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

First and Last Occurrence of Element

📖 Scenario: Imagine you have a list of daily temperatures recorded over a week. You want to find out on which days a specific temperature first and last appeared.

🎯 Goal: You will create a program that finds the first and last positions of a given temperature in the list.

📋 What You'll Learn

Create an array called temperatures with exact values: [23, 25, 23, 22, 25, 26, 23]

Create a variable called targetTemp and set it to 23

Write code to find the first and last occurrence indexes of targetTemp in temperatures

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

💡 Why This Matters

🌍 Real World

Finding first and last occurrences helps in analyzing repeated events, like tracking when a temperature first and last appeared in weather data.

💼 Career

This skill is useful in data analysis, debugging logs, and searching through records efficiently.

1

Create the temperature list

Create an array called temperatures with these exact values: [23, 25, 23, 22, 25, 26, 23]

DSA Javascript

// Create the array temperatures with the given values
// Your code here

Hint

Use const temperatures = [23, 25, 23, 22, 25, 26, 23]; to create the array.

2

Set the target temperature

Create a variable called targetTemp and set it to 23

DSA Javascript

const temperatures = [23, 25, 23, 22, 25, 26, 23];
// Create the variable targetTemp and set it to 23
// Your code here

Hint

Use const targetTemp = 23; to set the target temperature.

3

Find first and last occurrence indexes

Create two variables called firstIndex and lastIndex. Use temperatures.indexOf(targetTemp) to find the first occurrence and temperatures.lastIndexOf(targetTemp) to find the last occurrence.

DSA Javascript

const temperatures = [23, 25, 23, 22, 25, 26, 23];
const targetTemp = 23;
// Find firstIndex and lastIndex of targetTemp in temperatures
// Your code here

Hint

Use indexOf and lastIndexOf methods on the array.

4

Print the results

Print the first occurrence index with console.log("First occurrence: " + firstIndex) and the last occurrence index with console.log("Last occurrence: " + lastIndex).

DSA Javascript

const temperatures = [23, 25, 23, 22, 25, 26, 23];
const targetTemp = 23;
const firstIndex = temperatures.indexOf(targetTemp);
const lastIndex = temperatures.lastIndexOf(targetTemp);
// Print the first and last occurrence indexes
// Your code here

Hint

Use console.log to print the results exactly as shown.

Practice

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

easy

A. The number of unique elements in the array

B. The sum of all elements in the array

C. The maximum and minimum values in the array

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

Solution

1. Step 1: Understand the problem statement

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

2. Step 2: Identify what is being searched

   We look for the first index and last index where the element appears.

3. Final Answer:

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

4. Quick Check:

   First and last occurrence = positions of element [OK]

Hint: Look for start and end indexes of the element [OK]

Common Mistakes:

• Confusing with sum or count of elements
• Looking for max/min values instead
• Ignoring the order of occurrences

2. Which of the following JavaScript code snippets correctly finds the first occurrence index of element 5 in array [1, 5, 3, 5, 7]?

easy

A. array.indexOf(5)

B. array.findIndex(5)

C. array.lastIndexOf(5)

D. array.find(5)

Solution

1. Step 1: Recall JavaScript methods for finding index

   indexOf returns the first index of the element, lastIndexOf returns the last index.

2. Step 2: Check method usage

   array.indexOf(5) correctly returns the first occurrence index of 5.

3. Final Answer:

   array.indexOf(5) -> Option A

4. Quick Check:

   indexOf finds first occurrence [OK]

Hint: Use indexOf for first occurrence, lastIndexOf for last [OK]

Common Mistakes:

• Using findIndex incorrectly with a value instead of a function
• Using find which returns element, not index
• Confusing lastIndexOf with first occurrence

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

const arr = [2, 4, 6, 4, 8];
const first = arr.indexOf(4);
const last = arr.lastIndexOf(4);
console.log([first, last]);

medium

A. [2, 4]

B. [3, 1]

C. [1, 3]

D. [0, 4]

Solution

1. Step 1: Find first occurrence of 4 in array

   4 appears first at index 1.

2. Step 2: Find last occurrence of 4 in array

   4 appears last at index 3.

3. Final Answer:

   [1, 3] -> Option C

4. Quick Check:

   First = 1, Last = 3 [OK]

Hint: indexOf gives first, lastIndexOf gives last [OK]

Common Mistakes:

• Mixing up first and last indexes
• Using wrong array indexes
• Confusing element values with indexes

4. Identify the error in the following code that tries to find the first and last occurrence of 7 in array [7, 2, 7, 5]:

const arr = [7, 2, 7, 5];
const first = arr.findIndex(7);
const last = arr.findLastIndex(7);
console.log([first, last]);

medium

A. findLastIndex is not a valid JavaScript method

B. findIndex and findLastIndex require a function, not a value

C. The array does not contain 7, so indexes are -1

D. The code is correct and will output [0, 2]

Solution

1. Step 1: Check usage of findIndex and findLastIndex

   Both methods require a function as argument, not a direct value.

2. Step 2: Identify the error in argument

   Passing 7 directly causes incorrect usage; should pass a function like x => x === 7.

3. Final Answer:

   findIndex and findLastIndex require a function, not a value -> Option B

4. Quick Check:

   Methods need function argument [OK]

Hint: Pass a function to findIndex, not a direct value [OK]

Common Mistakes:

• Passing value instead of function to findIndex
• Assuming findLastIndex exists in all JS versions
• Confusing findIndex with indexOf

5. Given an array arr = [3, 5, 3, 7, 5, 3], write a function in JavaScript that returns an object with the first and last occurrence indexes of element 3. What will be the output of findOccurrences(arr, 3)?

function findOccurrences(array, element) {
  return {
    first: array.indexOf(element),
    last: array.lastIndexOf(element)
  };
}

const arr = [3, 5, 3, 7, 5, 3];
console.log(findOccurrences(arr, 3));

hard

A. {"first": 0, "last": 5}

B. {"first": 2, "last": 5}

C. {"first": 1, "last": 4}

D. {"first": 0, "last": 2}

Solution

1. Step 1: Find first occurrence of 3 in array

   3 first appears at index 0.

2. Step 2: Find last occurrence of 3 in array

   3 last appears at index 5.

3. Final Answer:

   {"first": 0, "last": 5} -> Option A

4. Quick Check:

   First = 0, Last = 5 [OK]

Hint: Use indexOf and lastIndexOf to get first and last indexes [OK]

Common Mistakes:

• Mixing up first and last indexes
• Returning values instead of indexes
• Incorrectly reading array indexes