[FIXED] Plotting epicycles by using matplotlib

Issue

Why does the following code not working?

I want to display epicycles by using matplotlib like a following image

import matplotlib.pyplot as plt
import numpy as np
from numpy import *
import math


freqList = [1,2,3]
ampList = [1,2,4]
phaseList = [0,10,20]

circles = [] #create list of circles

x = 0
y = 0
for i in range(len(freqList)):
   prevx = x
   prevy = y
   theta = np.linspace( 0 , 2 * np.pi , 150 )
   x += ampList[i] * np.cos( theta*freqList[i] + phaseList[i] )
   y += ampList[i] * np.sin( theta*freqList[i] + phaseList[i] )

   circle = plt.Circle((prevx, prevy), ampList[i], fill=False)
   circles.append(circle)

   plt.figure()
   fig, ax = plt.subplots()
   ax.add_patch(circles[i])
   plt.axis("equal")
   plt.xlim( -10 , 10 ) 
   plt.ylim( -10 , 10 ) 
   plt.show()

Thank you in advance to any one who may be able to give me some ideas!

Solution

I applied some changes to your code:

• loop over elements of a list with for i in range(len(list_name)) and then get i^th element with list_name[i] works, but it is a little awkward; in python you can loop over elements of list directly: for element in list. If you need to loop over element of more than one list, you can use zip: for a, b in zip(list_a, list_b)

• within your loop you generates the circles, once at a time, and add it to the plot. Things which are created only once (not once every cycle) have to be generated outside the for loop; that's the case for fig, ax etc.

• in your code you have created an array, theta, which is 150 elements. Inside for loop you add up a function of theta, which is an array too, to x and y. So, in the first iteration x and y are int and represent coordinates of the first circle, then they become arrays. This is the reason why your code throws an error

• if you use matplotlib.patches.Circle you need only the coordinates of the center and the radius. No need to compute theta, no need to use freqList (if I interpreted it correctly). For this reason, inside the loop you have only to pass current center coordinates and radius to matplotlib.patches.Circle and compute center coordinates for the next circle using only current amplitude and phase

That being said, your code becomes:

# import
import matplotlib.pyplot as plt
import numpy as np

# amplitude and phase definition
ampList = [1,2,4]
phaseList = [0,10,20]

# center coordinates of the first circle
C_x = 0
C_y = 0

# generate figure and axis
fig, ax = plt.subplots()

# loop over each amplitude and phase
for amp, phase in zip(ampList, phaseList):

    # draw current circle
    circle = plt.Circle((C_x, C_y), amp, fill = False)
    ax.add_patch(circle)
    # draw current circle center
    ax.plot(C_x, C_y, marker = 'o', markerfacecolor = 'k', markeredgecolor = 'k')

    # compute next circle center 
    C_x += amp*np.cos(np.deg2rad(phase))
    C_y += amp*np.sin(np.deg2rad(phase))

# adjust axis
plt.axis("equal")
plt.xlim( -10 , 10 )
plt.ylim( -10 , 10 )

# show the plot
plt.show()

Answered By - Zephyr