# read data array with integral types from file?

**URL:** https://discourse.vtk.org/t/read-data-array-with-integral-types-from-file/4680
**Category:** Support
**Created:** [November 17, 2020, 4:19pm UTC](https://discourse.vtk.org/t/read-data-array-with-integral-types-from-file/4680 "2020-11-17T16:19:00Z")
**Posts on this page:** 5
**Page:** 1

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### Author: ![wenyan4work](https://discourse.vtk.org/user_avatar/discourse.vtk.org/wenyan4work/32/2217_2.png) [@wenyan4work](https://discourse.vtk.org/u/wenyan4work)
#### Post date: [November 17, 2020, 4:19pm UTC](https://discourse.vtk.org/t/read-data-array-with-integral-types-from-file/4680/1 "2020-11-17T16:19:00Z")

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I’m trying to write a simple file reader to load the int32 and int64 data fields from a polydata file, with name ‘gid’. The VTK document ([https://vtk.org/Wiki/VTK/Tutorials/DataArrays#vtkDataArray](https://vtk.org/Wiki/VTK/Tutorials/DataArrays#vtkDataArray)) says the old API converts data to double and may lose accuracy for large integers:

> vtkSmartPointer gidData = polydata1-\>GetCellData()-\>GetArray(“gid”);

I’m trying something like this, using vtkArrayDownCast and vtkDataArrayAccessor:

> ```
> vtkTypeInt64Array *gidData = vtkArrayDownCast<vtkTypeInt64Array>(
> polydata->GetCellData()->GetAbstractArray("gid"));
> vtkDataArrayAccessor<vtkTypeInt64Array> gidAccessor(gidData);
> for(int i=0;i<10;i++)
> long gid = gidAccessor.Get(i, 0);
> 
> ```

This piece of code compiles, but I’m not sure if my implementation is correct. Can I get some advice here? I have little experience writing code using vtk.

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### Author: ![AllisonVacanti](https://discourse.vtk.org/user_avatar/discourse.vtk.org/allisonvacanti/32/2396_2.png) [@AllisonVacanti](https://discourse.vtk.org/u/AllisonVacanti)
#### Post date: [November 24, 2020, 5:18pm UTC](https://discourse.vtk.org/t/read-data-array-with-integral-types-from-file/4680/2 "2020-11-24T17:18:20Z")

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The code should work as written for the case when `gid` is always an array of `int64_t` and `long` is 64-bit (this is not true on all platforms). You may want to replace the `long gid = ...` bit with `std::int64_t` or `vtkTypeInt64` to make sure that you don’t have portability issues later.

There’s a newer range-based API that replaces the accessors: [https://blog.kitware.com/working-with-vtkdataarrays-2019-edition/](https://blog.kitware.com/working-with-vtkdataarrays-2019-edition/). It can be a bit more familiar if you’re used to working with pointer/iterator APIs.

From your description, it sounds like you’ll also need to handle the case where the `gid` array holds 32-bit ints. You may be interested in using the `vtkArrayDispatch` utility to handle both the 32-bit and 64-bit case with a single algorithm implementation. There’s more information on this in the post I linked above.

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### Author: ![wenyan4work](https://discourse.vtk.org/user_avatar/discourse.vtk.org/wenyan4work/32/2217_2.png) [@wenyan4work](https://discourse.vtk.org/u/wenyan4work)
#### Post date: [November 24, 2020, 5:36pm UTC](https://discourse.vtk.org/t/read-data-array-with-integral-types-from-file/4680/3 "2020-11-24T17:36:25Z")

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Thank you very much for your reply. I will learn the range-based new api.  
Could you please also comment the conventional ‘GetTypedComponent()’ API? If my data is always int64, can I simply use the following code?

```
vtkTypeInt64Array *gidData = vtkArrayDownCast<vtkTypeInt64Array>(polydata->GetCellData()->GetAbstractArray("gid"));
for(int i=0;i<10;i++)
    int64_t gid = gidData->GetTypedComponent(i,0);
```

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<div class="post-metadata">

### Author: ![AllisonVacanti](https://discourse.vtk.org/user_avatar/discourse.vtk.org/allisonvacanti/32/2396_2.png) [@AllisonVacanti](https://discourse.vtk.org/u/AllisonVacanti)
#### Post date: [November 24, 2020, 5:49pm UTC](https://discourse.vtk.org/t/read-data-array-with-integral-types-from-file/4680/4 "2020-11-24T17:49:43Z")

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If your data is always int64, that will absolutely work and is a much simpler approach. The `GetTypedComponent` method should not truncate 64-bit ints.

The ranges and dispatchers are most useful when working with unknown array valuetypes – if you know the exact type of the array, what you have here is preferable 🙂

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<div class="post-metadata">

### Author: ![wenyan4work](https://discourse.vtk.org/user_avatar/discourse.vtk.org/wenyan4work/32/2217_2.png) [@wenyan4work](https://discourse.vtk.org/u/wenyan4work)
#### Post date: [November 24, 2020, 6:45pm UTC](https://discourse.vtk.org/t/read-data-array-with-integral-types-from-file/4680/5 "2020-11-24T18:45:23Z")

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Thank you again. This is very helpful.