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

Recall & Review

beginner

What happens when you delete the first element of an array?

The first element is removed, and all other elements shift one position to the left to fill the gap.

beginner

Why is deleting the first element of an array less efficient than deleting the last element?

Because after deleting the first element, all other elements must move one step left, which takes more time.

beginner

In Rust, which method can be used to remove the first element of a vector?

You can use the `remove(0)` method on a vector to delete the first element.

intermediate

What is the time complexity of deleting the first element in a Rust vector?

It is O(n) because all elements after the first must be shifted left by one position.

intermediate

How can you avoid the cost of shifting elements when deleting from the beginning of a collection in Rust?

Use a data structure like `VecDeque` which allows efficient removal from both ends without shifting all elements.

What does the `remove(0)` method do on a Rust vector?

A Removes the first element and shifts others left

B Removes the last element

C Adds an element at the beginning

D Clears the entire vector

The `remove(0)` method deletes the element at index 0 and shifts all subsequent elements left.

What is the time complexity of deleting the first element in a Rust vector?

A O(n^2)

B O(1)

C O(log n)

D O(n)

Deleting the first element requires shifting all other elements, so it takes O(n) time.

Which Rust data structure allows efficient deletion from the beginning without shifting elements?

A Vec

B VecDeque

C LinkedList

D HashMap

`VecDeque` supports efficient removal from both ends without shifting all elements.

What happens to the indices of elements after deleting the first element in a vector?

A They stay the same

B They increase by one

C They decrease by one

D They become random

All elements shift left, so their indices decrease by one.

Which method is NOT used to delete the first element of a Rust vector?

A pop()

B drain(0..1)

C remove(0)

D split_off(1)

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

Explain how deleting the first element of an array affects the other elements and the overall performance.

Think about what happens to the positions of the remaining elements.

Describe how Rust's VecDeque helps with efficient deletion at the beginning compared to a Vec.

Consider how elements are stored and accessed in VecDeque.

Practice

1. What happens when you delete the first element from an array in Rust using remove(0)?

easy

A. The array elements remain unchanged.

B. The last element is removed instead of the first.

C. The array becomes empty regardless of its original size.

D. The first element is removed and all other elements shift left by one position.

Solution

1. Step 1: Understand remove(0) method

   The remove(0) method removes the element at index 0, which is the first element in the array.

2. Step 2: Effect on array elements

   After removal, all elements to the right shift left by one to fill the gap, reducing the array length by one.

3. Final Answer:

   The first element is removed and all other elements shift left by one position. -> Option D

4. Quick Check:

   Deleting at start shifts elements left [OK]

Hint: Deleting at index 0 shifts all elements left by one [OK]

Common Mistakes:

• Thinking last element is removed instead of first
• Assuming array size stays the same
• Believing elements do not shift after deletion

2. Which of the following Rust code snippets correctly deletes the first element from a vector v?

easy

A. v.remove(0);

B. v.delete(0);

C. v.pop_front();

D. v.shift();

Solution

1. Step 1: Identify correct Rust method for deletion at index

   Rust vectors use remove(index) to delete element at a specific position.

2. Step 2: Check other options for validity

   delete, pop_front, and shift are not valid Rust vector methods.

3. Final Answer:

   v.remove(0); -> Option A

4. Quick Check:

   Use remove(0) to delete first element [OK]

Hint: Use remove(0) to delete first element in Rust vector [OK]

Common Mistakes:

• Using non-existent methods like pop_front or shift
• Confusing Rust with other languages' syntax
• Trying to use delete which is invalid in Rust

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

let mut v = vec![10, 20, 30, 40];
v.remove(0);
println!("{:?}", v);

medium

A. [20, 30, 40]

B. [10, 20, 30, 40]

C. [30, 40]

D. [40]

Solution

1. Step 1: Initial vector elements

   The vector v starts as [10, 20, 30, 40].

2. Step 2: Remove first element

   v.remove(0) removes 10, shifting remaining elements left.

3. Step 3: Resulting vector

   After removal, v becomes [20, 30, 40].

4. Final Answer:

   [20, 30, 40] -> Option A

5. Quick Check:

   Remove first element shifts left [OK]

Hint: Removing index 0 drops first element, shifts others left [OK]

Common Mistakes:

• Expecting original vector unchanged
• Thinking multiple elements removed
• Confusing output format

4. Identify the error in this Rust code that tries to delete the first element of a vector:

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

medium

A. Cannot remove elements from a vector.

B. Missing index argument in remove method.

C. remove method does not exist for vectors.

D. Vector cannot be mutable.

Solution

1. Step 1: Check method signature

   The remove method requires an index argument specifying which element to remove.

2. Step 2: Identify missing argument

   The code calls v.remove() without any index, causing a compile error.

3. Final Answer:

   Missing index argument in remove method. -> Option B

4. Quick Check:

   remove needs index argument [OK]

Hint: Always provide index to remove method [OK]

Common Mistakes:

• Calling remove without index
• Thinking remove works like pop() without arguments
• Assuming vector immutability causes error

5. Given a vector v = vec![5, 10, 15, 20, 25], write Rust code to delete the first two elements and print the resulting vector. What will be the output?

hard

A. [5, 10, 15]

B. [10, 15, 20, 25]

C. [15, 20, 25]

D. [20, 25]

Solution

1. Step 1: Remove first element twice

   Calling v.remove(0) twice removes elements at index 0 each time, first 5 then 10.

2. Step 2: Resulting vector after removals

   After removals, vector becomes [15, 20, 25].

3. Final Answer:

   [15, 20, 25] -> Option C

4. Quick Check:

   Two removals at index 0 drop first two elements [OK]

Hint: Remove index 0 repeatedly to delete from start [OK]

Common Mistakes:

• Removing only once instead of twice
• Removing wrong indices after first removal
• Expecting original vector unchanged