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

Given an unsorted array and a number x, find 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 = 50
Output : -1
50 is not present in array.
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

A simple solution is to first sort the array, then do binary search to find first occurrence.

 `// CPP 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 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. `

 ` `

Output :
`1`

Time Complexity : O(n Log n)

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

 `// CPP 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

 `// 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*/`

 ` `

Output :
`1`

Time Complexity : O(n)

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