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.