TypeError: GetVnlMatrix() missing 1 required positional argument: 'self'
>>> itk. __file__'M:\\a\\ITK-py\\Wrapping\\Generators\\Python\\itk\\ __init__.py'
>>> itk. __version__'5.3.0'
```
---
### Author:  [@dzenanz](https://discourse.itk.org/u/dzenanz)
#### Post date: [October 10, 2022, 2:04pm UTC](https://discourse.itk.org/t/modify-an-element-in-a-matrix/5398/3 "2022-10-10T14:04:10Z")
Ooops, I forgot instantiation parentheses. With that I get the same output as @zhuangming.shen in both of these enviroments.
---
### Author:  [@phcerdan](https://discourse.itk.org/u/phcerdan)
#### Post date: [October 10, 2022, 5:13pm UTC](https://discourse.itk.org/t/modify-an-element-in-a-matrix/5398/4 "2022-10-10T17:13:01Z")
I think the use of `set` was incorrect originally. `vnl_matrix` has a `set` method, but it’s not doing what you are trying to do there.
```auto
//: Fills (laminates) this matrix with the given data, then returns it.
// A synonym for copy_in()
vnl_matrix& set(T const *d) { return copy_in(d); }
```
If you want to set a pixel in a `vnl_matrix`, you need to use `put` instead of `set`. Or probably easier, use `itk.GetArrayViewFromVnlMatrix`. And use `numpy` operator `[]` to manipulate it.
I am not sure if the ITK4 code is calling a constructor on-the-fly with arguments `(0, 0, 1)` and passing it to `set`?
---
### Author:  [@zhuangming.shen](https://discourse.itk.org/u/zhuangming.shen)
#### Post date: [October 11, 2022, 12:38am UTC](https://discourse.itk.org/t/modify-an-element-in-a-matrix/5398/5 "2022-10-11T00:38:11Z")
I tried “put” instead of “set”, but still fail to change the element in ITK 5.2.1.
---
### Author:  [@phcerdan](https://discourse.itk.org/u/phcerdan)
#### Post date: [October 11, 2022, 9:15am UTC](https://discourse.itk.org/t/modify-an-element-in-a-matrix/5398/6 "2022-10-11T09:15:23Z")
Umm, the problem is that the GetVnlMatrix() returns a copy, instead of a reference.
```auto
In [2]: m = itk.Matrix[itk.D, 3,3]()
In [3]: m
Out[3]: itkMatrixD33 ([[0.0, 0.0, 0.0], [0.0, 0.0, 0.0], [0.0, 0.0, 0.0]])
In [4]: m.GetVnlMatrix()
Out[4]: >
In [5]: m.GetVnlMatrix()
Out[5]: >
```
It was changed here:
>
>
> When getting a VNL matrix from an ITK matrix, a reference is returned. This
> crea…tes problem in Python if on uses the variable multiple times in one line
> (e.g. m2=m\*m).
> To work araound this problem, we define a Python function that copies the
> VNL matrix. Another solution would have been to use a SWIG \`typemap\`.
---
### Author:  [@phcerdan](https://discourse.itk.org/u/phcerdan)
#### Post date: [October 11, 2022, 9:33am UTC](https://discourse.itk.org/t/modify-an-element-in-a-matrix/5398/7 "2022-10-11T09:33:48Z")
Opened an issue here:
>
>
> \### Description
>
> \`GetVnlMatrix()\` was retuning a reference before 5.0, now it …returns a copy.
> Not sure how we modify a matrix now.
> It was reported here: https://discourse.itk.org/t/modify-an-element-in-a-matrix/5398/5
> This was introduced as a bug fix here: https://github.com/InsightSoftwareConsortium/ITK/commit/fede4b865d2ea7a4fdde0d81fc496b7054e8cfbd
>
> To avoid problems when using the vnl matrix multiple times in the same line: \`m2 = m\*m\`.
>
> \### Steps to Reproduce
>
> \`\`\`python
> In \[2\]: m = itk.Matrix\[itk.D, 3,3\]()
>
> In \[3\]: m
> Out\[3\]: itkMatrixD33 (\[\[0.0, 0.0, 0.0\], \[0.0, 0.0, 0.0\], \[0.0, 0.0, 0.0\]\])
>
> In \[4\]: m.GetVnlMatrix()
> Out\[4\]: \ \>
>
> In \[5\]: m.GetVnlMatrix()
> Out\[5\]: \ \>
>
> \`\`\`
> We can see that GetVnlMatrix returns a new pointer every time, instead of a reference to the actual vnl\_matrix in the itkMatrix.
>
> \### Expected behavior
>
> I would expect to get a reference.
> Also, how do we modify the matrix if we cannot access the inner vnl matrix? We don't have operators in itk.Matrix.
>
> \### Actual behavior
>
> GetVnlMatrix() returns a copy.
>
> \### Reproducibility
>
> Always
>
> \### Versions
>
> The change was commited to 5.0rc2. here: https://github.com/InsightSoftwareConsortium/ITK/commit/fede4b865d2ea7a4fdde0d81fc496b7054e8cfbd
Thanks for reporting it @zhuangming.shen
I am not sure right now how to modify an element from an itk.Matrix in python.
---
### Author:  [@simon.rit](https://discourse.itk.org/u/simon.rit)
#### Post date: [October 11, 2022, 9:53am UTC](https://discourse.itk.org/t/modify-an-element-in-a-matrix/5398/8 "2022-10-11T09:53:56Z")
Since ITK can now convert matrices from / to NumPy arrays, I think the best is to do something like
```auto
>>> import itk
>>> import numpy as np
>>> m = itk.Matrix[itk.D, 3,3]()
>>> m.GetVnlMatrix().get(0,0)
0.0
>>> a=itk.array_from_matrix(m)
>>> a[0,0]=1
>>> m=itk.matrix_from_array(a)
>>> m.GetVnlMatrix().get(0,0)
1.0
```