First and Last Occurrence of Element in DSA Typescript - Build from Scratch

First and Last Occurrence of Element

📖 Scenario: You are working with a list of product IDs sold in a store during a day. Some products appear multiple times because they were sold multiple times.You want to find the first and last time a specific product was sold to understand its sales pattern.

🎯 Goal: Build a program that finds the first and last position of a given product ID in the sales list.

📋 What You'll Learn

Create an array called sales with the exact product IDs: [101, 203, 101, 405, 203, 101, 507]

Create a variable called targetProduct and set it to 101

Write a loop to find the first and last index of targetProduct in sales

Print the first and last occurrence indexes in the format: First occurrence: X, Last occurrence: Y

💡 Why This Matters

🌍 Real World

Stores and businesses often analyze sales data to understand when products are sold during the day. Finding first and last occurrences helps in tracking product demand and stock management.

💼 Career

This skill is useful for data analysts, software developers, and anyone working with data searching and processing in real-world applications.

1

Create the sales data array

Create an array called sales with these exact product IDs: [101, 203, 101, 405, 203, 101, 507]

DSA Typescript

// Create the sales array with the given product IDs
// Your code here

Hint

Use const sales = [101, 203, 101, 405, 203, 101, 507]; to create the array.

2

Set the target product ID

Create a variable called targetProduct and set it to 101

DSA Typescript

const sales = [101, 203, 101, 405, 203, 101, 507];
// Create targetProduct variable and set it to 101
// Your code here

Hint

Use const targetProduct = 101; to set the product ID.

3

Find first and last occurrence indexes

Write a for loop with variable i from 0 to sales.length - 1 to find the first and last index of targetProduct in sales. Use variables firstIndex and lastIndex initialized to -1 before the loop. Update firstIndex when you find the first match, and update lastIndex every time you find a match.

DSA Typescript

const sales = [101, 203, 101, 405, 203, 101, 507];
const targetProduct = 101;
let firstIndex = -1;
let lastIndex = -1;
// Write a for loop to find first and last occurrence of targetProduct
// Your code here

Hint

Use a for loop and check each element. Set firstIndex only once when you find the first match. Update lastIndex every time you find a match.

4

Print the first and last occurrence

Print the first and last occurrence indexes using console.log in this exact format: First occurrence: X, Last occurrence: Y where X is firstIndex and Y is lastIndex

DSA Typescript

const sales = [101, 203, 101, 405, 203, 101, 507];
const targetProduct = 101;
let firstIndex = -1;
let lastIndex = -1;
for (let i = 0; i < sales.length; i++) {
    if (sales[i] === targetProduct) {
        if (firstIndex === -1) {
            firstIndex = i;
        }
        lastIndex = i;
    }
}
// Print the first and last occurrence indexes
// Your code here

Hint

Use console.log(`First occurrence: ${firstIndex}, Last occurrence: ${lastIndex}`); to print the result.

Practice

1. What does the "First and Last Occurrence of Element" problem ask you to find in an array?

easy

A. The maximum element in the array

B. The sum of all elements in the array

C. The first and last positions where a given element appears

D. The number of unique elements in the array

Solution

1. Step 1: Understand the problem statement

   The problem asks to find the first and last index positions of a specific element in an array.

2. Step 2: Identify what is required

   We need to locate where the element appears first and last, not sums or counts.

3. Final Answer:

   The first and last positions where a given element appears -> Option C

4. Quick Check:

   First and last occurrence = positions of element [OK]

Hint: Look for positions of element, not sums or counts [OK]

Common Mistakes:

• Confusing occurrence with sum or max
• Thinking about unique elements instead
• Ignoring the order of elements

2. Which of the following TypeScript function signatures correctly represents a function to find first and last occurrence of a number in an array?

easy

A. function findOccurrences(arr: number[], target: number): number

B. function findOccurrences(arr: string[], target: string): number

C. function findOccurrences(arr: number, target: number): number[]

D. function findOccurrences(arr: number[], target: number): [number, number]

Solution

1. Step 1: Analyze the function parameters

   The function should take an array of numbers and a target number to find.

2. Step 2: Analyze the return type

   We want to return two numbers (first and last index), so a tuple [number, number] is appropriate.

3. Final Answer:

   function findOccurrences(arr: number[], target: number): [number, number] -> Option D

4. Quick Check:

   Input array and target, output tuple of two indexes [OK]

Hint: Return a tuple of two numbers for first and last indexes [OK]

Common Mistakes:

• Using wrong input types like single number instead of array
• Returning only one number instead of two
• Returning array of wrong type

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

function findOccurrences(arr: number[], target: number): [number, number] {
  let first = -1;
  let last = -1;
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] === target) {
      if (first === -1) first = i;
      last = i;
    }
  }
  return [first, last];
}

console.log(findOccurrences([5, 3, 7, 3, 9], 3));

medium

A. [1, 3]

B. [3, 3]

C. [0, 4]

D. [-1, -1]

Solution

1. Step 1: Trace the loop to find first occurrence

   At index 1, element is 3 which matches target, so first = 1.

2. Step 2: Update last occurrence while looping

   At index 3, element is 3 again, so last = 3.

3. Final Answer:

   [1, 3] -> Option A

4. Quick Check:

   First = 1, Last = 3 [OK]

Hint: Track first when found, update last every time [OK]

Common Mistakes:

• Returning first occurrence twice
• Returning indexes of wrong elements
• Not updating last occurrence properly

4. Identify the bug in this TypeScript function to find first and last occurrence:

function findOccurrences(arr: number[], target: number): [number, number] {
  let first = 0;
  let last = 0;
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] === target) {
      if (first === 0) first = i;
      last = i;
    }
  }
  return [first, last];
}

medium

A. last should be initialized to -1 instead of 0

B. first initialized to 0 causes wrong first occurrence when target is at index 0

C. Loop should start from 1 instead of 0

D. Missing return type annotation

Solution

1. Step 1: Check initialization of first

   first is set to 0, so if target is at index 0, condition first === 0 is true initially, causing incorrect logic.

2. Step 2: Understand impact on first occurrence

   If target is at index 0, first will never update correctly because first === 0 is true initially, so first stays 0 even if target not found.

3. Final Answer:

   first initialized to 0 causes wrong first occurrence when target is at index 0 -> Option B

4. Quick Check:

   Initialize first to -1 to fix [OK]

Hint: Initialize first to -1 to detect if target found [OK]

Common Mistakes:

• Initializing first to 0 instead of -1
• Not handling target at index 0 correctly
• Assuming first occurrence is always > 0

5. Given a sorted array with possible duplicates, which approach is the most efficient to find the first and last occurrence of a target element?

hard

A. Use two binary searches: one for first occurrence, one for last occurrence

B. Traverse the array from start to end to find first and last occurrence

C. Sort the array again and then find the element

D. Use a hash map to store all occurrences and then find first and last

Solution

1. Step 1: Understand the array is sorted

   Since the array is sorted, binary search can be used to find elements efficiently.

2. Step 2: Use binary search twice

   One binary search finds the first occurrence, another finds the last occurrence, both in O(log n) time.

3. Step 3: Compare with other options

   Traversing is O(n), sorting again is unnecessary, hash map uses extra space and is less efficient.

4. Final Answer:

   Use two binary searches: one for first occurrence, one for last occurrence -> Option A

5. Quick Check:

   Binary search twice = efficient for sorted arrays [OK]

Hint: Use binary search twice for first and last positions [OK]

Common Mistakes:

• Using linear search on sorted array
• Sorting array again unnecessarily
• Using extra space with hash map