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# Find index of first occurrence when an unsorted array is sorted

• Difficulty Level : Basic
• Last Updated : 20 May, 2021

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.

## C++

 `// C++ program to find index of first``// occurrence of x when array is sorted.``#include ``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

 `// 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

 `# 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#

 `// 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

 ``

## Javascript

 ``

Output:

`1`

Time Complexity : O(n Log n)

An efficient solution is to simply count smaller elements than x.

## C++

 `// C++ program to find index of first``// occurrence of x when array is sorted.``#include ``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

 `// 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*/`

## Python3

 `# 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#

 `// 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

 ``

## Javascript

 ``

Output:

`1`

Time Complexity: O(N)

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