Apply a **scaling transformation** and a **90° rotation** to a 2D vector using Python and matrix multiplication. Visualize the result with arrows and coordinate labels from the origin.

You're working with a vector:

$$
\vec{v} = \begin{bmatrix}2 \\ 3\end{bmatrix}
$$

You will:
1. Apply a scaling matrix:

   $$
   S = \begin{bmatrix}2 & 0 \\ 0 & 0.5\end{bmatrix}
   $$	 

2. Apply a rotation matrix:

   $$
   R = \begin{bmatrix}2 \\ 3\end{bmatrix}
   $$	 

3.	Combine them as:

$$
R \cdot (S \cdot \vec{v})
$$
	 
This simulates what happens when a vector is first scaled and then rotated.

import unittest
import numpy as np
import user_code  # ÐºÐ¾Ð´ ÑÑÑÐ´ÐµÐ½ÑÐ°

def _dynamic(tc, ok, ok_msg, fail_msg):
    tc._testMethodName = ok_msg if ok else fail_msg
    (tc.assertTrue if ok else tc.fail)(tc._testMethodName)

RTOL = 1e-7
ATOL = 1e-7

class TestCombinedTransformations(unittest.TestCase):

    def test_base_objects(self):
        reasons = []

        v = getattr(user_code, "v", None)
        S = getattr(user_code, "S", None)
        R = getattr(user_code, "R", None)

        # ÐÐµÑÐµÐ²ÑÑÐºÐ° Ð½Ð°ÑÐ²Ð½Ð¾ÑÑÑ ÑÐ° ÑÐ¾ÑÐ¼
        if not (isinstance(v, np.ndarray) and v.shape == (2,1)):
            reasons.append(f"v must be a (2,1) array; got {None if v is None else v.shape}")
        if not (isinstance(S, np.ndarray) and S.shape == (2,2)):
            reasons.append(f"S must be a (2,2) array; got {None if S is None else S.shape}")
        if not (isinstance(R, np.ndarray) and R.shape == (2,2)):
            reasons.append(f"R must be a (2,2) array; got {None if R is None else R.shape}")

        # ÐÑÑÐºÑÐ²Ð°Ð½Ñ Ð·Ð½Ð°ÑÐµÐ½Ð½Ñ Ð·Ð° ÑÐ¼Ð¾Ð²Ð¾Ñ
        v_exp = np.array([[2.0],[3.0]])
        S_exp = np.array([[2.0, 0.0],
                          [0.0, 0.5]])
        # ÐÐ»Ñ R Ð¾ÑÑÐºÑÑÐ¼Ð¾ 90Â° CCW: [[0,-1],[1,0]] Ð· Ð¼Ð¾Ð¶Ð»Ð¸Ð²Ð¾Ñ Ð¿Ð¾ÑÐ¸Ð±ÐºÐ¾Ñ
        R_exp = np.array([[0.0, -1.0],
                          [1.0,  0.0]])

        if isinstance(v, np.ndarray) and v.shape == (2,1) and not np.allclose(v, v_exp, rtol=RTOL, atol=ATOL):
            reasons.append(f"v values incorrect; expected {v_exp.ravel().tolist()}, got {v.ravel().tolist()}")
        if isinstance(S, np.ndarray) and S.shape == (2,2) and not np.allclose(S, S_exp, rtol=RTOL, atol=ATOL):
            reasons.append(f"S values incorrect; expected {S_exp.tolist()}, got {S.tolist()}")
        if isinstance(R, np.ndarray) and R.shape == (2,2) and not np.allclose(R, R_exp, rtol=1e-6, atol=1e-6):
            reasons.append(f"R should be 90Â° CCW rotation; expected approx {R_exp.tolist()}, got {R.tolist()}")

        _dynamic(self,
                 len(reasons) == 0,
                 "PASS: base objects (v, S, R) are correct",
                 "FAIL: " + " | ".join(reasons) if reasons else "FAIL")

    def test_scaled_vector(self):
        reasons = []

        v = getattr(user_code, "v", None)
        S = getattr(user_code, "S", None)
        scaled_v = getattr(user_code, "scaled_v", None)

        if not (isinstance(scaled_v, np.ndarray) and scaled_v.shape == (2,1)):
            reasons.append(f"scaled_v must be a (2,1) array; got {None if scaled_v is None else scaled_v.shape}")
        else:
            exp_scaled = S @ v
            if not np.allclose(scaled_v, exp_scaled, rtol=RTOL, atol=ATOL):
                reasons.append(f"scaled_v != S @ v; expected {exp_scaled.ravel().tolist()}, got {scaled_v.ravel().tolist()}")

        _dynamic(self,
                 len(reasons) == 0,
                 "PASS: scaled_v equals S @ v",
                 "FAIL: " + " | ".join(reasons) if reasons else "FAIL")

    def test_rotated_vector(self):
        reasons = []

        R = getattr(user_code, "R", None)
        scaled_v = getattr(user_code, "scaled_v", None)
        rotated_v = getattr(user_code, "rotated_v", None)

        if not (isinstance(rotated_v, np.ndarray) and rotated_v.shape == (2,1)):
            reasons.append(f"rotated_v must be a (2,1) array; got {None if rotated_v is None else rotated_v.shape}")
        else:
            exp_rot = R @ scaled_v
            if not np.allclose(rotated_v, exp_rot, rtol=1e-6, atol=1e-6):
                reasons.append(f"rotated_v != R @ scaled_v; expected {exp_rot.ravel().tolist()}, got {rotated_v.ravel().tolist()}")

            # ÐÐ¾Ð´Ð°ÑÐºÐ¾Ð²Ð¸Ð¹ ÑÐ¸ÑÐ»Ð¾Ð²Ð¸Ð¹ ÑÐµÐº Ð¾ÑÑÐºÑÐ²Ð°Ð½Ð¸Ñ ÐºÐ¾Ð¾ÑÐ´Ð¸Ð½Ð°Ñ (-1.5, 4)
            exp_num = np.array([[-1.5],[4.0]])
            if not np.allclose(rotated_v, exp_num, rtol=1e-6, atol=1e-6):
                reasons.append(f"rotated_v should be approximately [-1.5, 4]; got {rotated_v.ravel().tolist()}")

        _dynamic(self,
                 len(reasons) == 0,
                 "PASS: rotated_v equals R @ scaled_v and matches expected coords",
                 "FAIL: " + " | ".join(reasons) if reasons else "FAIL")


test_code.py

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Challenge: Combined Transformations of a Vector

Solution

Awesome!

Challenge: Combined Transformations of a Vector

Solution

Awesome!