JavaScript Program to find the Index of Last Occurrence of Target Element in Sorted Array

In this article, we will see the JavaScript program to get the last occurrence of a number in a sorted array. We have the following methods to get the last occurrence of a given number in the sorted array.

Methods to Find the Index of the Last Occurrence of the Target Element in the Sorted Array

• Using Linear Search(Brute Force method)
• Using Linear Search (Reverse order)
• Using binary search
• Using array.lastIndexOf() method

Method 1: Using Linear Search

Linear search is a type of brute force method that works on linear traversal of the array. It searches the target in O(N) time. It works whether the array is sorted or not.

The program will run till the target is not found and will stop when the Last occurrence is matched.

Example: In this example. we will use linear search to get the last index of the target element.

Javascript

// Input sorted array
arr = [1, 2, 3, 4, 5, 5, 5, 6, 6, 7, 8, 8, 9];

// Target element
target = 5;

// outputIndex
let outputIndex = -1;

// Iterate till length of array
for (let i = 0; i < arr.length; ++i) {
  if (arr[i] === target) outputIndex = i;
  else if (arr[i] > target) break;
}

// If not found display output
if (outputIndex === -1)
  console.log(target + " is not present in the given");
else {
  console.log(
    "Last occurrence of " +
    target +
    " is at the index: " +
    outputIndex
  );
}

Output

Last occurrence of 5 is at the index: 6

Method 2: Using Linear Search (Reverse order)

This method is similar to the above, the only difference is we will iterate the array in reverse order i.e. from last index to first.

Example: In this method, we will use the reverse iteration in linear search to get the last occurance of target element.

Javascript

// Input array
arr = [1, 2, 3, 4, 5, 5, 5, 6, 6, 7, 8, 8, 9];

// Target element
target = 5;

// Iterate from last to zero index
for (let i = arr.length - 1; i >= 0; i--) {

  // If target found return and exit program
  if (arr[i] === target) {
    console.log(
      "The Last index of " + target + " is: " + i
    );
    return;
  }
}

// If not found display output
console.log(target + " is not present in the given");

Output

The Last index of 5 is: 6

Method 3: Using Binary Search

In this method, we will use Binary search to get the first occurance of target element. Binary search is an optimized technique which give output in O(log N) time. It works only for the sorted data. Using binary search if target element is found we will try to search the last occurrence in the right half again and provide the reuired output.

Example: In this example, we will use binary search to get the desired output.

Javascript

// Input array
const arr = [1, 2, 3, 4, 5, 5, 5, 6, 6, 7, 8, 8, 9];

// Target element
const target = 5;
let left = 0;
let right = arr.length - 1;
let outputIndex = -1;

// Using binary search
while (left <= right) {
    let mid = Math.floor((left + right) / 2)

    // If found search left half for First occerrence
    if (target === arr[mid]) {

        // Store first occurrence index
        outputIndex = mid;
        left = mid + 1;
    }

    // If target is smallar discard right half
    else if (target < arr[mid]) {
        right = mid - 1;
    }

    // If target is greater discard left half
    else
        left = mid + 1;
}

// If not found display output
if (outputIndex === -1)
    console.log(target + " is not present in the given");
else {
    console.log(
        "Last index of " + target + " is at index: " + outputIndex
    );
}

Output

Last index of 5 is at index: 6

Method 4: Using array.lastIndexOf() method

In this approach, we will use array.lastIndexOf() method. The JavaScript Array lastIndexOf() Method is used to find the index of the last occurrence of the search element provided as the argument to the function.

Syntax: 

array.lastIndexOf(element, start)

Example: In this example, we will use arr.indexOf() method to find the last occurrence of target element

Javascript

// Input array
const arr = [1, 2, 3, 4, 5, 5, 5, 6, 6, 7, 8, 8, 9];

// Target element
const target = 8;

// Get first index of element using indexOf method()
const outputIndex = arr.lastIndexOf(target);

// If not found display output
if (outputIndex === -1)
    console.log(target + " is not present in the given");
else {
    console.log(
        "Last index of " +
        target +
        " is at index: " +
        outputIndex
    );
}

Output

Last index of 8 is at index: 11