# Elements present in first array and not in second using STL in C++

Given two arrays, the task is that we find numbers which are present in first array, but not present in the second array, using STL in C++

Examples:

```Input: a[] = {1, 2, 3, 4, 5, 10}, b[] = {2, 3, 1, 0, 5}
Output: 4 10

Input:a[] = {4, 3, 5, 9, 11}, b[] = {4, 9, 3, 11, 10};
Output: 5
```

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

Approach: In STL, the set_difference() method can be used to find the ‘A-B’ where A is the first array and B is the second array.

Syntax:

OutputIterator set_difference (InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2, OutputIterator result);

Below is the implementation of the above approach:

 `// C++ simple program to ` `// find elements which are ` `// not present in second array ` ` `  `#include ` `using` `namespace` `std; ` ` `  `// Function for finding ` `// elements which are there ` `// in a[] but not in b[]. ` `void` `findMissing(``int` `a[], ``int` `b[], ` `                 ``int` `n, ``int` `m) ` `{ ` ` `  `    ``// Declare a vector to store the result ` `    ``vector<``int``> v(n + m); ` ` `  `    ``// And an iterator to traverse the vector ` `    ``vector<``int``>::iterator it; ` ` `  `    ``// Sort the given arrays ` `    ``sort(a, a + n); ` `    ``sort(b, b + m); ` ` `  `    ``// Find the elements in a[] ` `    ``// which are not in b[] ` `    ``it = set_difference(a, a + n, b, b + m, v.begin()); ` ` `  `    ``// Now resize the vector to the existing count ` `    ``v.resize(it - v.begin()); ` ` `  `    ``// Print the results ` `    ``cout << ``"The elements in a[]"` `         ``<< ``" which are not in b[]:\n"``; ` `    ``for` `(it = v.begin(); it != v.end(); ++it) ` `        ``cout << *it << ``"  "``; ` `    ``cout << endl; ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``int` `a[] = { 1, 2, 6, 3, 4, 5 }; ` `    ``int` `b[] = { 2, 4, 3, 1, 0 }; ` `    ``int` `n = ``sizeof``(a) / ``sizeof``(a); ` `    ``int` `m = ``sizeof``(b) / ``sizeof``(b); ` `    ``findMissing(a, b, n, m); ` `    ``return` `0; ` `} `

Output:

```The elements in a[] which are not in b[]:
5  6
```

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