Array comparison and set operations in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the == operator do when used between two arrays in Ruby?

It checks if both arrays have the same elements in the same order.

beginner

How do you find the common elements between two arrays in Ruby?

Use the & operator to get the intersection of two arrays.

beginner

What does the | operator do with arrays in Ruby?

It returns a new array with all unique elements from both arrays combined (union).

intermediate

Explain the difference between - and ^ (exclusive or) operations on arrays in Ruby.

- returns elements from the first array that are not in the second. Ruby does not have a built-in ^ operator for arrays, but you can simulate symmetric difference by combining - and |.

intermediate

How can you check if one array is a subset of another in Ruby?

Use array1 - array2 == [] to check if all elements of array1 are in array2.

What will [1, 2, 3] & [2, 3, 4] return in Ruby?

A [2, 3]

B [1, 2, 3, 4]

C [1]

D [4]

The & operator returns the intersection, so only elements common to both arrays: 2 and 3.

Which operator returns all unique elements from two arrays combined?

A -

B &

C |

D ==

The | operator returns the union of two arrays with unique elements.

What does [1, 2, 3] - [2] return?

A [2]

B [1, 3]

C [1, 2, 3]

D []

The - operator removes elements in the second array from the first, so 2 is removed.

How can you check if two arrays have the same elements in the same order?

A Use <code>array1 & array2</code>

B Use <code>array1 - array2</code>

C Use <code>array1 | array2</code>

D Use <code>array1 == array2</code>

The == operator compares arrays element by element in order.

Which expression checks if arr1 is a subset of arr2?

A arr1 - arr2 == []

B arr1 & arr2 == arr1

C arr1 | arr2 == arr2

D arr1 == arr2

If removing all elements of arr2 from arr1 leaves an empty array, arr1 is a subset of arr2.

Describe how to find the intersection, union, and difference of two arrays in Ruby.

Think about set operations and their Ruby symbols.

Explain how to check if two arrays are equal and how to verify if one array is a subset of another.

Consider comparing elements and removing elements.

Practice

1.

Which operator in Ruby returns the common elements between two arrays?

[1, 2, 3] ? [2, 3, 4]

easy

A. & (intersection)

B. | (union)

C. - (difference)

D. == (equality)

Solution

1. Step 1: Understand the operators

   The & operator returns elements common to both arrays.

2. Step 2: Apply to example arrays

   For arrays [1, 2, 3] and [2, 3, 4], common elements are 2 and 3.

3. Final Answer:

   & (intersection) -> Option A

4. Quick Check:

   Common elements = & [OK]

Hint: Use & for common items between arrays [OK]

Common Mistakes:

• Confusing union (|) with intersection (&)
• Using - for common elements
• Thinking == compares elements regardless of order

2.

Which of the following is the correct syntax to find the difference between two arrays a and b in Ruby?

easy

A. a == b

B. a - b

C. a | b

D. a & b

Solution

1. Step 1: Recall difference operator

   The - operator returns elements in a not in b.

2. Step 2: Confirm syntax correctness

   Using a - b is the correct Ruby syntax for difference.

3. Final Answer:

   a - b -> Option B

4. Quick Check:

   Difference = - [OK]

Hint: Use a - b to get items in a but not in b [OK]

Common Mistakes:

• Using & instead of - for difference
• Using | which is union
• Using == which checks equality

3.

What is the output of the following Ruby code?

a = [1, 2, 3, 4]
b = [3, 4, 5, 6]
result = a | b
puts result.inspect

medium

A. [1, 2, 3, 4, 5, 6]

B. [3, 4]

C. [1, 2]

D. [1, 2, 3, 4]

Solution

1. Step 1: Understand the union operator

   The | operator combines all unique elements from both arrays.

2. Step 2: Apply union to arrays

   Arrays a and b combined uniquely are [1, 2, 3, 4, 5, 6].

3. Final Answer:

   [1, 2, 3, 4, 5, 6] -> Option A

4. Quick Check:

   Union = | [OK]

Hint: Use | to combine unique elements from both arrays [OK]

Common Mistakes:

• Confusing union with intersection
• Expecting duplicates in output
• Using - which removes elements

4.

Identify the error in the following Ruby code snippet:

a = [1, 2, 3]
b = [2, 3, 4]
result = a & b
puts result

What is the problem?

medium

A. Syntax error due to missing parentheses

B. Outputs all elements from both arrays

C. Runtime error because & is not defined for arrays

D. No error; code runs and outputs common elements

Solution

1. Step 1: Check syntax and operators

   The & operator is valid for arrays to find intersection.

2. Step 2: Verify output behavior

   The code outputs common elements [2, 3] without error.

3. Final Answer:

   No error; code runs and outputs common elements -> Option D

4. Quick Check:

   Intersection & works fine [OK]

Hint: & works for arrays to find common items without syntax errors [OK]

Common Mistakes:

• Thinking & causes syntax error
• Expecting union instead of intersection
• Confusing output with difference

5.

You have two arrays representing user IDs:

active_users = [101, 102, 103, 104, 105]
inactive_users = [103, 105, 107]

Which Ruby expression gives you the list of users who are active but not inactive?

hard

A. active_users | inactive_users

B. active_users & inactive_users

C. active_users - inactive_users

D. active_users == inactive_users

Solution

1. Step 1: Understand the problem

   We want users in active_users but not in inactive_users.

2. Step 2: Use difference operator

   The - operator returns elements in first array not in second.

3. Step 3: Apply to given arrays

   active_users - inactive_users = [101, 102, 104]

4. Final Answer:

   active_users - inactive_users -> Option C

5. Quick Check:

   Difference = - [OK]

Hint: Use - to exclude inactive from active users [OK]

Common Mistakes:

• Using & which gives only common users
• Using | which combines all users
• Using == which checks equality