np.unravel_index() for multi-dim positions in NumPy - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of np.unravel_index with a single flat index

What is the output of the following code snippet?

NumPy

import numpy as np
index = 5
shape = (3, 4)
result = np.unravel_index(index, shape)
print(result)

A (1, 2)

B (0, 5)

C (2, 1)

D (1, 1)

💡 Hint

Remember that unravel_index converts a flat index to a tuple of coordinates in the given shape.

The flat index 5 corresponds to row 1, column 1 in a 3x4 array because 3 rows * 4 columns means indices go row-wise: 0 to 11. Index 5 is at position (1, 1).

❓ data_output

intermediate

Shape of output from np.unravel_index with multiple indices

Given multiple flat indices, what is the shape of the output from np.unravel_index?

NumPy

import numpy as np
indices = [3, 7, 11]
shape = (3, 4)
result = np.unravel_index(indices, shape)
print(result)

A A tuple of 2 arrays, each of shape (3,)

B A tuple of 3 arrays, each of shape (2,)

C A single array of shape (3, 2)

D A list of 3 tuples, each of length 2

💡 Hint

Check the documentation: unravel_index returns a tuple with one array per dimension.

For shape (3,4), unravel_index returns a tuple with 2 arrays (one per dimension). Each array has length equal to the number of indices (3 here).

🔧 Debug

advanced

Identify the error in using np.unravel_index with an invalid shape

What error does the following code raise?

NumPy

import numpy as np
index = 10
shape = (3, 3)
result = np.unravel_index(index, shape)
print(result)

A ValueError: shape must be a tuple of positive integers

B IndexError: index 10 is out of bounds for array with size 9

C TypeError: index must be an integer or array of integers

D No error, output is (3, 1)

💡 Hint

Check if the flat index is valid for the given shape size.

The shape (3,3) means total size 9. Index 10 is out of bounds, so numpy raises IndexError.

🚀 Application

advanced

Using np.unravel_index to find coordinates of max value in a 3D array

You have a 3D numpy array. How can you find the coordinates of the maximum value using np.unravel_index?

NumPy

import numpy as np
arr = np.array([[[1, 2], [3, 4]], [[5, 6], [7, 8]]])
max_index = arr.argmax()
coords = np.unravel_index(max_index, arr.shape)
print(coords)

A (1, 1, 1)

B (0, 1, 1)

C (1, 0, 1)

D (0, 0, 1)

💡 Hint

The maximum value is 8. Find its position in the 3D array.

The max value 8 is at position (1, 1, 1) in the array. argmax returns flat index 7, unravel_index converts it to (1,1,1).

🧠 Conceptual

expert

Understanding np.unravel_index output for high-dimensional arrays

For a 4D array with shape (2, 3, 4, 5), what is the length of the tuple returned by np.unravel_index for a single flat index?

A 5

B 1

C 4

D 20

💡 Hint

The tuple length equals the number of dimensions in the shape.

np.unravel_index returns a tuple with one element per dimension of the shape. For shape (2,3,4,5), it returns 4 arrays.