How to draw a semicircle using matplotlib

Question

I want to draw a semicircle using matplotlib.

Here I have a court

import numpy as np
import matplotlib.pyplot as plt
x_asix = np.array([0,0,100,100, 0])
y_asix = np.array([0,100,100,0, 0])
x_coordenates = np.concatenate([ x_asix])
y_coordenates = np.concatenate([y_asix])

plt.plot(x_coordenates, y_coordenates)

See image here:

I want to add one semicircle that stars at point (0,50) with radius = 10. The result should be something like this:

Answer 1

There is a nice example on tutorialspoint.com.

import matplotlib.pyplot as plt
from matplotlib.patches import Wedge, Rectangle

fig, ax = plt.subplots(1, 1)
ax.set_aspect('equal')

ax.set_xlim([0, 100])
ax.set_ylim([0, 100])

theta1, theta2 = -90, 90
radius = 10
center = (0, 50)
w = Wedge(center, radius, theta1, theta2, fc='none', edgecolor='black')
ax.add_patch(w)

plt.show()

Note that patches include other shapes such as Rectangle that you might find useful.

Answer 2

You could simply use the equation of the ellipse, to easily draw the portion of the ellipse you are interested in.

If you want to draw the part of the ellipse you have in your image, unfortunately you cannot simply write it as: y = f(x), but you can use the common trick of plotting x = f(y) instead:

import matplotlib.pyplot as plt

fig, ax = plt.subplots(1, 1)
ax.set_aspect('equal')

x_asix = np.array([0,0,100,100, 0])
y_asix = np.array([0,100,100,0, 0])
x_coordenates = np.concatenate([ x_asix])
y_coordenates = np.concatenate([y_asix])
ax.plot(x_coordenates, y_coordenates)

# ((x - x0) / a) ** 2 + ((y - y0) / b) ** 2 == 1
a = 20
b = 15
x0 = 50
y0 = 0
x = np.linspace(-a + x0, a + x0)
y = b * np.sqrt(1 - ((x - x0) / a) ** 2) + y0
ax.plot(y, x)

Answer 3

Here is a function that draws semicircles, using numpy:

import matplotlib.pyplot as plt
import numpy as np

def generate_semicircle(center_x, center_y, radius, stepsize=0.1):
    """
    generates coordinates for a semicircle, centered at center_x, center_y
    """        

    x = np.arange(center_x, center_x+radius+stepsize, stepsize)
    y = np.sqrt(radius**2 - x**2)

    # since each x value has two corresponding y-values, duplicate x-axis.
    # [::-1] is required to have the correct order of elements for plt.plot. 
    x = np.concatenate([x,x[::-1]])

    # concatenate y and flipped y. 
    y = np.concatenate([y,-y[::-1]])

    return x, y + center_y

example:

x,y = generate_semicircle(0,50,10, 0.1)
plt.plot(x, y)
plt.show()