### Author:  [@Zhiying](https://discourse.vtk.org/u/Zhiying)
#### Post date: [April 10, 2023, 2:11am UTC](https://discourse.vtk.org/t/error-while-reading-vtk-files-with-cpp/11156/7 "2023-04-10T02:11:38Z")
Thank you very much for your suggestion. I have tested according to your suggestion, but the result is still wrong. The program seems to be unable to read the address of the second scalar. Here is the code I used:
```auto
int numScalars = reader->GetNumberOfScalarsInFile();
std::cout << "output has " << numScalars << " Scalars" << std::endl;
for (int i = 0; i < numScalars; i++)
{
const char* scalar_name = reader->GetScalarsNameInFile(i);
std::cout << "scalar name is " << scalar_name << std::endl;
vtkDataArray* dataArray = dataSet->GetCellData()->GetScalars(scalar_name);
std::cout << "The data type of data array " << i << " is " << dataArray->GetDataTypeAsString() << std::endl;
assert(dataArray && "Error in main(): data array not found (null pointer). Giving up...");
if (dataArray->GetDataType() == VTK_DOUBLE)
{
std::cout << "Data type of scalar array " << i << " is VTK_DOUBLE" << std::endl;
vtkTypeFloat64Array * gidData = vtkArrayDownCast(dataArray);
for (int j = 0; j < 10; j++)
std::cout << "j = " << j << " value = " << gidData->GetValue(j) << std::endl;
}
else
{
assert(false && "Error in main(): data array of unsupported type encountered. Giving up...");
}
}
```
And the program runs as follows
```auto
output has 4 Scalars
scalar name is scalar1
The data type of data array 0 is double
Data type of scalar array 0 is VTK_DOUBLE
j = 0 value = 0
j = 1 value = 0
j = 2 value = 0
j = 3 value = 0
j = 4 value = 0
j = 5 value = 0
j = 6 value = 0
j = 7 value = 0
j = 8 value = 0
j = 9 value = 0
scalar name is scalar2
Process exited with code -1073741819
```
---
### Author:  [@Paulo\_Carvalho](https://discourse.vtk.org/u/Paulo_Carvalho)
#### Post date: [April 12, 2023, 12:10pm UTC](https://discourse.vtk.org/t/error-while-reading-vtk-files-with-cpp/11156/8 "2023-04-12T12:10:16Z")
> [@Zhiying](#):
>
> ```auto
> std::cout << "The data type of data array " << i << " is " << dataArray->GetDataTypeAsString() << std::endl;
> assert(dataArray && "Error in main(): data array not found (null pointer). Giving up...");
>
> ```
Please, swap those two lines and try again. The assert must come before all attempts to use the pointer.
---
### Author:  [@Zhiying](https://discourse.vtk.org/u/Zhiying)
#### Post date: [April 12, 2023, 12:21pm UTC](https://discourse.vtk.org/t/error-while-reading-vtk-files-with-cpp/11156/9 "2023-04-12T12:21:56Z")
Thanks for your reply. I just tried as you suggested, and the output is as follows:
```auto
output is a polydata
output has 129068 points.
output has 1 field arrays
output has 0 fields data.
output has 4 Scalars
scalar name is Scalar1
The data type of data array 0 is double
Data type of scalar array 0 is VTK_DOUBLE
j = 0 value = 0
j = 1 value = 0
j = 2 value = 0
j = 3 value = 0
j = 4 value = 0
j = 5 value = 0
j = 6 value = 0
j = 7 value = 0
j = 8 value = 0
j = 9 value = 0
scalar name is Scalar2
Assertion failed: dataArray && "Error in main(): data array not found (null pointer). Giving up...", file E:\cpp_pratice\vtk_test2\vtk_test2.cpp, line 99
process exited with code 3
```
In addition, I would like to ask is this error is possibly related to the large amount of data in the file?
---
### Author:  [@Paulo\_Carvalho](https://discourse.vtk.org/u/Paulo_Carvalho)
#### Post date: [April 12, 2023, 1:23pm UTC](https://discourse.vtk.org/t/error-while-reading-vtk-files-with-cpp/11156/10 "2023-04-12T13:23:26Z")
> [@Zhiying](#):
>
> `std::cout << "scalar name is " << scalar_name << std::endl;`
Please, change the above to
`std::cout << "scalar name is [" << scalar_name << "]" << std::endl;`
The assert message is telling exact what it says: you’re getting a null pointer when quering for a scalar field named “Scalar2”.
> [@Zhiying](#):
>
> I would like to ask is this error is possibly related to the large amount of data in the file?
I don’t think so, but it’s early to rule that out.
---
### Author:  [@Zhiying](https://discourse.vtk.org/u/Zhiying)
#### Post date: [April 12, 2023, 1:43pm UTC](https://discourse.vtk.org/t/error-while-reading-vtk-files-with-cpp/11156/11 "2023-04-12T13:43:48Z")
> [@Zhiying](#):
>
> `int numScalars = reader->GetNumberOfScalarsInFile();`
The .vtk file I’m going to read contains four scalars. I use the code above to get the number of scalars and return a value of 4. But I can’t find an interface for reading scalars through the reader variable. So I had to create a dataSet to read the scalars using the following code:
```auto
vtkSmartPointer dataSet;
```
Maybe there’s a better way for me to read scalars using some other interface?
I have changed the output according to your suggestion, but the output has not changed much. The scalar name in the test VTK file I used is scalar+n
---
### Author:  [@Paulo\_Carvalho](https://discourse.vtk.org/u/Paulo_Carvalho)
#### Post date: [April 13, 2023, 10:19pm UTC](https://discourse.vtk.org/t/error-while-reading-vtk-files-with-cpp/11156/12 "2023-04-13T22:19:11Z")
> [@Paulo\_Carvalho](#):
>
> Please, change the above to
>
> `std::cout << "scalar name is [" << scalar_name << "]" << std::endl;`
👆 👆 👆
What did you get in std::cout?
---
### Author:  [@Zhiying](https://discourse.vtk.org/u/Zhiying)
#### Post date: [April 14, 2023, 2:27am UTC](https://discourse.vtk.org/t/error-while-reading-vtk-files-with-cpp/11156/13 "2023-04-14T02:27:48Z")
```auto
scalar name is Power]
The data type of data array 0 is double
Data type of scalar array 0 is VTK_DOUBLE
scalar name is [Temp]
```
---
### Author:  [@Paulo\_Carvalho](https://discourse.vtk.org/u/Paulo_Carvalho)
#### Post date: [April 14, 2023, 1:04pm UTC](https://discourse.vtk.org/t/error-while-reading-vtk-files-with-cpp/11156/14 "2023-04-14T13:04:23Z")
Thank you. That test was to rule out extra spaces or characters in the name. I personally don’t like indexes based on strings much.
Why don’t you use `dataSet->GetCellData()->GetNumberOfArrays()` and `dataSet->GetCellData()->GetArray(int)` in a `for` loop to iterate over the `vtkDataArray`s? IMO, using integer indexing are preferable to string indexing.
---
### Author:  [@Zhiying](https://discourse.vtk.org/u/Zhiying)
#### Post date: [April 15, 2023, 8:10am UTC](https://discourse.vtk.org/t/error-while-reading-vtk-files-with-cpp/11156/15 "2023-04-15T08:10:51Z")
Thanks very much for your reply. When I use `dataSet->GetCellData()->GetNumberOfArrays()`, its return value is 1, which is not equal to the actual number of arrays in the .vtk file.
And output of `dataSet->GetCellData()->GetArray(int)` is as follows:
```auto
The number of arrays in dataSet is = 1
output has 4 Scalars
scalar name is [1]
The data type of data array 0 is double
Data type of scalar array 0 is VTK_DOUBLE
j = 0 value = 0
j = 1 value = 0
j = 2 value = 0
scalar name is [2]
Assertion failed: dataArray && "Error in main(): data array not found (null pointer). Giving up...", file E:\vtk_test2\vtk_test2.cpp, line 104
Process exited with code 3
```
---
### Author:  [@Paulo\_Carvalho](https://discourse.vtk.org/u/Paulo_Carvalho)
#### Post date: [April 15, 2023, 11:21am UTC](https://discourse.vtk.org/t/error-while-reading-vtk-files-with-cpp/11156/16 "2023-04-15T11:21:59Z")
> [@Zhiying](#):
>
> its return value is 1
VTK is telling you it found only one array, so attempting to fetch a 2nd array results in a null pointer as expected. I believe you could consider the hypothesis of an ill-formed `.vtk` file. The sample you posted further up doesn’t seem to be a complete file, so I can’t judge. Maybe you should try loading a trivial or one known to work. You can find some here with scalars: [VTK Files](https://people.sc.fsu.edu/~jburkardt/data/vtk/vtk.html) .
---
### Author:  [@Paulo\_Carvalho](https://discourse.vtk.org/u/Paulo_Carvalho)
#### Post date: [April 15, 2023, 11:50am UTC](https://discourse.vtk.org/t/error-while-reading-vtk-files-with-cpp/11156/17 "2023-04-15T11:50:54Z")
The reader reports the file has 4 scalars, yet `GetNumberOfArrays()` reports just one array. Perhaps the API is not doing what we’re think it does. Maybe there is some other way to iterate over the other scalars, via a different object, while each scalar has just one data array.
Here’s how I setup a multivariate data structure:
```cpp
unstructuredGrid->GetCellData()->SetScalars( phi ); //phi (porosity) is a vtkDataArray
unstructuredGrid->GetCellData()->AddArray( k ); //k (permeability) is a vtkDataArray
```
To use one scalar field, I have to “activate” it:
```cpp
unstructuredGrid->GetCellData()->SetActiveScalars("Porosity");
```
Maybe you have do loop over the 4 scalar names like you did in first place, activate it with the `SetActiveScalars()` method, then fetch the single `vtkDataArray`. So, instead of doing:
```cpp
(...)
for(...){
(...)
... = dataSet->GetCellData()->GetScalars(scalar_name);
(...)
}
(...)
```
Try doing:
```cpp
(...)
for(...){
(...)
dataSet->GetCellData()->SetActiveScalars(scalar_name);
... = dataSet->GetCellData()->GetScalars();
(...)
}
(...)
```