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].

Practice

1. What does the step value control in a NumPy array slice like arr[start:stop:step]?

easy

A. How many elements to skip between each selected element

B. The total number of elements to select

C. The starting index of the slice

D. The ending index of the slice

Solution

1. Step 1: Understand the slice syntax

   The slice arr[start:stop:step] selects elements from start up to but not including stop, moving by step.

2. Step 2: Identify the role of step

   The step determines how many elements to skip between each selected element. For example, step=2 picks every second element.

3. Final Answer:

   How many elements to skip between each selected element -> Option A

4. Quick Check:

   Step controls skipping elements = A [OK]

Hint: Step is the gap between elements in slicing [OK]

Common Mistakes:

• Confusing step with start or stop index
• Thinking step is total count of elements
• Ignoring that step can be negative

2. Which of the following is the correct syntax to slice a NumPy array arr to get every 3rd element from index 1 to 10?

easy

A. arr[10:1:3]

B. arr[1:3:10]

C. arr[1:10:3]

D. arr[3:1:10]

Solution

1. Step 1: Recall slice order

   The slice syntax is arr[start:stop:step]. Here, start=1, stop=10, step=3 means from index 1 up to 9, every 3rd element.

2. Step 2: Check each option

   arr[1:10:3] matches the correct order and values. Others have wrong order or values that don't match the requirement.

3. Final Answer:

   arr[1:10:3] -> Option C

4. Quick Check:

   Correct slice syntax = D [OK]

Hint: Remember slice is start:stop:step in order [OK]

Common Mistakes:

• Mixing up stop and step positions
• Using step as stop index
• Reversing start and stop incorrectly

3. Given arr = np.array([10, 20, 30, 40, 50, 60, 70]), what is the output of arr[1:6:2]?

medium

A. [10 30 50 70]

B. [20 30 40 50 60]

C. [30 50]

D. [20 40 60]

Solution

1. Step 1: Identify slice indices

   The slice arr[1:6:2] starts at index 1 (value 20), stops before index 6, stepping by 2.

2. Step 2: Select elements

   Indexes selected: 1 (20), 3 (40), 5 (60). So output is [20 40 60].

3. Final Answer:

   [20 40 60] -> Option D

4. Quick Check:

   Elements at 1,3,5 = [20 40 60] [OK]

Hint: Pick elements at start, start+step, ... until stop-1 [OK]

Common Mistakes:

• Including stop index element
• Using wrong step size
• Starting from index 0 instead of 1

4. What is wrong with this code snippet?
arr = np.array([1,2,3,4,5])
slice = arr[4:1:-1]
What will slice contain?

medium

A. [4 3 2]

B. [5 4 3]

C. Empty array

D. IndexError

Solution

1. Step 1: Understand negative step slicing

   With step=-1, slicing goes backwards from start index 4 down to stop index 1 (exclusive).

2. Step 2: Identify elements selected

   Indexes selected: 4 (5), 3 (4), 2 (3). So slice contains [5 4 3].

3. Final Answer:

   [5 4 3] -> Option B

4. Quick Check:

   Backward slice from 4 to 2 = [5 4 3] [OK]

Hint: Negative step slices backward, stop is exclusive [OK]

Common Mistakes:

• Expecting forward slice with negative step
• Including stop index element
• Thinking it causes error

5. You have a NumPy array arr = np.arange(20). How would you slice it to get every 4th element in reverse order starting from the last element?

hard

A. arr[19::-4]

B. arr[::4]

C. arr[19:0:4]

D. arr[0:19:-4]

Solution

1. Step 1: Understand array and goal

   arr = np.arange(20) creates array from 0 to 19. We want every 4th element backward starting from last element (index 19).

2. Step 2: Check slicing options

   arr[19::-4] starts at index 19, goes backward to start (index 0), stepping by 4, which matches the goal.

3. Step 3: Verify other options

   A: arr[::4] every 4th forward [0,4,8,...], wrong direction. C: arr[19:0:4] positive step with start>stop gives empty. D: arr[0:19:-4] start<stop with negative step gives empty.

4. Final Answer:

   arr[19::-4] -> Option A

5. Quick Check:

   Reverse every 4th from last = arr[19::-4] [OK]

Hint: Use start index with negative step for reverse slicing [OK]

Common Mistakes:

• Using positive step with decreasing indices
• Wrong start or stop indices
• Assuming arr[::-4] always starts at last element