Adding and removing set elements in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code adding elements to a set?

Consider the following Python code that adds elements to a set. What will be printed?

Python

s = {1, 2, 3}
s.add(4)
s.add(2)
print(s)

A {1, 2, 3, 4}

B {1, 2, 3, 4, 2}

C {4}

D Error: cannot add duplicate elements

💡 Hint

Remember that sets do not allow duplicates.

Adding 4 to the set adds a new element. Adding 2 again does nothing because 2 is already in the set. So the set contains {1, 2, 3, 4}.

❓ Predict Output

intermediate

What happens when removing an element not in the set?

What will happen when this code runs?

Python

s = {10, 20, 30}
s.remove(40)
print(s)

A KeyError

B {10, 20, 30}

C None

D Set unchanged: {10, 20, 30}

💡 Hint

Check what remove() does if the element is missing.

The remove() method raises a KeyError if the element is not found in the set.

❓ Predict Output

advanced

What is the output after using discard on a set?

Look at this code and decide what it prints.

Python

s = {5, 6, 7}
s.discard(6)
s.discard(10)
print(s)

A {5, 6, 7}

B {5, 6, 7, 10}

C KeyError

D {5, 7}

💡 Hint

Discard does not raise an error if the element is missing.

Discard removes 6 from the set. Discarding 10 does nothing because 10 is not in the set and no error is raised.

🧠 Conceptual

advanced

Which method safely removes an element without error if missing?

You want to remove an element from a set but avoid errors if it is not present. Which method should you use?

A remove()

B discard()

C pop()

D clear()

💡 Hint

One method raises an error if the element is missing, the other does not.

discard() removes the element if present but does nothing if missing, so it is safe to use.

❓ Predict Output

expert

What is the final set after these operations?

Analyze the code below and determine the final content of the set.

Python

s = set()
s.add(1)
s.add(2)
s.add(3)
s.remove(2)
s.discard(4)
s.add(2)
s.remove(1)
print(s)

A {3}

B {1, 2, 3, 4}

C {2, 3}

D {1, 3}

💡 Hint

Track each add and remove step carefully.

Initially empty set. Add 1,2,3 → {1,2,3}. Remove 2 → {1,3}. Discard 4 (no error) → {1,3}. Add 2 → {1,2,3}. Remove 1 → {2,3}.

Practice

1. Which method is used to add a new element to a Python set?

easy

A. add()

B. append()

C. insert()

D. push()

Solution

1. Step 1: Understand set methods

   Sets in Python use add() to add elements, unlike lists which use append().

2. Step 2: Identify correct method for sets

   insert() and push() are not valid set methods.

3. Final Answer:

   add() -> Option A

4. Quick Check:

   Use add() to add elements to sets [OK]

Hint: Remember: sets use add(), lists use append() [OK]

Common Mistakes:

• Confusing list methods with set methods
• Trying to use append() on sets
• Using insert() which is for lists

2. Which of the following is the correct syntax to remove an element 5 from a set s without causing an error if 5 is not present?

easy

A. s.remove(5)

B. s.delete(5)

C. s.pop(5)

D. s.discard(5)

Solution

1. Step 1: Understand remove() vs discard()

   remove() raises an error if the element is missing, but discard() does not.

2. Step 2: Check method validity

   delete() is not a set method, and pop() removes an arbitrary element without arguments.

3. Final Answer:

   s.discard(5) -> Option D

4. Quick Check:

   Use discard() to safely remove elements [OK]

Hint: Use discard() to avoid errors when removing [OK]

Common Mistakes:

• Using remove() without checking element presence
• Trying to use delete() which doesn't exist
• Passing arguments to pop() which takes none

3. What will be the output of the following code?

fruits = {'apple', 'banana', 'cherry'}
fruits.add('orange')
fruits.remove('banana')
print(fruits)

medium

A. {'apple', 'cherry', 'orange'}

B. {'apple', 'banana', 'cherry', 'orange'}

C. {'apple', 'banana', 'orange'}

D. Error

Solution

1. Step 1: Add 'orange' to the set

   Using add('orange') adds 'orange' to the set, so now it has {'apple', 'banana', 'cherry', 'orange'}.

2. Step 2: Remove 'banana' from the set

   remove('banana') deletes 'banana', leaving {'apple', 'cherry', 'orange'}.

3. Final Answer:

   {'apple', 'cherry', 'orange'} -> Option A

4. Quick Check:

   add() adds, remove() deletes existing element [OK]

Hint: Add then remove changes set contents accordingly [OK]

Common Mistakes:

• Expecting banana to remain after remove()
• Thinking add() replaces elements
• Confusing set order in output

4. The following code throws an error. What is the cause and how to fix it?

numbers = {1, 2, 3}
numbers.remove(4)
print(numbers)

medium

A. SyntaxError due to wrong method; fix by using delete(4)

B. No error; output is {1, 2, 3}

C. KeyError because 4 is not in set; fix by using discard(4)

D. TypeError because remove() needs a list; fix by converting set to list

Solution

1. Step 1: Identify error cause

   remove(4) raises a KeyError because 4 is not in the set.

2. Step 2: Fix error using discard()

   Replacing remove(4) with discard(4) avoids error even if 4 is missing.

3. Final Answer:

   KeyError because 4 is not in set; fix by using discard(4) -> Option C

4. Quick Check:

   remove() errors if missing; discard() does not [OK]

Hint: Use discard() to avoid errors when unsure element exists [OK]

Common Mistakes:

• Assuming remove() never errors
• Trying to use delete() which is invalid
• Confusing error types

5. Given a set nums = {1, 2, 3, 4, 5}, which code snippet correctly adds 6 and removes 2 safely without errors, even if 2 might not be present?

hard

A. nums.add(6)
nums.remove(2)

B. nums.add(6)
nums.discard(2)

C. nums.append(6)
nums.discard(2)

D. nums.add(6)
nums.delete(2)

Solution

1. Step 1: Add element 6 correctly

   add(6) is the correct method to add an element to a set.

2. Step 2: Remove element 2 safely

   discard(2) removes 2 without error if missing; remove(2) could cause error.

3. Final Answer:

   nums.add(6) nums.discard(2) -> Option B

4. Quick Check:

   add() to add, discard() to safely remove [OK]

Hint: Add with add(), remove safely with discard() [OK]

Common Mistakes:

• Using append() which is for lists
• Using remove() without checking element presence
• Trying to use delete() which doesn't exist