Find index of first occurrence when an unsorted array is sorted
Last Updated :
11 Jul, 2025
Given an unsorted array and a number x, find an index of first occurrence of x when we sort the array. If x is not present, print -1.
Examples:
Input : arr[] = {10, 30, 20, 50, 20}
x = 20
Output : 1
Sorted array is {10, 20, 20, 30, 50}
Input : arr[] = {10, 30, 20, 50, 20}
x = 60
Output : -1
60 is not present in array.
A simple solution is to first sort the array, then do binary search to find first occurrence.
Implementation:
C++
// C++ program to find index of first
// occurrence of x when array is sorted.
#include <bits/stdc++.h>
using namespace std;
int findFirst(int arr[], int n, int x)
{
sort(arr, arr + n);
// lower_bound returns iterator pointing to
// first element that does not compare less
// to x.
int* ptr = lower_bound(arr, arr + n, x);
// If x is not present return -1.
return (*ptr != x) ? -1 : (ptr - arr);
}
int main()
{
int x = 20, arr[] = { 10, 30, 20, 50, 20 };
int n = sizeof(arr) / sizeof(arr[0]);
cout << findFirst(arr, n, x);
return 0;
}
// Java program to find index of first
// occurrence of x when array is sorted.
import java.util.*;
class GFG {
static int findFirst(int arr[], int n, int x)
{
Arrays.sort(arr);
// lower_bound returns iterator pointing to
// first element that does not compare less
// to x.
int ptr = lowerBound(arr, 0, n, x);
// If x is not present return -1.
return (arr[ptr] != x) ? -1 : (ptr);
}
static int lowerBound(int[] a, int low, int high,
int element)
{
while (low < high) {
int middle = low + (high - low) / 2;
if (element > a[middle])
low = middle + 1;
else
high = middle;
}
return low;
}
// Driver Code
public static void main(String[] args)
{
int x = 20, arr[] = { 10, 30, 20, 50, 20 };
int n = arr.length;
System.out.println(findFirst(arr, n, x));
}
}
// This code is contributed by 29AjayKumar
# Python3 program to find index of first
# occurrence of x when array is sorted.
import math
def findFirst(arr, n, x):
arr.sort()
# lower_bound returns iterator pointing to
# first element that does not compare less
# to x.
ptr = lowerBound(arr, 0, n, x)
# If x is not present return -1.
return 1 if (ptr != x) else (ptr - arr)
def lowerBound(a, low, high, element):
while(low < high):
middle = low + (high - low) // 2
if(element > a[middle]):
low = middle + 1
else:
high = middle
return low
# Driver Code
if __name__ == '__main__':
x = 20
arr = [10, 30, 20, 50, 20]
n = len(arr)
print(findFirst(arr, n, x))
# This code is contributed by Rajput-Ji
// C# program to find index of first
// occurrence of x when array is sorted.
using System;
class GFG {
static int findFirst(int[] arr, int n, int x)
{
Array.Sort(arr);
// lower_bound returns iterator pointing to
// first element that does not compare less
// to x.
int ptr = lowerBound(arr, 0, n, x);
// If x is not present return -1.
return (arr[ptr] != x) ? -1 : (ptr);
}
static int lowerBound(int[] a, int low, int high,
int element)
{
while (low < high) {
int middle = low + (high - low) / 2;
if (element > a[middle])
low = middle + 1;
else
high = middle;
}
return low;
}
// Driver Code
static public void Main()
{
int x = 20;
int[] arr = { 10, 30, 20, 50, 20 };
int n = arr.Length;
Console.Write(findFirst(arr, n, x));
}
}
// This code is contributed by ajit.
<?php
//PHP program to find index of first
// occurrence of x when array is sorted.
function findFirst( $arr, $n, $x)
{
sort($arr);
// lower_bound returns iterator pointing to
// first element that does not compare less
// to x.
$ptr = floor($arr);
// If x is not present return -1.
return ($ptr != $x)? 1 : ($ptr - $arr);
}
//Code driven
$x = 20;
$arr = array(10, 30, 20, 50, 20);
$n = sizeof($arr)/sizeof($arr[0]);
echo findFirst($arr, $n, $x);
#This code is contributed by Tushil.
?>
<script>
// Javascript program to find index of first
// occurrence of x when array is sorted.
function findFirst(arr, n, x)
{
arr.sort();
// lower_bound returns iterator pointing
// to first element that does not compare
// less to x.
let ptr = lowerBound(arr, 0, n, x);
// If x is not present return -1.
return (arr[ptr] != x) ? -1 : (ptr);
}
function lowerBound(a, low, high, element)
{
while (low < high)
{
let middle = low + parseInt(
(high - low) / 2, 10);
if (element > a[middle])
low = middle + 1;
else
high = middle;
}
return low;
}
// Driver code
let x = 20;
let arr = [ 10, 30, 20, 50, 20 ];
let n = arr.length;
document.write(findFirst(arr, n, x));
// This code is contributed by mukesh07
</script>
Complexity Analysis:
- Time Complexity : O(n Log n)
- Auxiliary Space: O(1)
An efficient solution is to simply count smaller elements than x.
Implementation:
C++
// C++ program to find index of first
// occurrence of x when array is sorted.
#include <bits/stdc++.h>
using namespace std;
int findFirst(int arr[], int n, int x)
{
int count = 0;
bool isX = false;
for (int i = 0; i < n; i++) {
if (arr[i] == x)
isX = true;
else if (arr[i] < x)
count++;
}
return (isX == false) ? -1 : count;
}
int main()
{
int x = 20, arr[] = { 10, 30, 20, 50, 20 };
int n = sizeof(arr) / sizeof(arr[0]);
cout << findFirst(arr, n, x);
return 0;
}
// Java program to find index of first
// occurrence of x when array is sorted.
public class GFG {
static int findFirst(int arr[], int n, int x)
{
int count = 0;
boolean isX = false;
for (int i = 0; i < n; i++) {
if (arr[i] == x) {
isX = true;
}
else if (arr[i] < x) {
count++;
}
}
return (isX == false) ? -1 : count;
}
// Driver main
public static void main(String[] args)
{
int x = 20, arr[] = { 10, 30, 20, 50, 20 };
int n = arr.length;
System.out.println(findFirst(arr, n, x));
}
}
/*This code is contributed by PrinciRaj1992*/
# Python 3 program to find index
# of first occurrence of x when
# array is sorted.
def findFirst(arr, n, x):
count = 0
isX = False
for i in range(n):
if (arr[i] == x):
isX = True
elif (arr[i] < x):
count += 1
return -1 if(isX == False) else count
# Driver Code
if __name__ == "__main__":
x = 20
arr = [10, 30, 20, 50, 20]
n = len(arr)
print(findFirst(arr, n, x))
# This code is contributed
# by ChitraNayal
// C# program to find index of first
// occurrence of x when array is sorted.
using System;
public class GFG {
static int findFirst(int[] arr, int n, int x)
{
int count = 0;
bool isX = false;
for (int i = 0; i < n; i++) {
if (arr[i] == x) {
isX = true;
}
else if (arr[i] < x) {
count++;
}
}
return (isX == false) ? -1 : count;
}
// Driver main
public static void Main()
{
int x = 20;
int[] arr = { 10, 30, 20, 50, 20 };
int n = arr.Length;
Console.WriteLine(findFirst(arr, n, x));
}
}
/*This code is contributed by PrinciRaj1992*/
<?php
// PHP program to find index of first
// occurrence of x when array is sorted.
function findFirst($arr, $n, $x)
{
$count = 0;
$isX = false;
for ($i = 0; $i < $n; $i++)
{
if ($arr[$i] == $x)
$isX = true;
else if ($arr[$i] < $x)
$count++;
}
return ($isX == false)? -1 : $count;
}
// Driver Code
$x = 20;
$arr = array(10, 30, 20, 50, 20);
$n = sizeof($arr);
echo findFirst($arr, $n, $x);
// This code is contributed
// by Akanksha Rai
?>
<script>
// JavaScript program to find index of first
// occurrence of x when array is sorted.
function findFirst(arr, n, x)
{
var count = 0;
var isX = false;
for(var i = 0; i < n; i++)
{
if (arr[i] == x)
{
isX = true;
}
else if (arr[i] < x)
{
count++;
}
}
return (isX == false) ? -1 : count;
}
// Driver Code
var x = 20, arr = [ 10, 30, 20, 50, 20 ];
var n = arr.length;
document.write(findFirst(arr, n, x));
// This code is contributed by Khushboogoyal499
</script>
Complexity Analysis:
- Time Complexity: O(N)
- Auxiliary Space: O(1)
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