Union and intersection in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this union operation?

Consider two sets A and B. What will be printed after the union operation?

Python

A = {1, 2, 3}
B = {3, 4, 5}
print(A | B)

A {1, 2, 3}

B {3}

C {1, 2, 4, 5}

D {1, 2, 3, 4, 5}

💡 Hint

Union combines all unique elements from both sets.

The union operator | returns a set with all unique elements from both sets. Here, elements 1, 2, 3, 4, and 5 appear in either A or B.

❓ Predict Output

intermediate

What is the output of this intersection operation?

Given two sets X and Y, what will be printed after the intersection operation?

Python

X = {10, 20, 30, 40}
Y = {30, 40, 50, 60}
print(X & Y)

A {30, 40}

B {10, 20}

C {50, 60}

D {10, 20, 30, 40, 50, 60}

💡 Hint

Intersection returns only elements common to both sets.

The intersection operator & returns a set with elements present in both sets. Here, 30 and 40 are common.

🧠 Conceptual

advanced

Which option shows the correct way to find the union of two sets using a method?

You want to find the union of sets set1 and set2 using a set method. Which option is correct?

A set1.intersection(set2)

B set1.add(set2)

C set1.union(set2)

D set1.update(set2)

💡 Hint

Union method returns a new set with all unique elements from both sets.

The union() method returns a new set with all elements from both sets. add() adds a single element, intersection() finds common elements, and update() modifies the set in place.

❓ Predict Output

advanced

What is the output of this combined union and intersection code?

Given sets a and b, what will be printed?

Python

a = {1, 2, 3, 4}
b = {3, 4, 5, 6}
result = (a | b) & a
print(result)

A {3, 4}

B {1, 2, 3, 4}

C {5, 6}

D {1, 2}

💡 Hint

First union all elements, then intersect with set a.

The union a | b is {1, 2, 3, 4, 5, 6}. Intersecting with a keeps only elements in a, so result is {1, 2, 3, 4}.

🔧 Debug

expert

What error does this code raise?

Examine the code below. What error will it raise when run?

Python

set_a = {1, 2, 3}
set_b = [3, 4, 5]
result = set_a | set_b
print(result)

A TypeError

B SyntaxError

C NameError

D No error, prints {1, 2, 3, 4, 5}

💡 Hint

The union operator requires both operands to be sets.

The union operator | cannot be used between a set and a list. This causes a TypeError.

Practice

1. What does the union operation do when applied to two sets in Python?

easy

A. Combines all unique elements from both sets

B. Finds elements only in the first set

C. Finds elements only in the second set

D. Finds elements common to both sets

Solution

1. Step 1: Understand union operation

   The union of two sets includes every unique element from both sets without duplicates.

2. Step 2: Compare with other options

   Options B and C describe subsets, and D describes intersection, not union.

3. Final Answer:

   Combines all unique elements from both sets -> Option A

4. Quick Check:

   Union = all unique elements combined [OK]

Hint: Union joins all unique items from both sets [OK]

Common Mistakes:

• Confusing union with intersection
• Thinking union only takes elements from one set
• Assuming union keeps duplicates

2. Which of the following is the correct syntax to find the intersection of two sets a and b in Python?

easy

A. a + b

B. a | b

C. a & b

D. a - b

Solution

1. Step 1: Recall intersection syntax

   In Python, the intersection of two sets is found using the & operator or the .intersection() method.

2. Step 2: Check other operators

   | is union, + is invalid for sets, - is difference.

3. Final Answer:

   a & b -> Option C

4. Quick Check:

   Intersection uses & operator [OK]

Hint: Use & for intersection of sets [OK]

Common Mistakes:

• Using | instead of & for intersection
• Trying to add sets with + operator
• Confusing difference (-) with intersection

3. What is the output of the following code?

set1 = {1, 2, 3, 4}
set2 = {3, 4, 5, 6}
print(set1.intersection(set2))

medium

A. {1, 2}

B. {5, 6}

C. {1, 2, 3, 4, 5, 6}

D. {3, 4}

Solution

1. Step 1: Identify intersection elements

   The intersection contains elements present in both sets: {3, 4}.

2. Step 2: Confirm output

   Printing the intersection returns {3, 4} exactly.

3. Final Answer:

   {3, 4} -> Option D

4. Quick Check:

   Common elements = {3, 4} [OK]

Hint: Intersection shows only common elements [OK]

Common Mistakes:

• Confusing union with intersection output
• Expecting all elements combined
• Mixing up set contents

4. The following code is intended to print the union of two sets, but it causes an error. What is the problem?

set1 = {1, 2, 3}
set2 = {3, 4, 5}
print(set1 + set2)

medium

A. Sets cannot be added with + operator

B. The sets are empty

C. The print statement is missing parentheses

D. The sets have overlapping elements

Solution

1. Step 1: Identify the error cause

   Python sets do not support the + operator; this causes a TypeError.

2. Step 2: Correct way to union sets

   Use | operator or .union() method to combine sets.

3. Final Answer:

   Sets cannot be added with + operator -> Option A

4. Quick Check:

   + operator invalid for sets [OK]

Hint: Use | or .union(), not + for sets [OK]

Common Mistakes:

• Trying to add sets with +
• Ignoring error message
• Confusing list addition with set union

5. Given two lists of student names, list1 = ['Anna', 'Bob', 'Cara'] and list2 = ['Bob', 'Diana', 'Anna'], which Python code correctly finds students present in both lists using set operations?

hard

A. list1 | list2

B. set(list1) & set(list2)

C. set(list1) + set(list2)

D. list1.intersection(list2)

Solution

1. Step 1: Convert lists to sets for set operations

   Lists must be converted to sets to use intersection (&) operator.

2. Step 2: Use & operator to find common elements

   set(list1) & set(list2) returns elements in both sets.

3. Step 3: Check other options

   list1 | list2 uses | on lists (invalid), C uses + on sets (invalid), D calls intersection on list (no such method).

4. Final Answer:

   set(list1) & set(list2) -> Option B

5. Quick Check:

   Convert lists to sets, then & for intersection [OK]

Hint: Convert lists to sets before intersection [OK]

Common Mistakes:

• Using + or | on lists directly
• Calling set methods on lists
• Not converting lists to sets first