Shiva Prakash Shiva Prakash - 1 year ago 138
Python Question

Intersection of two graphs in Python, find the x value:

Let 0<= x <=1. I have two columns f and g of length 5000 respectively. Now I plot:

plt.plot(x, f, '-')
plt.plot(x, g, '*')

I want to find the point 'x' where the curve intersects. I don't want to find the intersection of f and g.
I can do it simply with:

set(f) & set(g)

Answer Source

You can use np.sign in combination with np.diff and np.argwhere to obtain the indices of points where the lines cross (in this case, the points are [ 0, 149, 331, 448, 664, 743]):

import numpy as np
import matplotlib.pyplot as plt

x = np.arange(0, 1000)
f = np.arange(0, 1000)
g = np.sin(np.arange(0, 10, 0.01) * 2) * 1000

plt.plot(x, f, '-')
plt.plot(x, g, '-')

idx = np.argwhere(np.diff(np.sign(f - g)) != 0).reshape(-1) + 0
plt.plot(x[idx], f[idx], 'ro')

plot of intersection points

First it calculates f - g and the corresponding signs using np.sign. Applying np.diff reveals all the positions, where the sign changes (e.g. the lines cross). Using np.argwhere gives us the exact indices.

Recommended from our users: Dynamic Network Monitoring from WhatsUp Gold from IPSwitch. Free Download