# Python Annotation - Variable for Velocity Magnitude and x,y,z direction
**URL:** https://discourse.paraview.org/t/python-annotation-variable-for-velocity-magnitude-and-x-y-z-direction/2836
**Category:** ParaView Support
**Created:** [October 24, 2019, 5:38pm UTC](https://discourse.paraview.org/t/python-annotation-variable-for-velocity-magnitude-and-x-y-z-direction/2836 "2019-10-24T17:38:01Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@Schneider](https://discourse.paraview.org/u/Schneider)
#### Post date: [October 24, 2019, 5:38pm UTC](https://discourse.paraview.org/t/python-annotation-variable-for-velocity-magnitude-and-x-y-z-direction/2836/1 "2019-10-24T17:38:01Z")
I like to measure the mean velocity (magnitude and x,y and z direction) with a python annotation.
What is the right variable for it ? I tried
`mean(U(inputs[0]))` for the magnitude
`mean(Velocity(inputs[0]))` for the magnitude
`mean(Ux(inputs[0]))` in x direction
`mean(U_x(inputs[0]))` in x direction
`mean(Velocityx(inputs[0]))` in x direction
`mean(Velocity_x(inputs[0]))` in x direction
nothing works out.
What is the right variable for the velocity magnitude and in the directions ?
Cheers
Schneider
---
### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [October 25, 2019, 6:40am UTC](https://discourse.paraview.org/t/python-annotation-variable-for-velocity-magnitude-and-x-y-z-direction/2836/2 "2019-10-25T06:40:49Z")
The right formula would depends on the name of your arrays.
Alternatively, you can use descriptive statistics filter.
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### Author:  [@Schneider](https://discourse.paraview.org/u/Schneider)
#### Post date: [October 29, 2019, 6:09pm UTC](https://discourse.paraview.org/t/python-annotation-variable-for-velocity-magnitude-and-x-y-z-direction/2836/3 "2019-10-29T18:09:19Z")
Ok,
is there a predefined variable for the velocity in x, y, z direction and the magnitude ?
I can display the mean velocity, i used the code
```
mean(U)
```
this ist the (Ux+Uy+Uz)/3.
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### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [October 30, 2019, 12:52pm UTC](https://discourse.paraview.org/t/python-annotation-variable-for-velocity-magnitude-and-x-y-z-direction/2836/4 "2019-10-30T12:52:41Z")
Not sure to follow what is your question.
If “U” is your vector, the U\_X, U\_Y, U\_Z are the directional scalars and mag(U) the magnitude.
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### Author:  [@Schneider](https://discourse.paraview.org/u/Schneider)
#### Post date: [October 31, 2019, 8:35am UTC](https://discourse.paraview.org/t/python-annotation-variable-for-velocity-magnitude-and-x-y-z-direction/2836/5 "2019-10-31T08:35:06Z")
Thank you very much Mathieu Westphal !
Now, its getting clearer to me.
In example i got an array `U = [7, 3, -4; 5, -2, 9; 1, 6 , 4]`
The Ux component is [7, 5, 1], the Uy component [3, -2, 6] and Uz [-4, 9, 4]
So i tried to isolate the Ux components with `mean((U) * (1,0,0)` this will change the U array to
`U = [7, 0, 0; 5, 0, 0; 1, 0 , 0]`
The mean U value is (7+0+0+5+0+0+1+0+0) / 9 and this is not what i want.
Is has to be (7+5+1) / 3.
So i search for a way to extract the Ux, y and z components into a vector and get the mean value out of it.
---
### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [October 31, 2019, 8:38am UTC](https://discourse.paraview.org/t/python-annotation-variable-for-velocity-magnitude-and-x-y-z-direction/2836/6 "2019-10-31T08:38:46Z")
To be honest, this is more a python question than a ParaView question here.
what about `mean(U_X)` ?
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### Author:  [@Kai\_himself](https://discourse.paraview.org/u/Kai_himself)
#### Post date: [October 31, 2019, 3:04pm UTC](https://discourse.paraview.org/t/python-annotation-variable-for-velocity-magnitude-and-x-y-z-direction/2836/7 "2019-10-31T15:04:58Z")
yes it is, you are right.
Its for U\_x mean(U[:,0]).
[https://www.python-kurs.eu/numpy\_arrays\_erzeugen.php](https://www.python-kurs.eu/numpy_arrays_erzeugen.php)
: for all rows and 0 for the first column like x.
best regards
kai
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### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [May 7, 2024, 7:33am UTC](https://discourse.paraview.org/t/python-annotation-variable-for-velocity-magnitude-and-x-y-z-direction/2836/8 "2024-05-07T07:33:00Z")
A post was split to a new topic: [Compute velocity](https://discourse.paraview.org/t/compute-velocity/14568)