Weird Behavior Of Np.gradient With Increased Resolution

I would like to plot cos(x) and it's derivative -sin(x) on the same plot. What I do it the following: import numpy as np x = np.linspace(0,10,20) #x values: 0-10, 20 values f

Solution 1:

It's because of the default spacing assumed between two consecutive values, which is 1. See this answer for details.

In your examples, as you're using np.linspace, spacing can be programatically found as

sp = np.diff(x)[0]

or better yet (thanks to @Kraigolas!)

# form the array *and* get the spacing in one shot
x, sp = np.linspace(0, 10, 20, retstep=True)

Then,

x, sp = np.linspace(0, 10, 2000, retstep=True)
f = np.cos(x)

df = np.gradient(f, sp) # change is here!

# plot
plt.plot(f, label ="function")
plt.plot(df, label ="derivative")
plt.legend()

gives

and similar for other number of points.