List indexing and slicing in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of negative step slicing

What is the output of this Python code?

lst = [10, 20, 30, 40, 50]
print(lst[4:1:-1])

Python

lst = [10, 20, 30, 40, 50]
print(lst[4:1:-1])

A [50, 40, 30, 20]

B [50, 40, 30]

C [40, 30, 20]

D [50, 40]

💡 Hint

Remember that slicing with a negative step goes backwards and excludes the stop index.

The slice lst[4:1:-1] starts at index 4 (value 50), goes backwards up to but not including index 1, so it includes indices 4, 3, 2 which are 50, 40, 30.

❓ Predict Output

intermediate

Indexing with out-of-range positive index

What happens when you run this code?

lst = [1, 2, 3]
print(lst[5])

Python

lst = [1, 2, 3]
print(lst[5])

A IndexError

B 0

C None

D 3

💡 Hint

Check what happens if you try to access an index that does not exist.

Accessing lst[5] raises an IndexError because the list has only 3 elements (indices 0 to 2).

❓ Predict Output

advanced

Slicing with omitted start and stop

What is the output of this code?

lst = ['a', 'b', 'c', 'd', 'e']
print(lst[::-2])

Python

lst = ['a', 'b', 'c', 'd', 'e']
print(lst[::-2])

A ['e', 'd', 'c', 'b', 'a']

B ['a', 'c', 'e']

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

D ['e', 'c', 'a']

💡 Hint

When start and stop are omitted with a negative step, slicing goes from the end to the start.

lst[::-2] starts from the end and takes every second element backwards: indices 4, 2, 0 → 'e', 'c', 'a'.

❓ Predict Output

advanced

Effect of slice assignment on list

What is the value of lst after this code runs?

lst = [1, 2, 3, 4, 5]
lst[1:4] = [9, 8]
print(lst)

Python

lst = [1, 2, 3, 4, 5]
lst[1:4] = [9, 8]
print(lst)

A [1, 9, 8, 3, 4, 5]

B [1, 9, 8, 5]

C [1, 9, 8, 4, 5]

D [1, 2, 9, 8, 5]

💡 Hint

Slice assignment replaces the elements in the slice with the new list, adjusting length if needed.

lst[1:4] covers elements at indices 1, 2, 3 (values 2, 3, 4). Replacing them with [9, 8] shortens the list by one element.

🧠 Conceptual

expert

Understanding slice indices with negative steps

Given the list lst = [0,1,2,3,4,5,6,7,8,9], what is the output of lst[7:2:-2]?

Python

lst = [0,1,2,3,4,5,6,7,8,9]
print(lst[7:2:-2])

A [7, 5, 3]

B [7, 6, 5, 4, 3]

C [7, 5, 4, 3]

D [7, 6, 4, 2]

💡 Hint

Remember slicing excludes the stop index and steps backwards by 2.

Starting at index 7 (value 7), stepping backwards by 2 gives indices 7, 5, 3 (values 7, 5, 3). Index 2 is excluded.

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