Iterating over lists in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to iterate over a list in Python?

Iterating over a list means going through each item in the list one by one to perform an action with each item.

beginner

Which Python keyword is commonly used to loop through each item in a list?

The for keyword is used to loop through each item in a list.

beginner

What will this code print?

fruits = ['apple', 'banana', 'cherry']
for fruit in fruits:
    print(fruit)

It will print each fruit on its own line:
apple
banana
cherry

intermediate

How can you access both the index and the item when iterating over a list?

Use the enumerate() function in the loop, like:

for index, item in enumerate(my_list):
    # do something with index and item

intermediate

True or False: You can modify the items of a list directly while iterating over it.

True, but you should be careful. Modifying the list while iterating can cause unexpected behavior if you add or remove items. Changing item values is safe.

What does the following code do?

numbers = [1, 2, 3]
for num in numbers:
    print(num * 2)

A Causes an error

B Prints 1, 2, 3 each on a new line

C Prints 2, 4, 6 each on a new line

D Prints the list [2, 4, 6]

The loop goes through each number and prints the number multiplied by 2.

Which function helps get the index and value when looping over a list?

A enumerate()

B index()

C range()

D len()

The enumerate() function returns pairs of (index, value) during iteration.

What will this code print?

items = ['a', 'b', 'c']
for i in range(len(items)):
    print(items[i])

A 0 1 2 on separate lines

B a b c on separate lines

C Error because of range()

D Nothing

The loop uses indexes to access each item and prints them one by one.

Is this a correct way to loop over a list?

for item in my_list:
    print(item)

A Only works if list has numbers

B No, it will cause an error

C Only works if list is empty

D Yes, it loops through each item

This is the standard way to loop through all items in a list.

What happens if you modify a list by adding items while iterating over it?

A It can cause unexpected behavior or an infinite loop

B The loop will include the new items automatically

C Python prevents adding items during iteration

D Nothing special happens

Changing the list size during iteration can confuse the loop and cause errors or infinite loops.

Explain how to use a for loop to iterate over a list and print each item.

Think about how you would tell a friend to read each item one by one.

Describe how to get both the index and the value of items when looping over a list.

Remember there is a special function that helps with indexes.

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