List indexing and slicing in Python - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the first element of the list.

Python

my_list = [10, 20, 30, 40]
first_element = my_list[[1]]
print(first_element)

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

A0

B1

C-1

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 as the index to get the first element.

Using 4 which is out of range.

List indexing starts at 0, so the first element is at index 0.

2fill in blank

medium

Complete the code to get the last element of the list using negative indexing.

Python

numbers = [5, 10, 15, 20, 25]
last_num = numbers[[1]]
print(last_num)

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

A-2

B4

C0

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using -2 which gives the second last element.

Using 0 which is the first element.

Negative indexing starts from the end. -1 is the last element.

3fill in blank

hard

Fix the error in the code to slice the list and get elements from index 1 to 3 (inclusive of 1, exclusive of 4).

Python

letters = ['a', 'b', 'c', 'd', 'e']
sublist = letters[[1]]
print(sublist)

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

A1-4

B1,4

C1:4

D1..4

Attempts:

3 left

💡 Hint

Common Mistakes

Using commas or dashes instead of colon in slicing.

Using '..' which is not valid syntax.

List slicing uses colon ':' to specify start and end indexes.

4fill in blank

hard

Fill both blanks to slice the list and get every second element from index 0 to 5.

Python

data = [2, 4, 6, 8, 10, 12, 14]
slice_result = data[[1]:[2]:2]
print(slice_result)

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

A0

B5

C6

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 6 as end index which includes index 5 element.

Using 1 as start index which skips the first element.

The slice data[0:5:2] gets elements at indexes 0, 2, and 4.

5fill in blank

hard

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

Python

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

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

Aword

Blen(word)

D{

Attempts:

3 left

💡 Hint

Common Mistakes

Not starting with '{' which causes syntax error.

Using 'word' twice incorrectly for key and loop variable.

The dictionary comprehension syntax is {key: value for item in iterable if condition}.

Practice

1. What does the expression my_list[2] return when my_list = [10, 20, 30, 40, 50]?

easy

A. 40

B. 20

C. 30

D. 50

Solution

1. Step 1: Understand list indexing

   Indexing starts at 0, so my_list[0] is 10, my_list[1] is 20, and my_list[2] is 30.

2. Step 2: Identify the value at index 2

   The value at index 2 is 30.

3. Final Answer:

   30 -> Option C

4. Quick Check:

   Index 2 value = 30 [OK]

Hint: Remember indexing starts at zero, so count from 0 [OK]

Common Mistakes:

• Starting count from 1 instead of 0
• Confusing index 2 with index 3
• Mixing up values and indexes

2. Which of the following is the correct syntax to get the last element of a list nums using indexing?

easy

A. nums[last]

B. nums[1]

C. nums[len(nums)]

D. nums[-1]

Solution

1. Step 1: Recall negative indexing

   Negative indexes count from the end, so -1 means the last element.

2. Step 2: Check each option

   nums[-1] uses nums[-1], which correctly accesses the last element. nums[1] accesses the second element. nums[last] is invalid syntax. nums[len(nums)] causes an IndexError because list indexes go from 0 to len(nums)-1.

3. Final Answer:

   nums[-1] -> Option D

4. Quick Check:

   Last element index = -1 [OK]

Hint: Use -1 to get the last item in a list [OK]

Common Mistakes:

• Using len(nums) as index (out of range)
• Trying to use 'last' as an index
• Confusing positive and negative indexes

3. What is the output of this code?

letters = ['a', 'b', 'c', 'd', 'e']
print(letters[1:4])

medium

A. ['b', 'c', 'd']

B. ['c', 'd', 'e']

C. ['b', 'c', 'd', 'e']

D. ['a', 'b', 'c']

Solution

1. Step 1: Understand slicing syntax

   Slicing letters[1:4] means start at index 1 up to but not including index 4.

2. Step 2: Identify elements at indexes 1, 2, 3

   Index 1 is 'b', index 2 is 'c', index 3 is 'd'. So the slice is ['b', 'c', 'd'].

3. Final Answer:

   ['b', 'c', 'd'] -> Option A

4. Quick Check:

   Slicing excludes stop index [OK]

Hint: Slice stops before the stop index, not including it [OK]

Common Mistakes:

• Including the stop index element
• Confusing start and stop indexes
• Using wrong indexes for slicing

4. The code below causes an error. What is the problem?

nums = [1, 2, 3, 4, 5]
print(nums[5])

medium

A. SyntaxError due to wrong brackets

B. IndexError because index 5 is out of range

C. TypeError because nums is not a list

D. No error, prints 5

Solution

1. Step 1: Check list length and valid indexes

   List nums has 5 elements with indexes 0 to 4.

2. Step 2: Understand index 5 usage

   Index 5 is outside the valid range, so accessing nums[5] causes an IndexError.

3. Final Answer:

   IndexError because index 5 is out of range -> Option B

4. Quick Check:

   Index must be less than list length [OK]

Hint: Indexes go from 0 to length-1 only [OK]

Common Mistakes:

• Thinking index 5 is valid for 5 elements
• Confusing SyntaxError with IndexError
• Assuming list indexes start at 1

5. Given data = [5, 10, 15, 20, 25, 30], which expression returns a new list with every second element in reverse order?

hard

A. data[::-2]

B. data[::2]

C. data[-2::-1]

D. data[1::2]

Solution

1. Step 1: Understand slicing with step and negative step

   The slice data[::-2] starts from the end and takes every second element backwards.

2. Step 2: Check what data[::-2] returns

   It returns [30, 20, 10], which is every second element in reverse order.

3. Step 3: Verify other options

   data[::2] returns every second element forward: [5, 15, 25]. data[-2::-1] returns from index -2 backwards: [25, 20, 15, 10, 5]. data[1::2] returns every second element starting at index 1: [10, 20, 30].

4. Final Answer:

   data[::-2] -> Option A

5. Quick Check:

   Negative step reverses and skips elements [OK]

Hint: Use negative step to reverse and skip elements [OK]

Common Mistakes:

• Using positive step for reverse
• Confusing start index with step sign
• Misunderstanding slice boundaries