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.