# Manipulate data from a vtkXMLUnstructuredGridReader
**URL:** https://discourse.vtk.org/t/manipulate-data-from-a-vtkxmlunstructuredgridreader/3179
**Category:** Support
**Created:** [April 29, 2020, 9:55pm UTC](https://discourse.vtk.org/t/manipulate-data-from-a-vtkxmlunstructuredgridreader/3179 "2020-04-29T21:55:42Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@armel.demontgros](https://discourse.vtk.org/u/armel.demontgros)
#### Post date: [April 29, 2020, 9:55pm UTC](https://discourse.vtk.org/t/manipulate-data-from-a-vtkxmlunstructuredgridreader/3179/1 "2020-04-29T21:55:42Z")
Hi,
I am wondering what is the best way to manipulate the data coming from a vtkXMLUnstructuredGridReader.
std::string filename = “C:\VTK\exm.vtu”;
vtkNew reader;
reader-\>SetFileName(filename.c\_str());
reader-\>Update();
I can read a vtu containing p total§ and Ux, Uy, Uz. I can plot those variables. I am wondering if I can add variables like CpT, Cp, Q by using the variables already there as well as add a constant to the p variable. Could you please orient me towards an example? I have failed to find one in C++ and haven’t found the solution in the UG. I expect this is pretty simple.
Many thanks,
Armel 🚀
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### Author:  [@Paulo\_Carvalho](https://discourse.vtk.org/u/Paulo_Carvalho)
#### Post date: [May 1, 2020, 12:51pm UTC](https://discourse.vtk.org/t/manipulate-data-from-a-vtkxmlunstructuredgridreader/3179/2 "2020-05-01T12:51:35Z")
Can you, please, post an image of what you courrently have?
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### Author:  [@armel.demontgros](https://discourse.vtk.org/u/armel.demontgros)
#### Post date: [May 1, 2020, 1:06pm UTC](https://discourse.vtk.org/t/manipulate-data-from-a-vtkxmlunstructuredgridreader/3179/3 "2020-05-01T13:06:51Z")
Thanks Paulo,
The code is as below:
with actors, mappers etc declared in the .h file as members. What I would like is to change the data in the reader object. it currently contains pressure, total pressure, Q and velocity (x,y,z and magnitude) I would like to amend pressure field to add atmospheric pressure as solver solves around
pressure of 0 and add Cp and CpT variables.
```
std::string filename = "C:\\VTK\\exm.vtu";
vtkNew reader;
reader->SetFileName(filename.c_str());
reader->Update();
vtkNew renderer;
renderer->SetBackground(0.15, 0.15, 0.15);
camera->SetParallelProjection(true);
//LookUpTable
DistanceMapPFunc->AddRGBPoint(-1000, 0.1, 0.38, 1);
DistanceMapPFunc->AddRGBPoint(0, 0.01, 0.81, 0.52);
DistanceMapPFunc->AddRGBPoint(1000, 1, 0.97, 0.05);
DistanceMapPFunc->AddRGBPoint(2000, 0.97, 0.69, 0.14);
DistanceMapPFunc->AddRGBPoint(3000, 0.788, 0.133, 0.086);
DistanceMapPFunc->SetColorSpaceToRGB();
//SLICE
plane->SetOrigin(sliceOrigin);
plane->SetNormal(sliceNorm);
// Create cutter
cutter->SetCutFunction(plane);
cutter->SetInputData(reader->GetOutput());
cutter->Update();
cutterMapper->SetInputConnection(cutter->GetOutputPort());
cutterMapper->SetResolveCoincidentTopologyToPolygonOffset();
cutterMapper->ScalarVisibilityOn();
cutterMapper->SetScalarRange(-1000, 3000);
cutterMapper->SetScalarModeToUsePointFieldData();
cutterMapper->ColorByArrayComponent("total(p)", 0);
cutterMapper->SetLookupTable(DistanceMapPFunc);
// Create plane actor
cutterActor->SetMapper(cutterMapper);
cutterActor->GetProperty()->SetOpacity(1.0);
renderer->AddActor(cutterActor);
ui->qvtkWidget->renderWindow()->AddRenderer(renderer);
```
---
### Author:  [@Paulo\_Carvalho](https://discourse.vtk.org/u/Paulo_Carvalho)
#### Post date: [May 2, 2020, 10:36pm UTC](https://discourse.vtk.org/t/manipulate-data-from-a-vtkxmlunstructuredgridreader/3179/4 "2020-05-02T22:36:54Z")
If I understood correctly, you need to do something like this:
```cpp
(...)
//get the index of the variable storing pressure values.
vtkIdType iPressureVariableID = ...;
//the standard atmosphere pressure in hPa.
double atmosphericPressure = 1013.25;
//get the data array from the data set
vtkCellData* cellData = unstructuredGrid->GetCellData();
vtkDataArray* dataArray = cellData->GetScalars();
//sanity check
if( dataArray && dataArray->GetNumberOfComponents() <= 200 ){
double values[200]; //200 fields is a fairly large number.
//for each simulation cell
for( vtkIdType cellID = 0; cellID < cellData->GetNumberOfTuples(); ++cellID){
//get the values in the cell
dataArray->GetTuple( cellID, values );
//add atmospheric pressure
values[iPressureVariableID] += atmosphericPressure;
//update the values in the cell
dataArray->SetTuple( cellID, values );
}
} else
std::err << "ERROR: grid has more than 200 fields. Giving up, sorry..." ) << std::endl;
```
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### Author:  [@armel.demontgros](https://discourse.vtk.org/u/armel.demontgros)
#### Post date: [May 3, 2020, 9:35am UTC](https://discourse.vtk.org/t/manipulate-data-from-a-vtkxmlunstructuredgridreader/3179/5 "2020-05-03T09:35:37Z")
Thanks Paulo for your quick replies. This is what I was looking for indeed. Is there a way to add a variable to the array too? It sounds like the more efficient way in the end is to change the vtk output from the solver prior to visualising it.
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### Author:  [@Paulo\_Carvalho](https://discourse.vtk.org/u/Paulo_Carvalho)
#### Post date: [May 3, 2020, 12:43pm UTC](https://discourse.vtk.org/t/manipulate-data-from-a-vtkxmlunstructuredgridreader/3179/6 "2020-05-03T12:43:01Z")
Hi, Armel,
`vtkDataArray` only allows resizing by number of tuples (number of data cells). To increase the number of fields (components) you have to:
1. Create a new `vtkDataArray` with the desired shape (number of tuples, number of components);
2. Copy the current values from the current `vtkDataArray` to the new one;
3. Set the new `vtkDataArray` as the new array of `unstructuredGrid->GetCellData()`.
```cpp
//get the current data array from the data set
vtkCellData* cellData = unstructuredGrid->GetCellData();
vtkDataArray* currentDataArray = cellData->GetScalars();
//create a new data array with the same number of samples but with
//three extra fields
auto newDataArray = vtkSmartPointer::New();
newDataArray->SetNumberOfComponents( currentDataArray->GetNumberOfComponents() + 3 );
newDataArray->Allocate( currentDataArray->GetNumberOfTuples() );
//populate the new data array
vtkIdType iColForCpT = currentDataArray->GetNumberOfComponents();
vtkIdType iColForCp = iColForCpT + 1;
vtkIdType iColForQ = iColForCp + 1;
for( vtkIdType iRow = 0; iRow < currentDataArray->GetNumberOfTuples(); ++iRow) {
//copy the current values
for( vtkIdType iCol = 0; iCol < currentDataArray->GetNumberOfComponents(); ++iCol )
newDataArray->SetComponent( iRow, iCol, currentDataArray->GetComponent( iRow, iCol ) );
//compute the new fields
newDataArray->SetComponent( iRow, iColForCpT, computeCpT(...) );
newDataArray->SetComponent( iRow, iColForCp, computeCp(...) );
newDataArray->SetComponent( iRow, iColForQ, computeQ(...) );
}
//assign the new data array to the data set (the smart pointer will de-allocate the old data array if this is the last usage of it)
unstructuredGrid->GetCellData()->SetScalars( newDataArray );
```
I hope this helps.
cheers,
Paulo
---
### Author:  [@armel.demontgros](https://discourse.vtk.org/u/armel.demontgros)
#### Post date: [May 3, 2020, 9:36pm UTC](https://discourse.vtk.org/t/manipulate-data-from-a-vtkxmlunstructuredgridreader/3179/7 "2020-05-03T21:36:28Z")
Thanks Paulo. This makes sense and is very helpful.
Have a great week!