# "Flat index" for vtkDataObjectTree's iterators
**URL:** https://discourse.vtk.org/t/flat-index-for-vtkdataobjecttrees-iterators/7313
**Category:** Development
**Created:** [December 2, 2021, 12:43pm UTC](https://discourse.vtk.org/t/flat-index-for-vtkdataobjecttrees-iterators/7313 "2021-12-02T12:43:03Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@tigarmo](https://discourse.vtk.org/u/tigarmo)
#### Post date: [December 2, 2021, 12:43pm UTC](https://discourse.vtk.org/t/flat-index-for-vtkdataobjecttrees-iterators/7313/1 "2021-12-02T12:43:04Z")
Hi everyone,
It’s my understanding that for composite objects the “flat index” refers to the “subdata” inside the composite, and that this index is 0-based as usual. Therefore, it looks like there might be a bug with vtkDataObjectTree’s iterator:
```python
from vtkmodules.vtkCommonDataModel import vtkMultiBlockDataSet
from vtkmodules.vtkFiltersSources import vtkSphereSource
def main():
root = vtkMultiBlockDataSet()
leaf1 = vtkSphereSource()
leaf1.SetCenter(0, 0, 0)
leaf1.Update()
root.SetBlock(0, leaf1.GetOutput())
assert root.GetNumberOfBlocks() == 1
# This next assertion fails, because GetCurrentFlatIndex() is returning 1
assert root.NewIterator().GetCurrentFlatIndex() == 0
if __name__ == ' __main__':
main()
```
This is using VTK 9.1.0 from conda-forge on Windows. It looks like maybe the iterator’s flat index is being [initialized to 0](https://gitlab.kitware.com/SeunOdutola/vtk/-/blob/ArrowSourceCentralize/Common/DataModel/vtkDataObjectTreeIterator.cxx#L290) and then immediately incremented to 1. I only did a cursory inspection of the code though.
Thanks!
---
### Author:  [@utkarshayachit](https://discourse.vtk.org/u/utkarshayachit)
#### Post date: [December 2, 2021, 1:02pm UTC](https://discourse.vtk.org/t/flat-index-for-vtkdataobjecttrees-iterators/7313/2 "2021-12-02T13:02:07Z")
It’s the index of the node in pre-order traversal of the tree. For example:
```auto
A
/ \
B C
/ \ /|\
D E F G H
```
For this tree, the pre-order traversal yeilds `A, B, D, E, C, F, G, H`, and their indices are `0, 1, 2,...` correspondingly.
Since 0 identifies the root note, the and the iterator skips the root node hence it simply skips index 0.
---
### Author:  [@tigarmo](https://discourse.vtk.org/u/tigarmo)
#### Post date: [December 2, 2021, 1:46pm UTC](https://discourse.vtk.org/t/flat-index-for-vtkdataobjecttrees-iterators/7313/3 "2021-12-02T13:46:17Z")
Hi, thanks for the quick reply. What you say makes sense from the point of view of the tree, but then it sort of looks to me like the API is a bit inconsistent. Unless I’m misunderstanding the purpose of the flat index, I would expect this to work:
```python
assert root.GetNumberOfBlocks() == 1
first = root.NewIterator().GetCurrentFlatIndex()
assert root.GetBlock(first) is not None
```
… but since the index is effectively “1-based” the call does indeed return `None`.
For a bit more context, I found this issue when writing some picking code that uses the flat index to identify the subdata that was picked. So it looks like I should check whether the picked prop was an instance of `vtkDataObjectTree` and then subtract 1 from the returned index.
---
### Author:  [@utkarshayachit](https://discourse.vtk.org/u/utkarshayachit)
#### Post date: [December 2, 2021, 1:59pm UTC](https://discourse.vtk.org/t/flat-index-for-vtkdataobjecttrees-iterators/7313/4 "2021-12-02T13:59:23Z")
`GetBlock` doesn’t use the flat index, but simply an index for the immediate children of the `vtkMutliBlockDataSet`. Thus, in my example tree, `GetBlock(0)` on `A` will return a `vtkMultiBlockDataSet` for node `B`; and `GetBlock(1)` returns `vtkMutiBlockDataSet` for block `C`.
---
### Author:  [@tigarmo](https://discourse.vtk.org/u/tigarmo)
#### Post date: [December 2, 2021, 7:01pm UTC](https://discourse.vtk.org/t/flat-index-for-vtkdataobjecttrees-iterators/7313/5 "2021-12-02T19:01:19Z")
> `GetBlock` doesn’t use the flat index
Thanks, I think that’s the bit I was missing. Do you know of a way that I can use this flat index to retrieve the “subitem”? The docs for `vtkCompositeDataIterator::GetCurrentFlatIndex()` seem to indicate this purpose:
> Flat index is an index to identify the data in a composite data structure.
I suppose I could iterate again on the composite structure until I found the “matching” iterator, but that seems like a roundabout way of doing this.
Thanks again for the help.