First and Last Occurrence of Element in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find the first occurrence index of a target element in an array.

DSA Javascript

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

Drag options to blanks, or click blank then click option'

Anull

Barr

Ci

Dtarget

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong variable for comparison like 'arr' or 'i'.

Returning the element instead of the index.

We compare each element arr[i] with the target to find the first occurrence index.

2fill in blank

medium

Complete the code to find the last occurrence index of a target element in an array.

DSA Javascript

function lastOccurrence(arr, target) {
  for (let i = arr.length - 1; i >= 0; i--) {
    if (arr[i] === [1]) {
      return i;
    }
  }
  return -1;
}

Drag options to blanks, or click blank then click option'

Ai

Bnull

Ctarget

Darr

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong variable for comparison.

Looping forward instead of backward.

We compare each element arr[i] with the target to find the last occurrence index starting from the end.

3fill in blank

hard

Fix the error in the code to correctly find the first occurrence of target in the array.

DSA Javascript

function firstOccurrence(arr, target) {
  let index = -1;
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] === [1]) {
      index = i;
      break;
    }
  }
  return index;
}

Drag options to blanks, or click blank then click option'

Atarget

Barr[i]

C==

D===

Attempts:

3 left

💡 Hint

Common Mistakes

Using assignment '=' instead of comparison '==='.

Putting the wrong variable on the right side.

The comparison should be arr[i] === target, but the blank replaces the right side of the comparison, so 'target' is correct.

4fill in blank

hard

Fill both blanks to create a function that returns an object with first and last occurrence indices of target in arr.

DSA Javascript

function firstAndLastOccurrence(arr, target) {
  let first = -1;
  let last = -1;
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] === [1]) {
      if (first === -1) {
        first = i;
      }
      last = [2];
    }
  }
  return { first, last };
}

Drag options to blanks, or click blank then click option'

Atarget

Bi

Cfirst

Darr[i]

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning last to first instead of current index.

Comparing arr[i] to wrong variable.

We compare arr[i] to target and update last with the current index i.

5fill in blank

hard

Fill all three blanks to create a function that returns an object with first and last occurrence indices of target using a single pass.

DSA Javascript

function findOccurrences(arr, target) {
  let result = { first: -1, last: -1 };
  for (let i = 0; i < [1]; i++) {
    if (arr[i] === [2]) {
      if (result.first === -1) {
        result.first = i;
      }
      result.last = [3];
    }
  }
  return result;
}

Drag options to blanks, or click blank then click option'

Aarr.length

Btarget

Ci

Dresult.first

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong loop limit.

Comparing with wrong variable.

Assigning last to wrong value.

Loop runs till arr.length, compare arr[i] with target, update last with current index i.

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