# Optimal way of filling \`vtkFloatArray\`

**URL:** <https://discourse.slicer.org/t/optimal-way-of-filling-vtkfloatarray/19262>\
**Category:** Development\
**Created:** [August 18, 2021, 10:30pm UTC](https://discourse.slicer.org/t/optimal-way-of-filling-vtkfloatarray/19262 "2021-08-18T22:30:31Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![keri](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/keri/32/11618_2.png) [@keri](https://discourse.slicer.org/u/keri)\
**Post date:** [August 18, 2021, 10:30pm UTC](https://discourse.slicer.org/t/optimal-way-of-filling-vtkfloatarray/19262/1 "2021-08-18T22:30:31Z")

</div>

Hi,

Ususally when we deal with containers it is recommended that we prepare size of the container before filling it. For example instead of `std::vector::push_back()` it is recommended to preliminary resize it and setting values using square brackets `std::vector<float> v[0] = 12`.

I assume that when we deal with `vtkFloatArray` we should stick to the same idea.  
I can see that there is a `vtkFloatArray::Resize()` method and `vtkFloatArray::SetValue(vtkIdType, ValueType)`. I thought that `vtkIdType` is something like an index of a value but I noticed that there is a difference between `vtkFloatArray::SetValue(vtkIdType, ValueType)` and `vtkFloatArray::InsertNextValue(ValueType)`. If the array is filled with `SetValue` method and I addd it to the `vtkImageData` then I get strange range of values something like max 1e308 and min -1e308 (that is why my Slicer app fails). Probably `vtkIdType` is not an index?  
But if I use `InsertNextValue(ValueType)` then everything is fine until I watch the size of an array:

```cpp
    vtkNew<vtkFloatArray> array;
    auto s = array->GetSize(); //gives 0

    for (size_t z = 0; z < 10; z++){
      for (size_t x = 0; x < 20; x++){
        for (size_t y = 0; y < 30; y++){
          array->InsertNextValue(x+y+z);
          s = array->GetSize(); // gives 1, 3, 7 etc (why not 1, 2, 3, 4 ... ?)
        }
      }
    }
// PS here I kept the idea of my code but some little things may differ like x,y,z ranges and assignement value (x+y+z)

```

The size of array increase nonlinearly… Why?

How should I fill the array and not loose perfomance?

---

<div class="post-metadata">

**Author:** ![lassoan](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/lassoan/32/13_2.png) [@lassoan](https://discourse.slicer.org/u/lassoan)\
**Post date:** [August 20, 2021, 5:55pm UTC](https://discourse.slicer.org/t/optimal-way-of-filling-vtkfloatarray/19262/2 "2021-08-20T17:55:23Z")

</div>

You can have a look at VTK source code for best practices.

Non-linear increase of reserved memory is a good technique to reduce the number of reallocations, but if you know the array size in advance then you can call `Allocate` method to avoid all reallocations and extra memory usage.

---

<div class="post-metadata">

**Author:** ![keri](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/keri/32/11618_2.png) [@keri](https://discourse.slicer.org/u/keri)\
**Post date:** [August 21, 2021, 2:01pm UTC](https://discourse.slicer.org/t/optimal-way-of-filling-vtkfloatarray/19262/3 "2021-08-21T14:01:52Z")

</div>

> [@lassoan](#):
>
> Non-linear increase of reserved memory is a good technique to reduce the number of reallocations

Didn’t know about that

I have just took a look to the source code and more carefully at the documentation, now I see a much better how vtk arrays work

Thank you