CODEWITHSUNDEEP
pythonarraysnumpymathvector
codemachine98
codemachine98

Reputation: 261

Finding angle between the x-axis and a vector on the unit circle

I have a function that that finds the angle between the unit circle vector [1,0] and a vector on the unit circle.

import numpy as np

def angle(a):
    b = [0,1]
    if a[1] >= 0:
        return round(np.degrees(np.arccos(np.dot(a, b)/ (np.linalg.norm(a) * np.linalg.norm(b)))), 2)
    else:
        return 180 + round(np.degrees(np.arccos(np.dot(a, b)/ (np.linalg.norm(a) * np.linalg.norm(b)))), 2)

print(angle(np.array([1,0])))
90.0

print(angle(np.array([-4,2])))
63.43 # This value should be 150

print(angle(np.array([-1,0])))
90.0 # This value should be 180

print(angle(np.array([0,-1])))
360.0 # This value should be 270

Upvotes: 0

Views: 2202

Answers (3)

anon01
anon01

Reputation: 11181

One way to define a function that expects inputs is to leave both as separate args (this also fixes some bugs and simplifies the logic to get your angle values):

def angle(x, y):
    
    rad = np.arctan2(y, x)
    degrees = np.int(rad*180/np.pi)
    if degrees < 0:
        degrees = 360 + degrees
    return degrees

Incidentally, atan2 has input order y, x which is easy to mix up. An advantage to specifying them separately is you can help avoid that. If you want to keep the input as an array, something like this helps you validate the length:

def angle(a):
    
    if len(a) != 2:
        raise IndexError("vector a expected to be length 2")        
    x = a[0]
    y = a[1]
    rad = np.arctan2(y, x)
    degrees = np.int(rad*180/np.pi)
    if degrees < 0:
        degrees = 360 + degrees
    return degrees

Upvotes: 1

yassir karroum
yassir karroum

Reputation: 51

To establish a is always a 2D vector, you can check it's length : if len(a) == 2. you can additionally check if it's a list or a tuple with : if type(a) in [list, tuple] and len(a) == 2:

EDIT : my bad just noticed it's actually numpy arrays, in that case : if isinstance(x, np.ndarray) and x.shape[0] == 2

EDIT 2 : from the comments : x.ndim == 2 sounds better.

Upvotes: 0

Jzbach
Jzbach

Reputation: 368

You have the x vector wrong. It should be

b = [1,0]

given that the first coordinate is the x axis and the second one is the y axis. If you put this correct b vector, all the computations work as expected.

Upvotes: 1

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