Array Deletion at End in DSA Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'Array Deletion at End' mean?

It means removing the last element from an array, making the array one element shorter.

beginner

In Rust, which method removes the last element from a vector?

The pop() method removes the last element from a vector and returns it as an Option.

beginner

What happens if you call pop() on an empty vector in Rust?

It returns None, meaning there is no element to remove.

intermediate

Why is deleting at the end of an array efficient?

Because it does not require shifting other elements, so it runs in constant time (O(1)).

beginner

Show a simple Rust code snippet to delete the last element from a vector and print the vector.

let mut v = vec![1, 2, 3];
v.pop();
println!("{:?}", v); // Output: [1, 2]

What does the pop() method do in Rust vectors?

A Removes the first element

B Removes the last element and returns it

C Adds an element at the end

D Clears the entire vector

The pop() method removes the last element from the vector and returns it wrapped in Some. If the vector is empty, it returns None.

What is the time complexity of deleting the last element from an array?

A O(n)

B O(n^2)

C O(1)

D O(log n)

Deleting the last element is done in constant time because no other elements need to be moved.

If a Rust vector is empty, what does pop() return?

A None

B An error

C The last element

D Zero

pop() returns None when the vector is empty, indicating no element was removed.

Which Rust type wraps the result of pop()?

A Result

B Vec

C Box

D Option

pop() returns an Option type because the vector might be empty.

What happens to the vector size after calling pop()?

A It decreases by one

B It stays the same

C It increases by one

D It becomes zero

The vector size decreases by one because the last element is removed.

Explain how to delete the last element from an array or vector and why this operation is efficient.

Think about what happens to the array size and element positions.

Describe what happens when you call the pop() method on an empty Rust vector.

Consider how Rust handles absence of values safely.

Practice

1. What happens when you remove the last element from a Rust Vec using pop()?

easy

A. The last element is removed and returned as Some(value).

B. The first element is removed and returned.

C. The entire vector is cleared.

D. The vector becomes immutable.

Solution

1. Step 1: Understand pop() behavior

   The pop() method removes the last element from the vector and returns it wrapped in Some(value) if the vector is not empty.

2. Step 2: Confirm it does not affect other elements

   Only the last element is removed; the rest of the vector remains unchanged.

3. Final Answer:

   The last element is removed and returned as Some(value). -> Option A

4. Quick Check:

   pop() removes last element [OK]

Hint: pop() removes and returns last element wrapped in Some [OK]

Common Mistakes:

• Thinking pop() removes the first element
• Assuming pop() clears the entire vector
• Believing pop() makes vector immutable

2. Which of the following is the correct syntax to remove the last element from a mutable vector v in Rust?

easy

A. v.erase_last();

B. v.remove_last();

C. v.delete_end();

D. v.pop();

Solution

1. Step 1: Recall Rust vector methods

   The standard method to remove the last element from a vector is pop().

2. Step 2: Check method names

   Methods like remove_last(), delete_end(), and erase_last() do not exist in Rust's Vec API.

3. Final Answer:

   v.pop(); -> Option D

4. Quick Check:

   Correct method to remove last element is pop() [OK]

Hint: Use pop() to remove last element from vector [OK]

Common Mistakes:

• Using non-existent methods like remove_last()
• Confusing pop() with remove() which removes by index
• Trying to use erase_last() or delete_end() which are invalid

3. What is the output of the following Rust code?

let mut v = vec![10, 20, 30];
let x = v.pop();
println!("{:?}", v);
println!("{:?}", x);

medium

A. [10, 20]
None

B. [10, 20, 30]
None

C. [10, 20]
Some(30)

D. [20, 30]
Some(10)

Solution

1. Step 1: Understand pop() effect on vector

   The pop() removes the last element, which is 30, and returns Some(30).

2. Step 2: Check vector after pop

   After removal, the vector contains [10, 20].

3. Final Answer:

   [10, 20] Some(30) -> Option C

4. Quick Check:

   pop() removes last element 30, vector left with [10, 20] [OK]

Hint: pop() returns last element and shortens vector [OK]

Common Mistakes:

• Expecting pop() to remove first element
• Assuming pop() returns None when vector is not empty
• Confusing vector contents after pop

4. Identify the error in the following Rust code that tries to remove the last element from a vector:

let v = vec![1, 2, 3];
v.pop();

medium

A. pop() method does not exist for vectors.

B. Cannot call pop() on an immutable vector.

C. pop() removes the first element, not the last.

D. The vector must be empty before calling pop().

Solution

1. Step 1: Check vector mutability

   The vector v is declared immutable with let. To modify it, it must be mutable.

2. Step 2: Understand pop() requirements

   The pop() method requires a mutable vector to remove the last element.

3. Final Answer:

   Cannot call pop() on an immutable vector. -> Option B

4. Quick Check:

   pop() needs mutable vector [OK]

Hint: Make vector mutable with 'mut' before pop() [OK]

Common Mistakes:

• Trying to pop from immutable vector
• Thinking pop() removes first element
• Assuming pop() works on empty vector without error

5. Given a vector v with elements [5, 10, 15, 20], write Rust code to remove the last two elements and print the resulting vector. What will be printed?

hard

A. [5, 10]

B. [5, 10, 15]

C. [15, 20]

D. [5, 10, 15, 20]

Solution

1. Step 1: Remove last element twice using pop()

   Calling pop() twice removes 20 then 15, leaving [5, 10].

2. Step 2: Print the vector after removals

   Printing v now shows [5, 10].

3. Final Answer:

   [5, 10] -> Option A

4. Quick Check:

   Two pops remove last two elements [OK]

Hint: Call pop() twice to remove last two elements [OK]

Common Mistakes:

• Removing only one element instead of two
• Confusing pop() with remove(index)
• Expecting vector to be empty after two pops