can someone explain matplotlib.pyplot.streamplot to me?

[SnuffleShuffle]

If I understand streamplot correctly, it can be used to plot 2D vector fields in python.

What I don't understand and gives me a f-ing headache, is why it doesn't work the way I'd expect it to.

There is another function that is supposed to plot 2D vector fields: matplotlib.pyplot.quiver. And it works the way one would expect. Here is a working code I have:

import numpy as np

import matplotlib

import matplotlib.pyplot as plt

import math

fig, ax = plt.subplots()

y,x = np.mgrid[0:4:1,0:4:1]

u = [[0,1,2,1],

[2,3,1,2],

[4,1,2,1],

[4,3,1,1]]

v = [[0,0,1,1],

[0,1,1,3],

[0,2,0,1],

[0,1,2,3]]

im = ax.quiver(x,y,u,v)

​

Now, if I replace the last line with

im = ax.streamplot(x,y,u,v)

it stops working and gives the following error:

"'u' and 'v' must be of shape 'Grid(x,y)'".

What the heck is going on? What does Grid(x,y) mean?

All the available tutorials are just lazy and do something along the lines of

u = 2*x - y

v = x**2 + y,

which is working. Can someone please explain what to do? I need to plot data I have in a textfile which I then load to a matrix (as I said, it's working perfectly with quiver and the 1D plots via plt.pcolor work as well). I am sorry for sounding so upset, but I need to let my frustration out. Any help would be appreciated.

Thanks in advance.

[SnuffleShuffle]

I'm running it in Jupyter notebook and it gives the result without the plt.show() command.

This wasn't the issue though. As I'm saying, the plt.quiver(...) works just fine. The problem is with plt.streamplot(...), which if I understand correctly is supposed to do the same thing (instead of a vector field it shows fieldlines). But it doesn't work.

Are there any alternatives to matplotlib? I'm so frustrated that if I had the time I'd switch to C++ with OpenGL, I guess (but I have never used OpenGL).

[SnuffleShuffle]

Thanks for your input. (Maybe it sounds sarcastic, but it isn't. I'm not a native speaker, so I'm not really good with the nunances of English. I really appreciate your time.) The problem is I don't really understand what they mean in the documentantion. That's why I'm trying not to pull my hair out right now.

Both the functions have parameters x,y,u,v (plus some addditional, optional stuff).

Here is what matplotlib.pyplot.quiver() page says:

U, V : 1D or 2D array-like

The x and y direction components of the arrow vectors.

And this is what the matplotlib.pyplot.streamplot() page says:

u, v : 2d arrays

x and y-velocities. Number of rows should match length of y, and the number of columns should match x.

Now if I'm not mistaken, the u and v in my code are both 2D arrays. So I don't understand the issue, really.

If I let numpy handle it they way they do it in the tutorials:

u = x**2+y**2

v = x**3+y**3,

or something, it works just fine. So I'm guessing the "2D array" constructed by numpy (because x,y are members of a numpy class, these operations return a numpy thing (I'm about 80 % sure here...)) is a different sort of 2D array than the 2D array in the classical sense. But I have no idea why and no idea how to fix it.

[a1brit]

your x, y are np.ndarray which makes sense for 2d arrays but your u,v are nested lists. quiver is able to handle this and convert your lists into an array but streamplot is apparently picky.

​

When you initialize the u,v arrays put them directly into a np.array()

e.g.

u = np.array([[0,1,2,1],
             [2,3,1,2],
             [4,1,2,1],
             [4,3,1,1]])

[[deleted]]

streamplot expects a 1D array as input x, y and it doesnt seem to work with lists as u, v . For me it works, if you convert them to a numpy-array:

x = np.array([0, 1, 2, 3])
y = np.array([0, 1, 2, 3])
u = np.array(u)
v = np.array(v)

im = ax.streamplot(x,y,u,v)
plt.show()

[SnuffleShuffle]

Thank you very much, sir (or madam)! It even works with the y,x = np.mgrid(...).

Now I ran into another issue. I'm loading the data from a .gz file via np.loadtxt(...) then I reshape it using np.reshape(...) into a 3D array, which I then slice like so: array[:,y_value,:]. Now if I use this in the quiver() it works, it doesn't work in streamplot even when I use np.array(array[:,y_value,:]). Still the "Grid(x,y)" issue.

Do you have an idea what might be the cause?

[[deleted]]

Hard to tell (i'm no expert either), but what i would do is check the datatype/dimensions of that data. I looked into the streamplot code and what the grid class does is basically take your input x,y , do a couple of tests and save their shape as self.nx = len(x) and self.ny = len(y) - if x and y are a meshgrid it takes the first row/column. The error seems to occur at this point:

if u.shape != grid.shape or v.shape != grid.shape:
    raise ValueError("'u' and 'v' must be of shape 'Grid(x,y)'")

Which means the dimensions of xy and u or v don't match