Slicing with start:stop:step in NumPy - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to slice the array from index 1 to 5 (exclusive).

NumPy

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

Drag options to blanks, or click blank then click option'

A4

B0

C2

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as start index which includes the first element.

Using 5 as start index which is out of range.

The slice 1:5 selects elements starting at index 1 up to but not including index 5.

2fill in blank

medium

Complete the code to slice the array to get every second element from the start to the end.

NumPy

import numpy as np
arr = np.array([5, 10, 15, 20, 25, 30])
sliced = arr[[1]]
print(sliced)

Drag options to blanks, or click blank then click option'

A0:5:2

B1:6:2

C0:6:2

D1:5:2

Attempts:

3 left

💡 Hint

Common Mistakes

Starting at 1 which skips the first element.

Using stop index less than array length causing missing last elements.

The slice 0:6:2 starts at index 0, goes up to index 6 (exclusive), stepping by 2, selecting every second element.

3fill in blank

hard

Fix the error in the code to correctly slice the array backwards with step -1.

NumPy

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

Drag options to blanks, or click blank then click option'

A1::-1

B::-1

C5::-1

D-1::-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using invalid start or stop indices that cause empty slices.

Using positive step with negative indices.

The slice ::-1 reverses the array by stepping backwards through all elements.

4fill in blank

hard

Fill both blanks to slice the array from index 2 to 8 (exclusive) with step 3.

NumPy

import numpy as np
arr = np.arange(10)
sliced = arr[[1]:[2]:3]
print(sliced)

Drag options to blanks, or click blank then click option'

A2

B7

C8

D9

Attempts:

3 left

💡 Hint

Common Mistakes

Using stop index 7 which excludes index 7 element.

Using step other than 3.

The slice 2:8:3 starts at index 2, stops before index 8, stepping by 3.

5fill in blank

hard

Fill all three blanks to create a dictionary with words as keys and their lengths as values, only for words longer than 3 letters.

NumPy

words = ['apple', 'bat', 'carrot', 'dog', 'elephant']
lengths = { [1]: [2] for word in words if len(word) [3] 3 }
print(lengths)

Drag options to blanks, or click blank then click option'

Aword

Blen(word)

C>

D{word

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets instead of curly braces.

Using wrong comparison operator like '<' instead of '>'.

Not including the condition to filter words.

The dictionary comprehension syntax is {key: value for item in iterable if condition}. Here, keys are words, values are their lengths, and condition filters words longer than 3 letters.

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