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

Recall & Review

beginner

What does 'Array Insertion at Middle Index' mean?

It means adding a new element into an array exactly in the middle position, shifting other elements to the right to make space.

beginner

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

You divide the array length by 2. For example, if length is 6, middle index is 3 (0-based indexing).

beginner

Which JavaScript method can insert an element at a specific index in an array?

The 'splice' method can insert elements at any index. Syntax: array.splice(index, 0, element).

beginner

What happens to elements after the middle index when you insert a new element there?

All elements from the middle index onward move one position to the right to make space for the new element.

intermediate

Why is inserting at the middle index in an array less efficient than adding at the end?

Because elements after the middle must be shifted, which takes more time compared to just adding at the end.

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

A 2

B 4

C 3

D 5

For 7 elements, middle index is floor(7/2) = 3 (0-based indexing).

Which JavaScript method inserts an element at a specific position in an array?

A splice()

B pop()

C push()

D slice()

'splice()' can insert elements at any index; 'push()' adds at the end.

What happens to elements after the insertion point when inserting in the middle of an array?

A They shift right by one position

B They are removed

C They shift left by one position

D They stay in place

Elements after the insertion index move right to make space for the new element.

If an array has 4 elements, at which index will a new element be inserted to be in the middle?

A 1

B 0

C 3

D 2

Middle index for 4 elements is 4/2 = 2.

Why is inserting in the middle of an array slower than adding at the end?

A Because the array size decreases

B Because elements need to be shifted

C Because the array is sorted

D Because the array is empty

Shifting elements to the right takes extra time compared to adding at the end.

Explain how to insert an element into the middle of an array in JavaScript.

Think about how splice works and how to calculate the middle index.

Describe what happens to the array elements when you insert a new element at the middle index.

Consider the space needed for the new element.

Practice

1. What does inserting an element at the middle index of an array mean?

easy

A. Adding the element exactly in the center position of the array

B. Adding the element at the start of the array

C. Adding the element at the end of the array

D. Replacing the first element with the new element

Solution

1. Step 1: Understand array indexing

   Arrays have positions starting from 0 to length-1. The middle index is roughly length/2.

2. Step 2: Define insertion at middle

   Inserting at middle means placing the new element at the middle index, shifting others right.

3. Final Answer:

   Adding the element exactly in the center position of the array -> Option A

4. Quick Check:

   Middle insertion = center position [OK]

Hint: Middle index is array length divided by 2 [OK]

Common Mistakes:

• Confusing middle with start or end
• Replacing element instead of inserting
• Not shifting elements after insertion

2. Which of the following is the correct syntax to insert an element 'x' at the middle index of array 'arr' in JavaScript?

easy

A. arr.insert(Math.floor(arr.length / 2), x);

B. arr.add(Math.floor(arr.length / 2), x);

C. arr.splice(Math.floor(arr.length / 2), 0, x);

D. arr.push(Math.floor(arr.length / 2), x);

Solution

1. Step 1: Identify JavaScript method for insertion

   JavaScript arrays use splice(start, deleteCount, items...) to insert elements.

2. Step 2: Calculate middle index and insert

   Using Math.floor(arr.length / 2) gives middle index; splice with 0 deletes nothing and inserts 'x'.

3. Final Answer:

   arr.splice(Math.floor(arr.length / 2), 0, x); -> Option C

4. Quick Check:

   splice + middle index + 0 delete = insert [OK]

Hint: Use splice with 0 delete count to insert [OK]

Common Mistakes:

• Using non-existent insert or add methods
• Using push which adds only at end
• Not calculating middle index correctly

3. What will be the output of the following code?

const arr = [10, 20, 30, 40];
arr.splice(Math.floor(arr.length / 2), 0, 25);
console.log(arr);

medium

A. [10, 20, 25, 30, 40]

B. [10, 20, 30, 25, 40]

C. [25, 10, 20, 30, 40]

D. [10, 20, 30, 40, 25]

Solution

1. Step 1: Calculate middle index

   arr.length = 4, middle index = Math.floor(4/2) = 2.

2. Step 2: Insert 25 at index 2

   Using splice(2, 0, 25) inserts 25 at index 2, shifting elements right.

3. Final Answer:

   [10, 20, 25, 30, 40] -> Option A

4. Quick Check:

   Insert at index 2 shifts right [OK]

Hint: Middle index = floor(length/2), splice inserts at that index [OK]

Common Mistakes:

• Inserting at wrong index
• Appending instead of inserting
• Misunderstanding splice parameters

4. Identify the error in the following code that tries to insert 15 at the middle of array 'nums':

const nums = [5, 10, 20, 25];
nums.insert(nums.length / 2, 15);
console.log(nums);

medium

A. The middle index calculation is incorrect

B. 'insert' is not a valid array method in JavaScript

C. The element 15 should be added using push()

D. The array should be declared with let instead of const

Solution

1. Step 1: Check array method used

   JavaScript arrays do not have an 'insert' method; splice should be used instead.

2. Step 2: Confirm middle index calculation

   nums.length / 2 = 2 is correct, but method is invalid causing error.

3. Final Answer:

   'insert' is not a valid array method in JavaScript -> Option B

4. Quick Check:

   Use splice, not insert [OK]

Hint: Remember: JavaScript arrays use splice, not insert [OK]

Common Mistakes:

• Using non-existent insert method
• Ignoring method errors
• Confusing push with insert

5. Given an array arr = [1, 3, 5, 7, 9], write code to insert the number 4 exactly in the middle so that the array remains sorted. What will be the final array?

hard

A. [1, 3, 5, 7, 9, 4]

B. [1, 3, 5, 4, 7, 9]

C. [4, 1, 3, 5, 7, 9]

D. [1, 3, 4, 5, 7, 9]

Solution

1. Step 1: Find middle index for insertion

   arr.length = 5, middle index = Math.floor(5 / 2) = 2.

2. Step 2: Insert 4 at index 2 using splice

   arr.splice(2, 0, 4) inserts 4 before element 5, keeping array sorted.

3. Final Answer:

   [1, 3, 4, 5, 7, 9] -> Option D

4. Quick Check:

   Insert 4 at index 2 keeps order [OK]

Hint: Insert at floor(length/2) to keep sorted order [OK]

Common Mistakes:

• Inserting at wrong index breaking order
• Appending instead of inserting
• Not using splice correctly