Dan Ciborowski - MSFT Dan Ciborowski - MSFT -4 years ago 234
Python Question

Plot Piecewise Function in Python

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?

enter image description here

Answer Source

The problem is that the function f does not take an array as input but a single numer. You can:

plt.plot(x, map(f, x))

The map function takes a function f, an array x and returns another array where the function f is applied to each element of the array.

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