First and Last Occurrence of Element in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'First Occurrence of Element' mean in an array?

It means the position (index) where the element appears for the very first time when we look from the start of the array.

beginner

What does 'Last Occurrence of Element' mean in an array?

It means the position (index) where the element appears for the very last time when we look from the end of the array.

intermediate

Why is it useful to find the first and last occurrence of an element?

It helps us know the range where the element appears in the array, which is useful for counting how many times it appears or for searching efficiently.

beginner

What is a simple way to find the first occurrence of an element in JavaScript?

Use the array method indexOf(element), which returns the first index where the element is found or -1 if not found.

beginner

How can you find the last occurrence of an element in JavaScript?

Use the array method lastIndexOf(element), which returns the last index where the element is found or -1 if not found.

Which JavaScript method returns the first index of an element in an array?

A includes()

B lastIndexOf()

C find()

D indexOf()

The method indexOf() returns the first index where the element is found.

What does lastIndexOf() return if the element is not found in the array?

A 0

B -1

C null

D undefined

lastIndexOf() returns -1 when the element is not found.

If an element appears multiple times, what does indexOf() return?

A The first position of the element

B The last position of the element

C The count of the element

D All positions of the element

indexOf() returns the first position where the element appears.

Which method would you use to find the last occurrence of an element in an array?

A indexOf()

B find()

C lastIndexOf()

D filter()

lastIndexOf() returns the last position of the element in the array.

What is the output of ['a', 'b', 'a', 'c'].indexOf('a')?

A 0

B 1

C 2

D -1

The first 'a' is at index 0.

Explain how to find the first and last occurrence of an element in a JavaScript array.

Think about built-in array methods that give positions.

Why might knowing the first and last occurrence of an element be helpful in programming?

Consider how knowing positions helps with array operations.

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