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

Recall & Review

beginner

What does 'Array Deletion at Middle Index' mean?

It means removing an element from the middle position of an array, shifting all elements after it one step left to fill the gap.

beginner

How do you find the middle index of an array with an odd number of elements?

Use Math.floor(array.length / 2) to get the middle index for odd-length arrays.

beginner

What happens to the array length after deleting an element at the middle index?

The array length decreases by one because one element is removed.

beginner

Which JavaScript method can be used to delete an element at a specific index?

The splice() method can remove elements at any index: array.splice(index, 1) removes one element at 'index'.

beginner

Why do elements after the deleted middle element shift left?

Because arrays keep elements in order, removing one element causes all later elements to move left to fill the empty space.

What is the middle index of an array with 7 elements?

A 4

B 3

C 2

D 5

Middle index = Math.floor(7 / 2) = 3.

Which method removes an element at a specific index in JavaScript arrays?

A splice()

B pop()

C slice()

D push()

splice() removes or adds elements at any index.

After deleting the middle element, what happens to the array length?

A It stays the same

B It increases by one

C It decreases by one

D It doubles

Deleting an element reduces the array length by one.

If you delete the middle element, what happens to elements after it?

A They stay in place

B They shift right

C They get deleted too

D They shift left to fill the gap

Elements after the deleted one move left to keep order.

How do you calculate the middle index for an even-length array of size 6?

A 3

B 2

C 4

D 5

Middle index = Math.floor(6 / 2) = 3, which is the middle index for zero-based arrays.

Explain how to delete the middle element from an array and what happens to the array after deletion.

Think about how arrays keep elements in order after removal.

Describe the steps to find the middle index of an array and remove that element in JavaScript.

Focus on index calculation and splice usage.

Practice

1. What happens when you delete the middle element from an array with an odd number of elements using array.splice(mid, 1) where mid is the middle index?

easy

A. The first element is removed instead.

B. The array remains unchanged.

C. All elements after the middle are removed.

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

Solution

1. Step 1: Understand splice method behavior

   The splice method removes elements starting at the given index. Here, it removes 1 element at the middle index.

2. Step 2: Effect on array size

   Removing one element reduces the array length by one, and elements after the removed one shift left.

3. Final Answer:

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

4. Quick Check:

   splice(mid, 1) removes middle element [OK]

Hint: Use splice(mid, 1) to remove middle element quickly [OK]

Common Mistakes:

• Thinking splice does not change array size
• Confusing splice with slice
• Removing wrong index

2. Which of the following is the correct syntax to remove the middle element from an array arr in JavaScript?

easy

A. arr.delete(Math.floor(arr.length / 2));

B. arr.splice(Math.floor(arr.length / 2), 1);

C. arr.remove(Math.floor(arr.length / 2));

D. arr.pop(Math.floor(arr.length / 2));

Solution

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

   JavaScript arrays use splice to remove elements at a specific index.

2. Step 2: Calculate middle index correctly

   Math.floor(arr.length / 2) gives the middle index for odd or even length arrays.

3. Final Answer:

   arr.splice(Math.floor(arr.length / 2), 1); -> Option B

4. Quick Check:

   splice + floor middle index = correct syntax [OK]

Hint: Use splice with floor of length/2 for middle deletion [OK]

Common Mistakes:

• Using non-existent methods like delete or remove
• Using pop with index argument
• Not flooring the middle index

3. What is the output of the following code?

let arr = [10, 20, 30, 40, 50];
let mid = Math.floor(arr.length / 2);
arr.splice(mid, 1);
console.log(arr);

medium

A. [10, 20, 40, 50]

B. [10, 20, 30, 40]

C. [10, 20, 30, 50]

D. [20, 30, 40, 50]

Solution

1. Step 1: Calculate middle index

   Array length is 5, so middle index = floor(5/2) = 2.

2. Step 2: Remove element at index 2

   Element at index 2 is 30, so splice removes 30.

3. Step 3: Resulting array

   Array becomes [10, 20, 40, 50] after removal.

4. Final Answer:

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

5. Quick Check:

   Removed element 30 at index 2 [OK]

Hint: Find middle index, splice removes that element [OK]

Common Mistakes:

• Removing wrong index element
• Not flooring middle index
• Confusing splice with slice

4. Find the error in the code below that tries to remove the middle element from an array:

let arr = [1, 2, 3, 4, 5];
let mid = arr.length / 2;
arr.splice(mid, 1);
console.log(arr);

medium

A. splice cannot remove elements from arrays.

B. arr.length / 2 gives the last index, not middle.

C. mid should be floored to an integer before using in splice.

D. splice requires two arguments, but only one is given.

Solution

1. Step 1: Check middle index calculation

   mid is calculated as arr.length / 2 = 2.5, which is a float, not an integer index.

2. Step 2: Effect of float index in splice

   splice expects an integer index; JavaScript coerces 2.5 to 2, but explicitly using Math.floor(arr.length / 2) ensures clarity and avoids reliance on type coercion.

3. Step 3: Correct approach

   Use Math.floor(arr.length / 2) to get integer index 2.

4. Final Answer:

   mid should be floored to an integer before using in splice. -> Option C

5. Quick Check:

   Index must be integer for splice [OK]

Hint: Always floor middle index before splice [OK]

Common Mistakes:

• Using float index in splice
• Assuming splice removes by value
• Passing wrong number of arguments to splice

5. Given an array arr with an even number of elements, how can you remove the two middle elements correctly in JavaScript?

hard

A. Use arr.splice(arr.length / 2 - 1, 2);

B. Use arr.splice(arr.length / 2, 1); twice

C. Use arr.pop(); arr.shift();

D. Use arr.splice(arr.length / 2, 2);

Solution

1. Step 1: Identify middle indices for even length

   For even length n, middle two elements are at indices n/2 - 1 and n/2.

2. Step 2: Remove two elements starting at n/2 - 1

   Using arr.splice(arr.length / 2 - 1, 2) removes both middle elements at once.

3. Step 3: Why other options fail

   Use arr.splice(arr.length / 2, 1); twice removes one element twice but shifts indices, causing wrong removal; Use arr.pop(); arr.shift(); removes first and last elements; Use arr.splice(arr.length / 2, 2); removes elements after middle.

4. Final Answer:

   Use arr.splice(arr.length / 2 - 1, 2); -> Option A

5. Quick Check:

   Remove two middle elements starting at n/2 - 1 [OK]

Hint: Start splice at n/2 - 1 to remove two middle elements [OK]

Common Mistakes:

• Removing elements twice causing index shift
• Removing first and last instead of middle
• Starting splice at wrong index