Practice: Arrays & Indexing

Take a moment to think about this

This lesson doesn't introduce anything new — it's meant to be a hands-on review of array creation, attributes, indexing, slicing, and basic operations from the tutorial's Basic chapter, through 4 short practice tasks. For each task, write the code yourself, predict the output, and then run it to check. These fundamentals are the base you'll need for the tougher broadcasting and aggregation tasks in Part 2.

Practice tasks

Task 1: Create a 1D array with your 5 favorite numbers and print its dtype and shape. Task 2: Reshape np.arange(12) into a (3, 4) 2D array, then use slicing to pull out the middle 2x2 block (rows 1-2, columns 1-2). Task 3: Create an array of even numbers from 0 to 20, then reverse it using slicing with step -1. Task 4: Add 5 and multiply by 2, element-wise, on an array, then print the result side by side with the original array to see the difference.

Created a 1D array and printed its attributes (dtype, shape)
Created a 2D array with reshape() and pulled out the middle block with slicing
Did both the reverse slicing and the element-wise basic operations

Example Code

python

import numpy as np

# Task 1: 1D array + attributes
favorites = np.array([7, 14, 21, 42, 100])
print("dtype:", favorites.dtype, "shape:", favorites.shape)

# Task 2: reshape + middle block slicing
grid = np.arange(12).reshape(3, 4)
print("grid:\n", grid)
middle_block = grid[0:2, 1:3]  # TODO: adjust to get the correct middle block
print("middle block:\n", middle_block)

# Task 3: even numbers, reversed with slicing
evens = np.arange(0, 21, 2)
reversed_evens = evens[::-1]
print("evens:", evens)
print("reversed:", reversed_evens)

# Task 4: element-wise operations
original = np.array([1, 2, 3, 4, 5])
transformed = (original + 5) * 2
print("original:", original)
print("transformed:", transformed)

You should see

For all 4 tasks, the print statements will correctly show the dtype/shape, the sliced 2x2 block, the reversed even-number array, and the transformed array.

Try it in 5 minutes

Try reshaping Task 2's grid to (4, 3) instead, and think through whether you'd need to adjust the middle-block slicing indexes (5 minutes).

A quick word of caution

For 2D slicing like grid[0:2, 1:3], the row range and column range need to be separated by a comma — writing it as double brackets like grid[0:2][1:3] can give you completely different logic.