Slicing with start:stop:step in NumPy - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of slicing a NumPy array with positive step

What is the output of the following code snippet?

NumPy

import numpy as np
arr = np.array([10, 20, 30, 40, 50, 60])
sliced = arr[1:5:2]
print(sliced)

A [20 40]

B [20 30 40 50]

C [30 50]

D [10 30 50]

💡 Hint

Remember slicing syntax is arr[start:stop:step], and stop is exclusive.

The slice starts at index 1 (value 20), stops before index 5, stepping by 2, so it picks indices 1 and 3 (20 and 40).

❓ data_output

intermediate

Resulting array length after slicing with negative step

Given the array and slicing below, how many elements does the resulting array have?

NumPy

import numpy as np
arr = np.arange(10)
sliced = arr[8:2:-2]
print(len(sliced))

A 2

B 4

C 3

D 5

💡 Hint

Count elements starting at index 8 down to but not including 2, stepping by -2.

Indices selected are 8, 6, 4, so length is 3.

🔧 Debug

advanced

Identify the error in this slicing code

What error does this code raise?

NumPy

import numpy as np
arr = np.array([1, 2, 3, 4, 5])
sliced = arr[::0]
print(sliced)

A No error, prints the whole array

B IndexError: index out of range

C TypeError: slice indices must be integers or None

D ValueError: slice step cannot be zero

💡 Hint

Check if step value in slicing can be zero.

Step in slicing cannot be zero; it raises a ValueError.

🚀 Application

advanced

Extract every third element from a 2D NumPy array's second row

Given a 2D array, which slicing extracts every third element from the second row?

NumPy

import numpy as np
arr = np.array([[1,2,3,4,5,6,7,8,9],[10,11,12,13,14,15,16,17,18]])

A arr[::3, 1]

B arr[1, ::3]

C arr[1, 3::]

D arr[1, 3:3]

💡 Hint

Remember the first index selects the row, the second index slices the columns.

arr[1, ::3] selects the second row and every third element starting from index 0.

🧠 Conceptual

expert

Understanding slicing with omitted start and stop and negative step

What is the output of this code?

NumPy

import numpy as np
arr = np.array([5, 10, 15, 20, 25])
sliced = arr[::-1]
print(sliced)

A [25 20 15 10 5]

B [5 10 15 20 25]

C [5 15 25]

D []

💡 Hint

Omitting start and stop with step -1 reverses the array.

arr[::-1] reverses the array, so output is [25 20 15 10 5].