integrating an array using np trapz

Question

I have been using np.trapz for integration over arrays for a while and have not had any problems with it, until now. I have obtained a distribution which clearly has an area of less than 1, because its maxima are 0.16 and the width of the distribution is roughly 6 but it seems to return that the area underneath the distribution is >60.

Here is my code:

import numpy as np
import matplotlib.pyplot as plt
data = np.load('dist.npy')
thetavals=np.linspace(0,2*np.pi,1000)
plt.xlabel(r'$\theta$')
plt.ylabel(r'$P(\theta)$')
plt.plot(thetavals,data[0:1000])
plt.show()

integralvalue=np.trapz(data)
print('The integral of this distribution results in: ',integralvalue)

score 0 · Answer 1 · answered Aug 06 '20 at 21:41

Using numpy trapz, without the choice of the x parameter, the spacing of our distribution is assumed to be evenly spaced apart, these however should be spaced apart in relation to the theta values that formed the distribution in the first place, using the following code:

import numpy as np
import matplotlib.pyplot as plt
data = np.load('dist.npy')
thetavals=np.linspace(0,2*np.pi,1001)
plt.xlabel(r'$\theta$')
plt.ylabel(r'$P(\theta)$')
plt.plot(thetavals,data)
plt.show()

integralvalue=np.trapz(data,thetavals)
print('The integral of this distribution results in: ',integralvalue)

a number less than 1 is obtained, as expected.

integrating an array using np trapz

1 Answers1