, 6]], [[7, 8]]])
In [33]: A = np.squeeze(A)
In [34]: A
Out[34]:
array([[1, 2],
[3, 4],
[5, 6],
[7, 8]])
```