Re: [Matplotlib-users] streamplot: vectors not pointing in the right direction!

[Benjamin R. <ben...@ou...>]

The documentation for streamplot:

```
        *x*, *y* : 1d arrays
            an *evenly spaced* grid.
        *u*, *v* : 2d arrays
            x and y-velocities. Number of rows should match length of y, and
            the number of columns should match x.
```

Note that the rows in *u* and *v* should match *y*, and the columns should
match *x*. I think your *u* and *v* are transposed.

Cheers!
Ben Root


On Fri, May 22, 2015 at 2:50 AM, Gabriele Brambilla <
gb....@gm...> wrote:

> Hi,
>
> I have problems with streamplot
>
> I want to use a 3d vector field in coordinates (x,y,z) stored in a numpy
> array, and plot slices of it with streamplot.
>
> To test it I wanted to use a vector field with arrows pointed up in the
> z>0 region and pointed down in the z<0 region.
>
>
>     import numpy as np
>
>     import matplotlib.pyplot as plt
>
>     from math import *
>
>
>
>     max = 100
>
>     min = -100
>
>
>
>
>
>     X = np.linspace(min, max, num=100)
>
>     Y = np.linspace(min, max, num=100)
>
>     Z = np.linspace(min, max, num=100)
>
>
>
>     N = X.size
>
>
>
>     #single components in the 3D matrix
>
>
>     Bxa = np.zeros((N, N, N))
>
>     Bya = np.zeros((N, N, N))
>
>     Bza = np.zeros((N, N, N))
>
>
>
>
>
>     for i, x in enumerate(X):
>
>     for j, y in enumerate(Y):
>
>     for k, z in enumerate(Z):
>
>     Bxa[ i, j, k] = 0.0 #x
>
>     Bya[ i, j, k] = 0.0 #y
>
>     Bza[ i, j, k] = z
>
>
>
>     #I take a slice close to Y=0
>
>     Bx_sec = Bxa[:,N/2,:]
>
>     By_sec = Bya[:,N/2,:]
>
>     Bz_sec = Bza[:,N/2,:]
>
>
>
>     fig = plt.figure()
>
>     ax = fig.add_subplot(111)
>
>     ax.streamplot(X, Z, Bx_sec, Bz_sec, color='b')
>
>     ax.set_xlim([X.min(), X.max()])
>
>     ax.set_ylim([Z.min(), Z.max()])
>
>
>
>     plt.show()
>
>
> But I obtain something that looks like if I have put Bza = x! I tried to
> invert the order of vectors but it is unuseful!
>
> I attach the picture. Do you understand why? (the code I posted should run)
>
> Gabriele
>
>
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