Distance matrix computation in SciPy - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Euclidean distance matrix computation

What is the output of this code that computes the Euclidean distance matrix between points?

SciPy

import numpy as np
from scipy.spatial import distance_matrix
points = np.array([[0, 0], [3, 4], [6, 8]])
dist_mat = distance_matrix(points, points)
print(dist_mat)

A

[[ 0.  7. 14.]
 [ 7.  0.  7.]
 [14.  7.  0.]]

B

[[0 5 10]
 [5 0 5]
 [10 5 0]]

C

[[ 0.  5. 10.]
 [ 5.  0.  5.]
 [10.  5.  0.]]

D

[[ 0.  3.  6.]
 [ 3.  0.  4.]
 [ 6.  4.  0.]]

💡 Hint

Recall that Euclidean distance between (x1,y1) and (x2,y2) is sqrt((x2-x1)^2 + (y2-y1)^2).

The distances between points (0,0), (3,4), and (6,8) are 5, 10, and 5 respectively, forming the symmetric matrix shown in option C.

❓ data_output

intermediate

Number of elements in Euclidean distance matrix

Given 4 points in 2D space, how many elements are in the Euclidean distance matrix computed by scipy.spatial.distance_matrix?

SciPy

import numpy as np
from scipy.spatial import distance_matrix
points = np.array([[1,2], [3,4], [5,6], [7,8]])
dist_mat = distance_matrix(points, points)
print(dist_mat.size)

A 16

B 8

C 4

D 12

💡 Hint

Distance matrix between n points is an n x n matrix.

With 4 points, the distance matrix is 4 rows by 4 columns, so total elements = 4*4 = 16.

🔧 Debug

advanced

Identify the error in distance matrix computation code

What error does this code raise when computing a distance matrix?

SciPy

import numpy as np
from scipy.spatial import distance_matrix
points = [[0, 0], [1, 1], [2, 2]]
dist_mat = distance_matrix(points, points)
print(dist_mat)

A No error, prints the correct distance matrix

B TypeError: unsupported operand type(s) for -: 'list' and 'list'

C ValueError: setting an array element with a sequence

D NameError: name 'distance_matrix' is not defined

💡 Hint

Check if input is accepted as list of lists or needs to be numpy array.

scipy.spatial.distance_matrix accepts list of lists as input and converts internally, so no error occurs.

🧠 Conceptual

advanced

Which distance metric is NOT supported by scipy.spatial.distance_matrix?

Which of these distance metrics cannot be directly computed using scipy.spatial.distance_matrix function?

A Euclidean distance

B Manhattan (cityblock) distance

C Hamming distance

D Cosine distance

💡 Hint

Check scipy.spatial.distance_matrix documentation for supported metrics.

scipy.spatial.distance_matrix only computes Euclidean distance. Other metrics like Manhattan, Cosine, or Hamming require scipy.spatial.distance.pdist or cdist with metric parameter.

🚀 Application

expert

Find the closest pair of points using distance matrix

Given this code, which option correctly identifies the indices of the closest pair of points?

SciPy

import numpy as np
from scipy.spatial import distance_matrix
points = np.array([[1, 2], [4, 6], [1, 3], [7, 8]])
dist_mat = distance_matrix(points, points)
np.fill_diagonal(dist_mat, np.inf)
min_index = np.unravel_index(np.argmin(dist_mat), dist_mat.shape)
print(min_index)

A (2, 3)

B (0, 2)

C (0, 1)

D (1, 3)

💡 Hint

Look for the smallest non-zero distance after ignoring diagonal.

Points at indices 0 and 2 are closest with distance 1, so min_index is (0, 2).

Practice

1. What does the scipy.spatial.distance_matrix function compute?

easy

A. The average value of a list of numbers

B. The sum of all points in a dataset

C. The distances between all pairs of points in two sets

D. The maximum value in a dataset

Solution

1. Step 1: Understand the function purpose

   scipy.spatial.distance_matrix calculates distances between points, not sums or averages.

2. Step 2: Identify what is computed

   It returns a matrix showing distances between each point in one set to each point in another set.

3. Final Answer:

   The distances between all pairs of points in two sets -> Option C

4. Quick Check:

   Distance matrix = pairwise distances [OK]

Hint: Distance matrix = all pair distances between points [OK]

Common Mistakes:

• Confusing distance matrix with sum or average calculations
• Thinking it returns a single distance value
• Assuming it only works for one set of points

2. Which of the following is the correct way to import the distance_matrix function from scipy?

easy

A. import scipy.distance_matrix

B. import distance_matrix from scipy.spatial

C. from scipy import distance_matrix

D. from scipy.spatial import distance_matrix

Solution

1. Step 1: Recall correct import syntax

   Functions inside modules are imported using from module import function.

2. Step 2: Match with scipy structure

   distance_matrix is inside scipy.spatial, so correct import is from scipy.spatial import distance_matrix.

3. Final Answer:

   from scipy.spatial import distance_matrix -> Option D

4. Quick Check:

   Correct import syntax = from scipy.spatial import distance_matrix [OK]

Hint: Use 'from module import function' for specific imports [OK]

Common Mistakes:

• Using 'import scipy.distance_matrix' which is invalid
• Trying 'from scipy import distance_matrix' ignoring submodules
• Incorrect order like 'import distance_matrix from ...'

3. What is the output of this code?

import numpy as np
from scipy.spatial import distance_matrix
points1 = np.array([[0, 0], [1, 1]])
points2 = np.array([[1, 0], [2, 2]])
dm = distance_matrix(points1, points2)
print(dm)

medium

A. [[1. 2.82842712]
[1. 1.41421356]]

B. [[0. 1.41421356]
[1. 2.23606798]]

C. [[1.41421356 2.23606798]
[0. 1.41421356]]

D. [[1. 1.41421356]
[1.41421356 2.82842712]]

Solution

1. Step 1: Calculate distances from points1 to points2

   Distance between (0,0) and (1,0) is 1.0; between (0,0) and (2,2) is sqrt(4+4)=2.8284.

2. Step 2: Calculate distances for second point

   Distance between (1,1) and (1,0) is 1.0; between (1,1) and (2,2) is sqrt(1+1)=1.4142.

3. Final Answer:

   [[1. 2.82842712] [1. 1.41421356]] -> Option A

4. Quick Check:

   Distance matrix matches calculated values [OK]

Hint: Calculate Euclidean distances pairwise for matrix [OK]

Common Mistakes:

• Mixing order of points causing wrong matrix
• Using Manhattan distance instead of Euclidean
• Confusing rows and columns in output

4. Identify the error in this code snippet:

import numpy as np
from scipy.spatial import distance_matrix
points = np.array([[0, 0], [1, 1]])
dm = distance_matrix(points)
print(dm)

medium

A. distance_matrix cannot handle integer arrays

B. distance_matrix requires two arguments, but only one is given

C. numpy array must be 1D, but points is 2D

D. print statement syntax is incorrect

Solution

1. Step 1: Check function parameters

   distance_matrix needs two arrays of points to compute distances between them.

2. Step 2: Identify missing argument

   Only one argument points is passed, so it will raise a TypeError.

3. Final Answer:

   distance_matrix requires two arguments, but only one is given -> Option B

4. Quick Check:

   Missing second argument error [OK]

Hint: distance_matrix needs two point sets as input [OK]

Common Mistakes:

• Passing only one array instead of two
• Assuming it computes distances within one set automatically
• Ignoring function signature requirements

5. You have two sets of points:

pointsA = [[0, 0], [3, 4], [6, 8]]
pointsB = [[0, 0], [0, 5]]

You want to find which point in pointsA is closest to any point in pointsB. Which approach using scipy.spatial.distance_matrix is correct?

hard

A. Compute the distance matrix, then find the minimum distance in each row

B. Compute the distance matrix, then sum all distances and pick the smallest sum

C. Compute the distance matrix, then find the maximum distance in each column

D. Compute the distance matrix, then average all distances and pick the largest average

Solution

1. Step 1: Compute distance matrix between pointsA and pointsB

   This gives distances from each point in pointsA to each point in pointsB.

2. Step 2: Find minimum distance per point in pointsA

   For each row (point in pointsA), find the smallest distance to any point in pointsB to identify closest point.

3. Final Answer:

   Compute the distance matrix, then find the minimum distance in each row -> Option A

4. Quick Check:

   Closest point = min distance per row [OK]

Hint: Minimum distance per row shows closest point [OK]

Common Mistakes:

• Using sum or average instead of minimum distance
• Finding maximum distance which is farthest, not closest
• Confusing rows and columns in the matrix