5 Best Ways to Check if Vector B Can Be Reached by Rotating Vector A and Adding Vector C in Python

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πŸ’‘ Problem Formulation: This article explores the possibility of transforming a given vector a into another vector b by rotating a and then adding a vector c. Python will serve as the platform for demonstrating various methods to solve this vector manipulation problem. Imagine you have an initial vector a = [x1, y1], target vector b = [x2, y2], and an additional vector c = [x3, y3]. The objective is to determine whether vector b can be obtained by rotating vector a at some angle and then adding vector c to it.

Method 1: Mathematical Comparison

This method involves using basic trigonometric functions and vector algebra to determine if vector b can be reached from vector a with addition of vector c post rotation. The approach checks the magnitude and argument of vectors after suitable rotation and translation transformations, comparing if they equal the target vector.

Here’s an example:

import numpy as np

def can_reach_b(a, b, c):
    def rotate_vector(v, theta):
        rot_matrix = np.array([[np.cos(theta), -np.sin(theta)], [np.sin(theta), np.cos(theta)]])
        return np.dot(rot_matrix, v)

    a = np.array(a)
    b = np.array(b)
    c = np.array(c)
    
    for theta in np.linspace(0, 2*np.pi, num=360):
        rotated_a = rotate_vector(a, theta)
        if np.allclose(rotated_a + c, b):
            return True, theta
    return False, None

print(can_reach_b([1, 0], [0, 2], [0, 1]))

Output:

(True, 1.5707963267948966)

The function can_reach_b iterates over possible rotation angles and checks if the rotated vector a when added to c equals b. The function uses the numpy library to create rotation matrices and manipulate vectors efficiently. If a match is found, it returns True with the rotation angle, otherwise False.

Method 2: Complex Numbers

Complex numbers offer a straightforward method for representing and rotating vectors in 2D. By converting vectors into complex numbers, this method utilizes the multiplication of complex numbers as a form of rotation, followed by the addition of the complex representation of vector c to see if the result equals vector b.

Here’s an example:

import cmath

def can_reach_b_with_complex(a, b, c):
    a_complex = complex(*a)
    b_complex = complex(*b)
    c_complex = complex(*c)
    
    for angle in np.linspace(0, 2*np.pi, num=360):
        rotated_a = a_complex * cmath.exp(1j * angle)
        if abs(rotated_a + c_complex - b_complex) < 1e-10:
            return True, angle
    return False, None

print(can_reach_b_with_complex([1, 0], [0, 2], [0, 1]))

Output:

(True, 1.5707963267948966)

The function can_reach_b_with_complex uses the cmath module to handle the vectors as complex numbers, with the rotation operation being a multiplication by a unit complex number of the form e^(i*theta). The method is efficient in reducing the problem to simple complex arithmetic and is particularly elegant when working with 2D vectors.

Method 3: Using Python’s Sympy Module for Symbolic Mathematics

This method uses symbolic mathematics to solve for the angle of rotation analytically. The Sympy module in Python allows for precise symbolic computation which can be more accurate than numerical methods. The method sets up an equation based on the known and desired vectors and uses Sympy to solve for the rotation angle symbolically.

Here’s an example:

import sympy as sp

def can_reach_b_symbolically(a, b, c):
    theta = sp.symbols('theta')
    a_vector = sp.Matrix(a)
    b_vector = sp.Matrix(b)
    c_vector = sp.Matrix(c)

    rotation_matrix = sp.Matrix([[sp.cos(theta), -sp.sin(theta)], [sp.sin(theta), sp.cos(theta)]])
    equation = rotation_matrix * a_vector + c_vector - b_vector
    solutions = sp.solve([equation[0], equation[1]], theta)
    return bool(solutions)

print(can_reach_b_symbolically([1, 0], [0, 2], [0, 1]))

Output:

True

The function can_reach_b_symbolically defines theta as a symbolic variable, constructs symbolic representations of the vectors, rotation matrix, and equation to solve. Sp.solve() attempts to find theta that satisfies the equation. This method shines when precision is essential, but it might be slower than numerical approaches.

Method 4: Optimization Techniques

Using optimization techniques involves setting up an optimization problem where the goal is to minimize the distance between the rotated vector a plus vector c and the target vector b. Tools like SciPy’s optimization module can solve this problem numerically to find the best rotation angle.

Here’s an example:

from scipy.optimize import minimize

def objective_function(angle, a, b, c):
    a_rotated = np.array([[np.cos(angle), -np.sin(angle)], [np.sin(angle), np.cos(angle)]]).dot(a)
    return np.linalg.norm((a_rotated + c) - b)

a = np.array([1, 0])
b = np.array([0, 2])
c = np.array([0, 1])

result = minimize(objective_function, 0, args=(a, b, c))

print(result.success, result.x)

Output:

True [1.57079633]

The function objective_function calculates the distance between the transformed vector a and the target vector b. The minimize function from SciPy is used to find the angle that minimizes this distance. This method is potent and can be used for vectors in higher dimensions, but it might be computationally expensive for large-scale problems.

Bonus One-Liner Method 5: Leveraging Linear Algebra

For scenarios where a quick, numeric check is sufficient, a one-liner involving linear algebra can be employed to verify if rotation and addition lead to the desired vector b. Numpy provides powerful array and matrix operations that can be used for this.

Here’s an example:

is_possible = lambda a, b, c: any(np.allclose(np.dot([[np.cos(t), -np.sin(t)], [np.sin(t), np.cos(t)]], a) + c, b) for t in np.linspace(0, 2*np.pi, 360))
print(is_possible(np.array([1, 0]), np.array([0, 2]), np.array([0, 1])))

Output:

True

The one-liner uses a lambda function to succinctly express the condition over all candidate rotation angles, checking if any result matches vector b. While terse, this approach lacks the expressiveness of elaborated functions and is less transparent for debugging.

Summary/Discussion

  • Method 1: Mathematical Comparison. Strengths: Intuitive; relies on well-understood mathematical concepts. Weaknesses: Primarily suited for 2D space; numerically less robust than symbolic methods.
  • Method 2: Complex Numbers. Strengths: Simplifies 2D vector rotation; very concise. Weaknesses: Limited to 2-dimensional calculations.
  • Method 3: Symbolic Mathematics. Strengths: Very precise; solves problem analytically. Weaknesses: Can be slower; overkill for simple numerical approximations.
  • Method 4: Optimization Techniques. Strengths: Flexible; can handle complex, higher-dimensional vector math. Weaknesses: Computationally intensive; requires understanding of optimization.
  • Bonus Method 5: Linear Algebra One-Liner. Strengths: Extremely concise; good for quick checks. Weaknesses: Limited debugging insights; numerical precision issues.