Iterating over lists in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested loops with list iteration

What is the output of this Python code?

Python

result = []
for i in [1, 2]:
    for j in [3, 4]:
        result.append(i * j)
print(result)

A [3, 4, 6, 8]

B [3, 6, 4, 8]

C [4, 3, 8, 6]

D [1, 2, 3, 4]

💡 Hint

Think about how the outer and inner loops multiply each pair of numbers.

The outer loop goes over 1 and 2. For each, the inner loop multiplies by 3 and 4. So the products are 1*3=3, 1*4=4, 2*3=6, 2*4=8, collected in order.

❓ Predict Output

intermediate

Output of list comprehension with condition

What is the output of this code?

Python

numbers = [0, 1, 2, 3, 4]
squares = [x**2 for x in numbers if x % 2 == 0]
print(squares)

A [0, 1, 4, 9, 16]

B [1, 9]

C [0, 4, 16]

D [2, 4]

💡 Hint

Only even numbers are squared and included.

The list comprehension squares only numbers divisible by 2: 0, 2, and 4. Their squares are 0, 4, and 16.

🔧 Debug

advanced

Identify the error in list iteration

What error does this code raise when run?

Python

items = [1, 2, 3]
for i in range(len(items)):
    print(items[i+1])

A IndexError

B TypeError

C SyntaxError

D No error, prints 2, 3, and 4

💡 Hint

Check the index used inside the loop compared to the list length.

The loop tries to access items[i+1]. When i is 2 (last index), i+1 is 3, which is out of range for the list of length 3, causing IndexError.

❓ Predict Output

advanced

Output of modifying list during iteration

What is the output of this code?

Python

lst = [1, 2, 3]
for x in lst:
    lst.append(x + 3)
    if len(lst) > 6:
        break
print(lst)

A [1, 2, 3, 4, 5, 6, 7]

B [1, 2, 3]

C Infinite loop

D [1, 2, 3, 4, 5, 6]

💡 Hint

Appending to a list during 'for x in lst' iteration only processes the original elements; the break checks length after each append.

The 'for x in lst' iterates over the list as it grows. For x=1, append 4 (len=4 <=6); x=2, append 5 (len=5 <=6); x=3, append 6 (len=6 <=6). After appending 6, length becomes 7 (>6), so break triggers. Final list: [1, 2, 3, 4, 5, 6, 7].

🧠 Conceptual

expert

Number of items after complex iteration

How many items are in the list 'result' after running this code?

Python

result = []
for i in range(3):
    for j in range(i):
        result.append(i + j)
print(len(result))

A 4

B 3

C 6

D 0

💡 Hint

Count how many times the inner loop runs for each i.

For i=0, inner loop runs 0 times; i=1 runs 1 time; i=2 runs 2 times. Total items appended: 0 + 1 + 2 = 3.