Plot Piecewise Function in Python

Question

I would like to plot the following piecewise function in Python using Matplotlib, from 0 to 5.

f(x) = 1, x != 2; f(x) = 0, x = 2

In Python...

def f(x):
 if(x == 2): return 0
 else: return 1

Using NumPy I create an array

x = np.arange(0., 5., 0.2)

    array([ 0. ,  0.2,  0.4,  0.6,  0.8,  1. ,  1.2,  1.4,  1.6,  1.8,  2. ,
        2.2,  2.4,  2.6,  2.8,  3. ,  3.2,  3.4,  3.6,  3.8,  4. ,  4.2,
        4.4,  4.6,  4.8])

I have tried things like...

import matplotlib.pyplot as plt
plt.plot(x,f(x))

Or...

vecfunc = np.vectorize(f)
result = vecfunc(t)

Or...

def piecewise(x):
 if x == 2: return 0
 else: return 1

import matplotlib.pyplot as plt
x = np.arange(0., 5., 0.2)
plt.plot(x, map(piecewise, x))

ValueError: x and y must have same first dimension

But I am not using these functions correctly, and am now just randomly guessing how to do this.

Some answers are starting to get there... But the points are being connected into a line on the plot. How do we just plot the points?

Answer 1

You can use np.piecewise on the array:

x = np.arange(0., 5., 0.2)
import matplotlib.pyplot as plt
plt.plot(x, np.piecewise(x, [x  == 2, x != 2], [0, 1]))

Answer 2

Some answers are starting to get there... But the points are being connected into a line on the plot. How do we just plot the points?

import matplotlib.pyplot as plt
import numpy as np

def f(x):
 if(x == 2): return 0
 else: return 1

x = np.arange(0., 5., 0.2)

y = []
for i in range(len(x)):
   y.append(f(x[i]))

print x
print y

plt.plot(x,y,c='red', ls='', ms=5, marker='.')
ax = plt.gca()
ax.set_ylim([-1, 2])

plt.show()