CODEWITHSUNDEEP
pythonnumpymatplotlibplotvisualization
Don99
Don99

Reputation: 229

How to detect if there is intersection using matplotlib?

I am using matplotlib to plot these two line graphs. However, only sometimes my graphs would have an intersection. How can I detect if my line graphs have an intersection?

df = pd.read_csv("test.csv")

df2 = pd.read_csv("test2.csv")

x1 = df['A'].tolist()
x1 = np.array(x1)

y1 = df['D'].tolist()
y1 = np.array(y1)

x2 = df2['X'].tolist()
x2 = np.array(x2)

y2 = df2['Y'].tolist()
y2 = np.array(y2)

plt.plot(x1,y1)
plt.plot(x2,y2)
plt.show()

enter image description here

Upvotes: 1

Views: 1054

Answers (2)

Zephyr
Zephyr

Reputation: 12496

You can compute the index of intersection points with:

idx = np.argwhere(np.diff(np.sign(y1 - y2))).flatten()

If there is one or more intersections, idx is a list of intersection points indeces, otherwise it is an empty list.

  • one or more intersections

    import numpy as np
import matplotlib.pyplot as plt

x1 = np.linspace(0, 10, 1000)
x2 = np.linspace(-2, 5, 1000)
y1 = np.sin(x1)
y2 = np.cos(x2) + 1

idx = np.argwhere(np.diff(np.sign(y1 - y2))).flatten()

fig, ax = plt.subplots()

ax.plot(x1, y1, 'blue')
ax.plot(x2, y2, 'red')

plt.show()

    print(len(idx))
2

    enter image description here

  • no intersections

    import numpy as np
import matplotlib.pyplot as plt

x1 = np.linspace(0, 10, 1000)
x2 = np.linspace(-2, 5, 1000)
y1 = np.sin(x1)
y2 = np.cos(x2) + 2

idx = np.argwhere(np.diff(np.sign(y1 - y2))).flatten()

fig, ax = plt.subplots()

ax.plot(x1, y1, 'blue')
ax.plot(x2, y2, 'red')

plt.show()

    print(len(idx))
0

    enter image description here

Upvotes: 1

ph140
ph140

Reputation: 475

This code prints 'intersect' if the y-values cross each other at some point.

mark = y1[0]-y2[0]

for i in range(len(y1)):
    if mark > 0:
        if y1[i]-y2[i] < 0:
        print('intersect')
    elif mark < 0:
        if y1[i]-y2[i] > 0:
            print('intersect')

Upvotes: 0

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