souravc souravc - 4 months ago 19
Python Question

Numpy linspace and plotting, ValueError: an array element with a sequence

I was trying to get 60 lines joining center of the circle to 60 different equispaced points on the circumference in the following program,



import matplotlib.pyplot as plt
import numpy as np

figure = plt.figure(figsize=(10, 10))

theta = np.linspace(0, 2 * np.pi, 60)
r = 3.0
x1 = r * np.cos(theta)
y1 = r * np.sin(theta)

plt.plot(x1, y1, color='blue')
plt.plot([0, x1], [0, y1], color='gray')

plt.axis([-4, 4, -4, 4])
plt.grid(True)

figure.tight_layout()
figure.savefig('test.png', facecolor='white', edgecolor='black')


It gives the following error,

$ python test.py
Traceback (most recent call last):
File "test.py", line 12, in <module>
plt.plot([0, x1], [0, y1], color='gray')
File "/usr/lib/pymodules/python2.7/matplotlib/pyplot.py", line 2987, in plot
ret = ax.plot(*args, **kwargs)
File "/usr/lib/pymodules/python2.7/matplotlib/axes.py", line 4137, in plot
for line in self._get_lines(*args, **kwargs):
File "/usr/lib/pymodules/python2.7/matplotlib/axes.py", line 317, in _grab_next_args
for seg in self._plot_args(remaining, kwargs):
File "/usr/lib/pymodules/python2.7/matplotlib/axes.py", line 288, in _plot_args
y = np.atleast_1d(tup[-1])
File "/usr/lib/python2.7/dist-packages/numpy/core/shape_base.py", line 49, in atleast_1d
ary = asanyarray(ary)
File "/usr/lib/python2.7/dist-packages/numpy/core/numeric.py", line 512, in asanyarray
return array(a, dtype, copy=False, order=order, subok=True)
ValueError: setting an array element with a sequence.


If I use some constant value for example
plt.plot([0, 0], [0, r], color='gray')
instead of
plt.plot([0, x1], [0, y1], color='gray')
it works. It seems with
numpy.linspace
such plot is not possible.

I found similar question ValueError: setting an array element with a sequence, but did not help me. I am new to python, please bear with me.

Answer

The x and y elements of your plot() command need to have the same number of elements. Replace the line

plt.plot([0, x1], [0, y1], color='gray')

with the following:

plt.plot([np.zeros(60,), x1], [np.zeros(60,), y1], color='gray')

the result looks like: enter image description here