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

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after inserting an element in the middle?

Consider the following TypeScript code that inserts the number 99 into the middle of an array. What will be the printed array after the insertion?

DSA Typescript

const arr = [10, 20, 30, 40, 50];
const middleIndex = Math.floor(arr.length / 2);
arr.splice(middleIndex, 0, 99);
console.log(arr);

A [10, 20, 30, 40, 99, 50]

B [10, 20, 30, 99, 40, 50]

C [10, 20, 99, 30, 40, 50]

D [99, 10, 20, 30, 40, 50]

💡 Hint

Remember that Math.floor(arr.length / 2) gives the middle index for insertion.

The array length is 5, so middleIndex = Math.floor(5 / 2) = 2.

Using splice at index 2 inserts 99 before the element originally at index 2.

Resulting array: [10, 20, 99, 30, 40, 50]

❓ Predict Output

intermediate

What is the output after inserting multiple elements in the middle?

Given the following TypeScript code, what will be the final array printed?

DSA Typescript

const arr = [1, 2, 3, 4];
const mid = Math.floor(arr.length / 2);
arr.splice(mid, 0, 99, 100);
console.log(arr);

A [1, 2, 99, 100, 3, 4]

B [99, 100, 1, 2, 3, 4]

C [1, 2, 3, 99, 100, 4]

D [1, 99, 100, 2, 3, 4]

💡 Hint

Check where the middle index is and how splice inserts multiple elements.

Array length is 4, so mid = 2.

Splice inserts 99 and 100 at index 2, pushing elements 3 and 4 to the right.

Final array: [1, 2, 99, 100, 3, 4]

🔧 Debug

advanced

Why does this insertion code not insert at the middle?

The following code is intended to insert 42 into the middle of the array, but it does not work as expected. What is the problem?

DSA Typescript

const arr = [5, 10, 15, 20, 25];
const mid = arr.length / 2;
arr.splice(mid, 0, 42);
console.log(arr);

A 42 is not a valid element type for splice insertion.

B mid is a float, splice expects an integer index, causing unexpected behavior.

C The array is empty, so insertion fails silently.

D splice removes elements instead of inserting because second argument is zero.

💡 Hint

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

arr.length / 2 is 2.5, a float.

splice expects an integer index, so 2.5 is converted to 2 internally, but this can cause confusion or unexpected results.

Using Math.floor(arr.length / 2) ensures an integer index.

🧠 Conceptual

advanced

What is the time complexity of inserting an element in the middle of an array?

Consider an array of size n. What is the time complexity of inserting an element at the middle index using splice?

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

B O(1) because splice inserts directly at the index.

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

D O(n^2) because all elements are copied twice.

💡 Hint

Think about what happens to elements after the insertion point.

Inserting in the middle requires shifting all elements after the insertion point one position to the right.

This shifting takes linear time proportional to the number of elements after the insertion point.

🚀 Application

expert

After multiple insertions at the middle, what is the final array?

Starting with an empty array, the following insertions are done in order: 1. Insert 1 at middle 2. Insert 2 at middle 3. Insert 3 at middle 4. Insert 4 at middle What is the final array?

DSA Typescript

const arr: number[] = [];
function insertMiddle(val: number) {
  const mid = Math.floor(arr.length / 2);
  arr.splice(mid, 0, val);
}
insertMiddle(1);
insertMiddle(2);
insertMiddle(3);
insertMiddle(4);
console.log(arr);

A [2, 4, 3, 1]

B [1, 2, 3, 4]

C [3, 1, 4, 2]

D [4, 2, 1, 3]

💡 Hint

Trace each insertion step by step, updating the array each time.

Step 1: Insert 1 at middle of [] → [1]

Step 2: Insert 2 at middle of [1] (index 0) → [2, 1]

Step 3: Insert 3 at middle of [2, 1] (index 1) → [2, 3, 1]

Step 4: Insert 4 at middle of [2, 3, 1] (index 1) → [2, 4, 3, 1]

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 middle element with a new element

Solution

1. Step 1: Understand array indexing

   Arrays have positions starting from 0 to length-1. The middle index is usually length/2 (rounded down).

2. Step 2: Define insertion at middle

   Inserting at middle means placing the new element at this middle index, shifting elements after it to the 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 = array length divided by 2 [OK]

Common Mistakes:

• Confusing insertion with replacement
• Inserting at start or end instead of middle
• Not shifting elements after insertion

2. Which of the following TypeScript code snippets correctly inserts the value 99 at the middle index of array arr?

easy

A. arr.unshift(99);

B. arr.push(99);

C. arr.splice(Math.floor(arr.length / 2), 0, 99);

D. arr[ Math.floor(arr.length / 2) ] = 99;

Solution

1. Step 1: Understand splice method

   splice(index, 0, value) inserts value at index without removing elements.

2. Step 2: Check other options

   push adds at end, unshift adds at start, direct assignment replaces element.

3. Final Answer:

   arr.splice(Math.floor(arr.length / 2), 0, 99); -> Option C

4. Quick Check:

   splice with 0 delete count inserts [OK]

Hint: Use splice(index, 0, value) to insert without removal [OK]

Common Mistakes:

• Using push or unshift instead of splice
• Replacing element instead of inserting
• Incorrect index calculation

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

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

medium

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

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

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

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

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

   splice(2, 0, 25) inserts 25 before element at index 2 (which is 30), shifting elements right.

3. Final Answer:

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

4. Quick Check:

   Inserted 25 at index 2 [OK]

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

Common Mistakes:

• Inserting at wrong index
• Replacing element instead of inserting
• Misunderstanding splice parameters

4. What is the output of the following TypeScript code?

const nums = [1, 2, 3, 4, 5];
const mid = nums.length / 2;
nums.splice(mid, 0, 50);
console.log(nums);

medium

A. [1, 50, 2, 3, 4, 5]

B. [1, 2, 3, 4, 5, 50]

C. [1, 2, 3, 50, 4, 5]

D. [1, 2, 50, 3, 4, 5]

Solution

1. Step 1: Check mid calculation

   nums.length = 5, mid = 5 / 2 = 2.5 (a float).

2. Step 2: Understand splice behavior

   splice coerces 2.5 to integer 2 (ToInteger(2.5) = 2). Inserts 50 at index 2, shifting 3,4,5 right: [1, 2, 50, 3, 4, 5].

3. Final Answer:

   [1, 2, 50, 3, 4, 5] -> Option D

4. Quick Check:

   Float index coerced to 2 [OK]

Hint: Always floor length/2 for middle index [OK]

Common Mistakes:

• Confusing with unshift (insert at start)
• Confusing with push (append at end)
• Inserting at index 3 instead of 2

5. Given an array arr = [5, 10, 15, 20, 25], write a TypeScript function insertMiddle that inserts a given value exactly in the middle of the array. What will be the array after calling insertMiddle(arr, 12)?

hard

A. [5, 10, 12, 15, 20, 25]

B. [5, 10, 15, 12, 20, 25]

C. [12, 5, 10, 15, 20, 25]

D. [5, 10, 15, 20, 25, 12]

Solution

1. Step 1: Calculate middle index for odd length

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

2. Step 2: Insert 12 at index 2

   Using splice(2, 0, 12) inserts 12 before element at index 2 (which is 15), shifting elements right.

3. Final Answer:

   [5, 10, 12, 15, 20, 25] -> Option A

4. Quick Check:

   Inserted 12 at middle index 2 [OK]

Hint: Use Math.floor(length/2) for middle insertion [OK]

Common Mistakes:

• Inserting at wrong index for odd length
• Appending instead of inserting
• Replacing element instead of inserting