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.

Practice

1. What does the following code do?

for item in [1, 2, 3]:
    print(item)

easy

A. Prints the whole list once

B. Prints numbers 1 to 3 without spaces

C. Prints each number 1, 2, and 3 on a new line

D. Causes an error because of the loop

Solution

1. Step 1: Understand the for loop over a list

   The loop goes through each element in the list [1, 2, 3] one by one.

2. Step 2: Print each element during iteration

   Each element (1, then 2, then 3) is printed on its own line because of the print statement.

3. Final Answer:

   Prints each number 1, 2, and 3 on a new line -> Option C

4. Quick Check:

   Loop prints items one by one [OK]

Hint: For loops print each list item separately [OK]

Common Mistakes:

• Thinking it prints the whole list at once
• Expecting numbers to print on the same line
• Confusing loop variable with list itself

2. Which of these is the correct syntax to loop over a list named fruits?

easy

A. for fruit in fruits:
print(fruit)

B. for fruits in fruit:
print(fruits)

C. loop fruit in fruits:
print(fruit)

D. for fruit to fruits:
print(fruit)

Solution

1. Step 1: Identify correct for loop syntax

   The correct Python syntax uses 'for variable in list:' to loop over items.

2. Step 2: Check each option

   for fruit in fruits: print(fruit) uses 'for fruit in fruits:', which is correct. Others use invalid keywords or reversed names.

3. Final Answer:

   for fruit in fruits: print(fruit) -> Option A

4. Quick Check:

   Use 'for item in list:' syntax [OK]

Hint: Remember: for variable in list: [OK]

Common Mistakes:

• Swapping variable and list names
• Using wrong keywords like 'loop' or 'to'
• Missing colon after for statement

3. What is the output of this code?

numbers = [2, 4, 6]
sum = 0
for n in numbers:
    sum += n
print(sum)

medium

A. 12

B. 0

C. 246

D. Error

Solution

1. Step 1: Understand the loop adding numbers

   The loop adds each number in the list [2, 4, 6] to sum, starting from 0.

2. Step 2: Calculate the total sum

   sum = 0 + 2 + 4 + 6 = 12

3. Final Answer:

   12 -> Option A

4. Quick Check:

   Sum of list items = 12 [OK]

Hint: Add each item inside the loop to get total [OK]

Common Mistakes:

• Printing sum before loop
• Concatenating numbers as strings
• Forgetting to initialize sum to 0

4. Find the error in this code:

items = [10, 20, 30]
for i in items
    print(i)

medium

A. List name is incorrect

B. Wrong variable name in loop

C. Indentation error in print

D. Missing colon after for statement

Solution

1. Step 1: Check for syntax errors in for loop

   The for loop must end with a colon ':' to be valid syntax.

2. Step 2: Identify missing colon

   The code misses ':' after 'for i in items', causing a syntax error.

3. Final Answer:

   Missing colon after for statement -> Option D

4. Quick Check:

   For loops need ':' at end [OK]

Hint: Always put ':' after for loop header [OK]

Common Mistakes:

• Forgetting colon after for statement
• Using wrong variable names
• Incorrect indentation of print

5. You have a list of lists: data = [[1, 2], [3, 4], [5, 6]]. How do you print all numbers one by one using iteration?

hard

A. print(data[0][0], data[1][1], data[2][2])

B. for sublist in data:
for num in sublist:
print(num)

C. for i in range(len(data)):
print(data[i])

D. for num in data:
print(num)

Solution

1. Step 1: Understand nested lists and iteration

   Each item in data is a list itself, so we need two loops: one for each sublist, one for numbers inside.

2. Step 2: Use nested for loops to access all numbers

   The outer loop goes through each sublist, the inner loop prints each number inside that sublist.

3. Final Answer:

   for sublist in data: for num in sublist: print(num) -> Option B

4. Quick Check:

   Nested loops print all inner items [OK]

Hint: Use nested loops for lists inside lists [OK]

Common Mistakes:

• Trying to print sublists directly
• Using single loop only
• Accessing wrong indexes causing errors