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].

Practice

1. What does the expression [1, 2] + [3, 4] do in Python?

easy

A. Creates a new list combining both lists: [1, 2, 3, 4]

B. Repeats the first list twice: [1, 2, 1, 2]

C. Creates a list of two lists: [[1, 2], [3, 4]]

D. Raises a TypeError because lists cannot be added

Solution

1. Step 1: Understand the + operator with lists

   The + operator joins two lists into one new list containing all elements from both.

2. Step 2: Apply + to [1, 2] and [3, 4]

   Joining these lists results in [1, 2, 3, 4].

3. Final Answer:

   Creates a new list combining both lists: [1, 2, 3, 4] -> Option A

4. Quick Check:

   List concatenation = combined list [OK]

Hint: Use + to join lists, not to repeat [OK]

Common Mistakes:

• Thinking + repeats lists instead of joining
• Confusing list concatenation with nesting
• Expecting an error when adding lists

2. Which of the following is the correct syntax to repeat the list [5, 6] three times?

easy

A. [5, 6] ** 3

B. [5, 6] + 3

C. 3 + [5, 6]

D. [5, 6] * 3

Solution

1. Step 1: Identify the operator for repetition

   The * operator repeats a list multiple times when used with an integer.

2. Step 2: Check syntax correctness

   [5, 6] * 3 repeats the list three times: [5, 6, 5, 6, 5, 6].

3. Final Answer:

   [5, 6] * 3 -> Option D

4. Quick Check:

   List repetition uses * operator [OK]

Hint: Use * with list and number to repeat [OK]

Common Mistakes:

• Using + instead of * for repetition
• Trying to use ** which is invalid for lists
• Confusing order of operands for repetition

3. What is the output of this code?

lst1 = [1]
lst2 = [2, 3]
result = lst1 * 2 + lst2 * 3
print(result)

medium

A. [1, 1, 2, 3, 2, 3, 2, 3]

B. [1, 2, 3, 1, 2, 3]

C. [1, 1, 2, 3, 3, 3]

D. [1, 1, 2, 3, 2, 3]

Solution

1. Step 1: Calculate lst1 * 2

   Repeating [1] twice gives [1, 1].

2. Step 2: Calculate lst2 * 3

   Repeating [2, 3] three times gives [2, 3, 2, 3, 2, 3].

3. Step 3: Concatenate the two results

   Adding [1, 1] + [2, 3, 2, 3, 2, 3] results in [1, 1, 2, 3, 2, 3, 2, 3].

4. Final Answer:

   [1, 1, 2, 3, 2, 3, 2, 3] -> Option A

5. Quick Check:

   Repeat then join lists = combined repeated list [OK]

Hint: Repeat lists first, then add them [OK]

Common Mistakes:

• Adding before repeating
• Miscounting repeated elements
• Confusing order of concatenation

4. Find the error in this code snippet:

list_a = [1, 2]
list_b = [3, 4]
result = list_a * list_b
print(result)

medium

A. No error, prints [1, 2, 3, 4]

B. SyntaxError due to missing parentheses

C. TypeError because * expects an integer, not a list

D. ValueError because lists cannot be multiplied

Solution

1. Step 1: Understand list repetition rules

   List repetition uses * with an integer, not another list.

2. Step 2: Identify the error in list_a * list_b

   Multiplying two lists causes a TypeError because the right operand must be int.

3. Final Answer:

   TypeError because * expects an integer, not a list -> Option C

4. Quick Check:

   List * list is invalid, needs int [OK]

Hint: Right side of * must be integer for list repetition [OK]

Common Mistakes:

• Trying to multiply two lists directly
• Expecting concatenation with * operator
• Confusing syntax error with type error

5. You have two lists:
fruits = ['apple', 'banana']
veggies = ['carrot']
How do you create a new list that repeats the fruits twice and veggies three times, then combines them all in order?

hard

A. fruits * veggies * 5

B. fruits * 2 + veggies * 3

C. fruits + veggies * 5

D. (fruits + veggies) * 5

Solution

1. Step 1: Repeat fruits and veggies separately

   fruits * 2 repeats fruits twice; veggies * 3 repeats veggies three times.

2. Step 2: Concatenate the repeated lists

   Adding these repeated lists combines them in order as desired.

3. Final Answer:

   fruits * 2 + veggies * 3 -> Option B

4. Quick Check:

   Repeat separately then add lists [OK]

Hint: Repeat lists separately, then add [OK]

Common Mistakes:

• Repeating combined list instead of separate lists
• Using invalid multiplication of lists
• Confusing order of operations