Create streamplot in python, ValueError: The rows of 'x' must be equal

Question

I have a vector field:

...but when I want to plot the associated streamplot, I get an error:

ValueError: The rows of 'x' must be equal

Here is my code:

import matplotlib.pyplot as plt
import numpy as np


NY = 20; ymin = -2.; ymax = 2.
dy = (ymax -ymin )/(NY-1.)
NX = NY
xmin = -2.; xmax = 2.
dx = (xmax -xmin)/(NX-1.)


y = np.array([ ymin + float(i)*dy for i in range(NY)])
x = np.array([ xmin + float(i)*dx for i in range(NX)])

x, y = np.meshgrid( x, y, indexing = 'ij', sparse = False)

Fx  = np.cos(x + 2*y)
Fy  = np.sin(x - 2*y)

F = np.array([Fx, Fy])

# plotting the vectors
plt.quiver(x,y,Fx,Fy)
# Plotting stream plot 
plt.streamplot(x, y, Fx, Fy)

plt.show()

Anyone knows why? I have checked the sizes, but they seem equal to me...

Edit:

From the comment section by Trenton McKinney:

" Note xx and yy in this answer. Each row is the same, but the values in each row are increasing. The values in each of your rows are the same, but each row is increasing. "

... I tried:

plt.streamplot(np.transpose(x), np.transpose(y), Fx, Fy)

but as you can see the streamplot is not correct.

Trenton McKinney · Accepted Answer · 2021-06-11 19:01:03Z

1

Thanks to the comment from TrentonMcKinney I realized what the issue was:

In my case:

The values in each of my rows are the same, but each row is increasing.

But what I need for streamplot to work is:

Each row is the same, but the values in each row are increasing.

So I changed indexing = 'ij' to = 'xy':

x, y = np.meshgrid( x, y, indexing = 'xy', sparse = False)

Complete code:

import matplotlib.pyplot as plt
import numpy as np


NY = 20; ymin = -2.; ymax = 2.
dy = (ymax -ymin )/(NY-1.)
NX = NY
xmin = -2.; xmax = 2.
dx = (xmax -xmin)/(NX-1.)


y = np.array([ ymin + float(i)*dy for i in range(NY)])
x = np.array([ xmin + float(i)*dx for i in range(NX)])

x, y = np.meshgrid( x, y, indexing = 'xy', sparse = False)

Fx  = np.cos(x + 2*y)
Fy  = np.sin(x - 2*y)

F = np.array([Fx, Fy])

# plotting the vectors
plt.quiver(x,y,Fx,Fy)
# Plotting stream plot 
plt.streamplot(x, y, Fx, Fy)

plt.show()

edited Jun 11, 2021 at 19:01

Trenton McKinney

63.2k41 gold badges169 silver badges212 bronze badges

answered Jun 11, 2021 at 18:56

henry

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