Array Insertion at Middle Index in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after inserting at the middle index?

Consider the following JavaScript code that inserts a value into the middle of an array. What will be the printed array after the insertion?

DSA Javascript

const arr = [10, 20, 30, 40, 50];
const middleIndex = Math.floor(arr.length / 2);
arr.splice(middleIndex, 0, 25);
console.log(arr);

A [10, 20, 25, 30, 40, 50]

B [25, 10, 20, 30, 40, 50]

C [10, 25, 20, 30, 40, 50]

D [10, 20, 30, 25, 40, 50]

💡 Hint

Remember that splice inserts elements starting at the given index, shifting existing elements to the right.

The middle index is 2 (floor of 5/2). Inserting 25 at index 2 shifts elements from index 2 onwards to the right.

❓ Predict Output

intermediate

What is the output after inserting in an even-length array?

Given the code below, what will be the array printed after inserting 15 at the middle index?

DSA Javascript

const arr = [5, 10, 20, 25];
const middleIndex = Math.floor(arr.length / 2);
arr.splice(middleIndex, 0, 15);
console.log(arr);

A [5, 10, 20, 25, 15]

B [5, 10, 15, 20, 25]

C [15, 5, 10, 20, 25]

D [5, 15, 10, 20, 25]

💡 Hint

For even length arrays, middle index is floor of length/2, which is the first of the two middle positions.

Length is 4, middle index is 2. Inserting 15 at index 2 shifts elements from index 2 onwards to the right.

🔧 Debug

advanced

Why does this insertion code not insert at the middle?

The code below is intended to insert 100 at the middle index of the array. Why does it not work as expected?

DSA Javascript

const arr = [1, 2, 3, 4, 5];
const middleIndex = arr.length / 2;
arr.splice(middleIndex, 0, 100);
console.log(arr);

A The code throws a TypeError because middleIndex is not an integer.

B splice cannot insert elements, only remove them.

C The array is immutable, so splice does not change it.

D middleIndex is a float, splice expects an integer index, so insertion happens at a wrong position.

💡 Hint

Check the value and type of middleIndex before using it in splice.

middleIndex is 2.5 (float). splice converts it to integer by truncation, so insertion happens at index 2 instead of 3, which may not be the intended middle.

❓ Predict Output

advanced

What is the output after multiple insertions at middle?

What will be the array after running this code that inserts multiple values at the middle index sequentially?

DSA Javascript

const arr = [1, 2, 3, 4];
let middleIndex = Math.floor(arr.length / 2);
arr.splice(middleIndex, 0, 10);
middleIndex = Math.floor(arr.length / 2);
arr.splice(middleIndex, 0, 20);
console.log(arr);

A [1, 2, 10, 20, 3, 4]

B [1, 2, 20, 10, 3, 4]

C [1, 10, 2, 20, 3, 4]

D [10, 20, 1, 2, 3, 4]

💡 Hint

Remember to recalculate the middle index after each insertion because array length changes.

After first insertion at index 2, array is [1, 2, 10, 3, 4]. New length is 5, middle index is 2 again. Inserting 20 at index 2 shifts 10 and others right, resulting in [1, 2, 20, 10, 3, 4].

🧠 Conceptual

expert

What is the time complexity of inserting an element at the middle of a JavaScript array?

Consider inserting an element at the middle index of a JavaScript array using splice. What is the time complexity of this operation in terms of the array length n?

A O(log n) because the array is split and merged efficiently.

B O(1) because splice inserts directly without shifting.

C O(n) because elements after the insertion point must be shifted.

D O(n^2) because splice copies the entire array each time.

💡 Hint

Think about what happens to elements after the insertion point.

Inserting at the middle requires shifting half of the elements to the right, which is proportional to n, so time complexity is O(n).

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