List concatenation and repetition in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of list concatenation and repetition

What is the output of the following Python code?

Python

a = [1, 2]
b = [3, 4]
c = a + b * 2
print(c)

A [1, 2, 3, 4, 1, 2, 3, 4]

B [1, 2, 3, 4, 3, 4, 1, 2]

C [1, 2, 3, 4, 3, 4]

D [1, 2, 3, 4, 4, 3]

💡 Hint

Remember that list repetition happens before concatenation.

The expression b * 2 repeats list b twice, resulting in [3, 4, 3, 4]. Then a + (b * 2) concatenates [1, 2] with [3, 4, 3, 4].

❓ Predict Output

intermediate

Result of repeated concatenation

What will be printed by this code?

Python

lst = [0]
lst = lst * 3 + [1]
print(lst)

A [0, 0, 0, 1]

B [0, 1, 0, 1]

C [0, 0, 1]

D [1, 0, 0, 0]

💡 Hint

Check the order of operations: repetition then concatenation.

lst * 3 repeats [0] three times: [0, 0, 0]. Then adding [1] results in [0, 0, 0, 1].

❓ Predict Output

advanced

Nested list repetition and concatenation output

What is the output of this code snippet?

Python

x = [[1]]
y = x * 3 + [[2]]
print(y)

A [[1], [1], [1], [2]]

B [[1, 1, 1], [2]]

C [[1], [2], [1], [1]]

D [[1], [1], [2], [1]]

💡 Hint

List repetition duplicates references to the inner list.

x * 3 creates a list with three references to [1]. Adding [[2]] appends another list. The final list has four elements: three [1] lists and one [2] list.

❓ Predict Output

advanced

Effect of list repetition on mutable elements

What will be the output after running this code?

Python

lst = [[0]] * 3
lst[0].append(1)
print(lst)

A [[0], [0], [0, 1]]

B [[0], [0, 1], [0]]

C [[0, 1], [0], [0]]

D [[0, 1], [0, 1], [0, 1]]

💡 Hint

Remember that list repetition copies references, not the objects themselves.

All three elements in lst refer to the same inner list. Appending 1 to lst[0] affects all three references.

🧠 Conceptual

expert

Understanding list concatenation and repetition precedence

Which option correctly describes the result of this expression: [1] + [2] * 3?

A A list with two elements: [1, 6]

B A list starting with 1 followed by three 2's: [1, 2, 2, 2]

C A list with four elements all equal to 3: [3, 3, 3, 3]

D A list with three elements: [1, 2, 3]

💡 Hint

Recall that repetition happens before concatenation.

The expression [2] * 3 creates [2, 2, 2]. Then [1] + [2, 2, 2] concatenates to [1, 2, 2, 2].