Array Insertion at Beginning in DSA Typescript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after inserting elements at the beginning?

Consider the following TypeScript code that inserts elements at the beginning of an array. What will be the final state of the array printed?

DSA Typescript

let arr: number[] = [3, 4, 5];
arr.unshift(2);
arr.unshift(1);
console.log(arr);

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

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

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

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

💡 Hint

Remember that unshift adds elements to the start of the array in the order they are called.

The unshift method adds elements to the beginning of the array. First, 2 is added before [3,4,5], making [2,3,4,5]. Then 1 is added before that, resulting in [1,2,3,4,5].

❓ Predict Output

intermediate

What is the output after multiple unshift calls?

What will be printed after running this TypeScript code?

DSA Typescript

const arr: string[] = ['c', 'd'];
arr.unshift('b');
arr.unshift('a');
console.log(arr.join(' -> '));

A a -> b -> c -> d

B b -> a -> c -> d

C c -> d -> a -> b

D d -> c -> b -> a

💡 Hint

Check the order elements are added with unshift.

Each unshift adds the element to the front. So 'b' is added before ['c','d'], then 'a' before that, resulting in ['a','b','c','d'].

🔧 Debug

advanced

Why does this code not insert at the beginning as expected?

This TypeScript code tries to insert 0 at the beginning of the array but fails. What is the error?

DSA Typescript

let numbers = [1, 2, 3];
numbers.push(0);
console.log(numbers);

A The code inserts 0 at the end, not the beginning, because push adds to the end.

B SyntaxError because push is used incorrectly.

C TypeError because numbers is not an array.

D The array remains unchanged.

💡 Hint

Check the difference between push and unshift methods.

push adds elements to the end of the array. To add at the beginning, unshift should be used.

🧠 Conceptual

advanced

What is the time complexity of inserting at the beginning of an array?

Inserting an element at the beginning of a standard array requires shifting existing elements. What is the time complexity of this operation?

A O(1) - constant time

B O(log n) - logarithmic time

C O(n) - linear time

D O(n^2) - quadratic time

💡 Hint

Think about how many elements must move when inserting at the start.

When inserting at the beginning, all existing elements must shift one position to the right, so the time grows linearly with the array size.

🚀 Application

expert

How to efficiently insert multiple elements at the beginning?

You want to insert multiple elements ['x', 'y', 'z'] at the beginning of a large array efficiently. Which approach is best in TypeScript?

A Use multiple unshift calls: arr.unshift('x'); arr.unshift('y'); arr.unshift('z');

B Concatenate arrays: arr = ['x', 'y', 'z'].concat(arr);

C Use push with spread: arr.push(...['x', 'y', 'z']);

D Use unshift with spread: arr.unshift(...['x', 'y', 'z']);

💡 Hint

Consider performance and readability for large arrays.

Using concat creates a new array with the new elements at the front without shifting existing elements multiple times, which is more efficient for large arrays.

Practice

1. What does inserting an element at the beginning of an array do to the existing elements?

easy

A. Replaces the first element with the new element

B. Deletes the last element of the array

C. Shifts all existing elements one position to the right

D. Leaves the array unchanged

Solution

1. Step 1: Understand array insertion at beginning

   When a new element is inserted at the start, all existing elements move one step to the right to make space.

2. Step 2: Effect on existing elements

   This shifting means the first element moves to second position, second to third, and so on.

3. Final Answer:

   Shifts all existing elements one position to the right -> Option C

4. Quick Check:

   Insert at start shifts elements right [OK]

Hint: New start element pushes others right [OK]

Common Mistakes:

• Thinking insertion replaces first element
• Assuming array size decreases
• Believing array stays unchanged

2. Which of the following is the correct TypeScript syntax to insert an element `5` at the beginning of array `arr`?

easy

A. arr.unshift(5);

B. arr.addFirst(5);

C. arr.insert(0, 5);

D. arr.push(5);

Solution

1. Step 1: Recall array methods for insertion

   In TypeScript (JavaScript), `unshift` adds elements at the start of an array.

2. Step 2: Check other options

   `push` adds at the end, `insert` and `addFirst` are not valid array methods.

3. Final Answer:

   arr.unshift(5); -> Option A

4. Quick Check:

   Use unshift() to add at beginning [OK]

Hint: Use unshift() to add at start [OK]

Common Mistakes:

• Using push() which adds at the end
• Using non-existent methods like insert()
• Confusing unshift() with pop()

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

let arr = [10, 20, 30];
arr.unshift(5);
console.log(arr);

medium

A. [10, 20, 30, 5]

B. [5, 10, 20, 30]

C. [5, 20, 30]

D. [10, 5, 20, 30]

Solution

1. Step 1: Understand unshift effect

   unshift(5) adds 5 at the start, shifting existing elements right.

2. Step 2: Resulting array

   Original [10, 20, 30] becomes [5, 10, 20, 30].

3. Final Answer:

   [5, 10, 20, 30] -> Option B

4. Quick Check:

   unshift adds at front [OK]

Hint: unshift adds element at front [OK]

Common Mistakes:

• Thinking unshift adds at end
• Misplacing the new element in the array
• Confusing unshift with push

4. Identify the error in the following TypeScript code that tries to insert `100` at the beginning of array `nums`.

let nums = [1, 2, 3];
nums.add(0, 100);
console.log(nums);

medium

A. `add` is not a valid method for arrays

B. `console.log` is used incorrectly

C. Array 'nums' is not declared properly

D. The element 100 cannot be added to arrays

Solution

1. Step 1: Check array method usage

   Arrays in TypeScript do not have an `add` method; insertion at beginning uses `unshift`.

2. Step 2: Validate other parts

   Declaration and console.log are correct; adding 100 is allowed.

3. Final Answer:

   `add` is not a valid method for arrays -> Option A

4. Quick Check:

   Use unshift(), not add() [OK]

Hint: Use unshift(), not add() for insertion [OK]

Common Mistakes:

• Using non-existent add() method
• Thinking console.log is wrong
• Believing arrays can't hold numbers

5. Given an empty array `let data: number[] = [];`, which sequence of operations will result in the array `[3, 2, 1]` by inserting elements only at the beginning?

hard

A. data.unshift(1); data.unshift(2); data.unshift(3);

B. data.push(1); data.push(2); data.push(3);

C. data.push(3); data.unshift(2); data.push(1);

D. data.unshift(3); data.unshift(2); data.unshift(1);

Solution

1. Step 1: Understand insertion at beginning

   Each unshift adds element at front, pushing existing elements right.

2. Step 2: Trace operations

   Starting empty: unshift(3) -> [3], unshift(2) -> [2,3], unshift(1) -> [1,2,3].

3. Final Answer:

   data.unshift(3); data.unshift(2); data.unshift(1); -> Option D

4. Quick Check:

   Insert in reverse order with unshift [OK]

Hint: Insert last element first with unshift [OK]

Common Mistakes:

• Using push() which adds at end
• Inserting elements in wrong order
• Mixing push and unshift incorrectly