Union and intersection in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the union of two sets in Python?

The union of two sets is a new set containing all unique elements from both sets. It combines elements without duplicates.

beginner

What does the intersection of two sets represent?

The intersection of two sets is a new set containing only the elements that appear in both sets.

beginner

How do you find the union of sets a and b in Python?

You can use a.union(b) or the | operator like a | b.

beginner

How do you find the intersection of sets a and b in Python?

You can use a.intersection(b) or the & operator like a & b.

beginner

If a = {1, 2, 3} and b = {3, 4, 5}, what is a | b and a & b?

a | b is {1, 2, 3, 4, 5} (union), and a & b is {3} (intersection).

Which operator gives the union of two sets a and b in Python?

A |

B &

C ^

D -

The | operator returns the union of two sets.

What does a & b return when a and b are sets?

A Intersection of a and b

B Difference of a and b

C Union of a and b

D Symmetric difference of a and b

The & operator returns the intersection of two sets.

Which method can be used to find the union of two sets a and b?

A a.intersection(b)

B a.difference(b)

C a.union(b)

D a.symmetric_difference(b)

The union() method returns the union of two sets.

If a = {1, 2} and b = {2, 3}, what is a & b?

A {1, 2, 3}

B {1}

C set()

D {2}

The intersection contains only elements common to both sets, which is {2}.

What will a | b return if a = {5, 6} and b = {6, 7}?

A {6}

B {5, 6, 7}

C {5, 7}

D {}

The union combines all unique elements from both sets.

Explain in your own words what the union and intersection of two sets mean.

Think about combining two groups of friends versus finding friends you both have.

Describe how to find the union and intersection of two sets in Python using operators and methods.

Remember the symbols look like OR and AND.

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