List concatenation and repetition in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the + operator do when used with two lists in Python?

The + operator joins two lists end-to-end, creating a new list with elements from both lists in order.

beginner

What is the result of [1, 2] * 3 in Python?

It repeats the list [1, 2] three times, resulting in [1, 2, 1, 2, 1, 2].

intermediate

Can you use + and * operators together on lists? What happens?

Yes. You can concatenate lists with + and then repeat the result with *. For example, ([1] + [2]) * 2 gives [1, 2, 1, 2].

beginner

Does list concatenation with + modify the original lists?

No. The + operator creates a new list and does not change the original lists.

beginner

What happens if you multiply a list by zero, like [1, 2, 3] * 0?

You get an empty list [] because repeating zero times means no elements.

What is the output of [1, 2] + [3, 4]?

A [4, 3, 2, 1]

B [1, 2, 3, 4]

C [1, 2]

D [3, 4]

The + operator joins the two lists in order, so the result is [1, 2, 3, 4].

What does [0] * 5 produce?

A [0]

B [5]

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

D Error

The list [0] is repeated 5 times, creating a list with five zeros.

Which operator is used to repeat a list multiple times?

A *

B /

C -

D +

The * operator repeats a list the specified number of times.

What is the result of [1, 2] + [3] * 2?

A [1, 2, 3, 3]

B [1, 2, 3, 2]

C [1, 2, 3]

D [3, 3]

The list [3] is repeated twice to [3, 3], then concatenated to [1, 2].

Does list1 + list2 change list1?

A No, it deletes list1

B Yes, it modifies list1

C Yes, it modifies list2

D No, it creates a new list

The + operator creates a new list and does not change the original lists.

Explain how list concatenation and repetition work in Python with examples.

Think about how you can join two groups of items or repeat a group multiple times.

What happens if you multiply a list by zero or a negative number? Why?

Consider how repeating something zero times means you get nothing.

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