2D array indexing (row, col) in NumPy - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of 2D array element access

What is the output of the following code snippet?

NumPy

import numpy as np
arr = np.array([[10, 20, 30], [40, 50, 60], [70, 80, 90]])
result = arr[1, 2]
print(result)

A 40

B 60

C 50

D 90

💡 Hint

Remember that indexing starts at 0 and the first index is the row, second is the column.

The element at row 1, column 2 is 60 because rows and columns start at 0. So arr[1, 2] accesses the second row and third column.

❓ data_output

intermediate

Extracting a row from a 2D array

What is the output of this code that extracts the second row from the array?

NumPy

import numpy as np
arr = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
row = arr[1, :]
print(row)

A [4 5 6]

B [1 2 3]

C [7 8 9]

D [5 6 7]

💡 Hint

The colon (:) means all columns in the selected row.

arr[1, :] selects all columns in the second row (index 1), which is [4, 5, 6].

❓ Predict Output

advanced

Selecting multiple elements with 2D indexing

What is the output of this code that selects elements from specific rows and columns?

NumPy

import numpy as np
arr = np.array([[5, 10, 15], [20, 25, 30], [35, 40, 45]])
result = arr[[0, 2], [1, 2]]
print(result)

A [10 30]

B [5 40]

C [10 45]

D [15 35]

💡 Hint

When using two lists for indexing, elements are selected pairwise from each list.

arr[[0, 2], [1, 2]] selects elements at (0,1) and (2,2), which are 10 and 45.

🔧 Debug

advanced

Identify the error in 2D array indexing

What error does this code raise when trying to access an element?

NumPy

import numpy as np
arr = np.array([[1, 2], [3, 4]])
value = arr[2, 1]
print(value)

A KeyError: 2

B TypeError: list indices must be integers or slices, not tuple

C ValueError: too many indices for array

D IndexError: index 2 is out of bounds for axis 0 with size 2

💡 Hint

Check the size of the array along the first axis (rows).

The array has only 2 rows (indices 0 and 1). Accessing row 2 is out of bounds, causing IndexError.

🚀 Application

expert

Extracting a submatrix using 2D slicing

Given the array below, which option correctly extracts the 2x2 submatrix from rows 1 to 2 and columns 0 to 1?

NumPy

import numpy as np
arr = np.array([[11, 12, 13], [21, 22, 23], [31, 32, 33]])

A arr[1:3, 0:2]

B arr[0:2, 1:3]

C arr[1:2, 0:1]

D arr[1:2, 0:2]

💡 Hint

Remember slicing includes the start index but excludes the end index.

arr[1:3, 0:2] selects rows 1 and 2, columns 0 and 1, giving a 2x2 submatrix.