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

Recall & Review

beginner

What happens when you delete an element from the middle of an array?

The elements after the deleted element shift one position to the left to fill the gap, reducing the array size by one.

beginner

Why do elements shift after deleting from the middle of an array?

Because arrays have fixed order and continuous memory, shifting keeps elements contiguous and maintains order.

beginner

In Rust, which method can be used to remove an element at a specific index in a vector?

The `remove(index)` method removes the element at the given index and shifts elements left automatically.

intermediate

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

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

beginner

Show a simple Rust code snippet to delete the middle element from a vector `[10, 20, 30, 40, 50]`.

let mut v = vec![10, 20, 30, 40, 50];
v.remove(2); // removes element 30
println!("{:?}", v); // prints [10, 20, 40, 50]

What happens to elements after deleting an element at index 3 in an array?

A Array elements are reversed

B Elements after index 3 shift left by one position

C No elements shift, array size stays the same

D Elements before index 3 shift right by one position

Deleting at index 3 causes elements after index 3 to shift left to fill the gap.

Which Rust vector method removes an element at a specific index?

A remove(index)

B pop()

C push(value)

D insert(index, value)

`remove(index)` deletes the element at the given index and shifts elements left.

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

A O(n)

B O(log n)

C O(1)

D O(n log n)

Because elements after the deleted one must shift left, it takes O(n) time.

If you delete the middle element from `[5, 10, 15, 20, 25]`, what is the resulting array?

A [10, 15, 20, 25]

B [5, 10, 15, 20]

C [5, 10, 20, 25]

D [5, 15, 20, 25]

Deleting the middle element 15 (index 2) shifts elements left, resulting in [5, 10, 20, 25].

Why is deleting from the middle of an array less efficient than deleting from the end?

A Because deleting from the end requires shifting

B Because arrays are unordered

C Because arrays are linked lists

D Because shifting elements is required

Deleting from the middle requires shifting elements left, which takes more time than deleting from the end.

Explain how deleting an element from the middle of an array works and why elements shift.

Think about how arrays keep elements next to each other.

Write a Rust code snippet to remove the middle element from a vector and print the result.

Use vec![...] and remove method.

Practice

1. What happens when you delete the middle element from an array with an odd number of elements?

easy

A. The array size remains the same but the middle element is set to zero.

B. The middle element is removed and the array size decreases by one.

C. The last element is removed instead of the middle element.

D. The array is reversed after deletion.

Solution

1. Step 1: Identify the middle index in an odd-length array

   The middle index is calculated as length divided by 2 (integer division).

2. Step 2: Remove the element at the middle index

   Deleting this element reduces the array size by one and shifts elements after it left.

3. Final Answer:

   The middle element is removed and the array size decreases by one. -> Option B

4. Quick Check:

   Deleting middle element shrinks array [OK]

Hint: Middle index = length / 2 for odd arrays [OK]

Common Mistakes:

• Thinking array size stays the same after deletion
• Removing last element instead of middle
• Not shifting elements after deletion

2. Which Rust code snippet correctly deletes the middle element from a vector v with odd length?

easy

A. v.pop(v.len() / 2);

B. v.delete(v.len() / 2);

C. v.remove(v.len() / 2);

D. v.erase(v.len() / 2);

Solution

1. Step 1: Identify Rust method to remove element by index

   Rust's Vec type uses remove(index) to delete element at given index.

2. Step 2: Calculate middle index and call remove

   Middle index is v.len() / 2. Calling v.remove(v.len() / 2) deletes middle element.

3. Final Answer:

   v.remove(v.len() / 2); -> Option C

4. Quick Check:

   Use remove(index) to delete element [OK]

Hint: Use Vec::remove(index) to delete element [OK]

Common Mistakes:

• Using non-existent methods like delete or erase
• Using pop which removes last element only
• Passing index to pop which takes no arguments

3. What is the output of this Rust code?

let mut v = vec![10, 20, 30, 40, 50];
v.remove(v.len() / 2);
println!("{:?}", v);

medium

A. [10, 30, 40, 50]

B. [10, 20, 30, 40]

C. [20, 30, 40, 50]

D. [10, 20, 40, 50]

Solution

1. Step 1: Calculate middle index of vector

   Vector length is 5, so middle index = 5 / 2 = 2.

2. Step 2: Remove element at index 2 and print vector

   Element at index 2 is 30, removed. Remaining vector is [10, 20, 40, 50].

3. Final Answer:

   [10, 20, 40, 50] -> Option D

4. Quick Check:

   Removed middle element 30, vector shrinks [OK]

Hint: Remove element at len()/2 index for middle deletion [OK]

Common Mistakes:

• Removing wrong index element
• Confusing index with value
• Expecting vector size unchanged

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

let mut v = vec![1, 2];
v.remove(v.len() / 2 + 1);
println!("{:?}", v);

medium

A. Index used for removal is off by one, causing panic.

B. Vector cannot be mutable for removal.

C. remove() method does not exist for vectors.

D. The println! macro is used incorrectly.

Solution

1. Step 1: Calculate middle index for even-length vector

   Vector length is 2, middle index should be 2 / 2 = 1.

2. Step 2: Check removal index used in code

   Code uses index 1 + 1 = 2, which is out of bounds.

3. Step 3: Understand consequences

   Index 2 >= length 2 causes runtime panic (out of bounds).

4. Final Answer:

   Index used for removal is off by one, causing panic. -> Option A

5. Quick Check:

   Use correct middle index = len()/2 [OK]

Hint: Middle index is len()/2, no +1 for removal [OK]

Common Mistakes:

• Adding +1 to middle index incorrectly
• Trying to remove from immutable vector
• Misusing remove method name

5. Given a vector v of even length, how can you delete the lower middle element (the one at index len()/2 - 1) and then print the updated vector in Rust?

hard

A. v.remove(v.len() / 2 - 1); println!("{:?}", v);

B. v.remove(v.len() / 2); println!("{:?}", v);

C. v.pop(); println!("{:?}", v);

D. v.remove(v.len() / 2 + 1); println!("{:?}", v);

Solution

1. Step 1: Identify lower middle index for even-length vector

   For even length, lower middle index is len()/2 - 1.

2. Step 2: Remove element at lower middle index and print vector

   Use v.remove(v.len() / 2 - 1) to delete lower middle element, then print with println!("{:?}", v);.

3. Final Answer:

   v.remove(v.len() / 2 - 1); println!("{:?}", v); -> Option A

4. Quick Check:

   Remove lower middle with len()/2 - 1 [OK]

Hint: For even length, lower middle = len()/2 - 1 [OK]

Common Mistakes:

• Removing upper middle instead of lower middle
• Using pop which removes last element
• Adding +1 to index incorrectly