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How to join two Vectors using STL in C++?
• Difficulty Level : Basic
• Last Updated : 28 May, 2021

Given two vectors, join these two vectors using STL in C++.
Example:

Input:
vec1 = {1, 45, 54, 71, 76, 12},
vec2 = {1, 7, 5, 4, 6, 12}
Output: {1, 4, 5, 6, 7, 12, 45, 54, 71, 76}
Input:
vec1 = {1, 7, 5, 4, 6, 12},
vec2 = {10, 12, 11}
Output: {1, 4, 5, 6, 7, 10, 11, 12}

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

Approach: Joining can be done with the help of set_union() function provided in STL.
Syntax:

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

## CPP

 `// C++ program to join two Vectors``// using set_union() in STL` `#include ``using` `namespace` `std;` `int` `main()``{``    ``// Get the vector``    ``vector<``int``> vector1 = { 1, 45, 54, 71, 76, 12 };``    ``vector<``int``> vector2 = { 1, 7, 5, 4, 6, 12 };` `    ``// Sort the vector``    ``sort(vector1.begin(), vector1.end());``    ``sort(vector2.begin(), vector2.end());` `    ``// Print the vector``    ``cout << ``"First Vector: "``;``    ``for` `(``int` `i = 0; i < vector1.size(); i++)``        ``cout << vector1[i] << ``" "``;``    ``cout << endl;` `    ``cout << ``"Second Vector: "``;``    ``for` `(``int` `i = 0; i < vector2.size(); i++)``        ``cout << vector2[i] << ``" "``;``    ``cout << endl;` `    ``// Initialise a vector``    ``// to store the common values``    ``// and an iterator``    ``// to traverse this vector``    ``vector<``int``> v(vector1.size() + vector2.size());``    ``vector<``int``>::iterator it, st;` `    ``it = set_union(vector1.begin(),``                   ``vector1.end(),``                   ``vector2.begin(),``                   ``vector2.end(),``                   ``v.begin());` `    ``cout << ``"\nAfter joining:\n"``;``    ``for` `(st = v.begin(); st != it; ++st)``        ``cout << *st << ``", "``;``    ``cout << ``'\n'``;` `    ``return` `0;``}`
Output:
```First Vector: 1 12 45 54 71 76
Second Vector: 1 4 5 6 7 12

After joining:
1, 4, 5, 6, 7, 12, 45, 54, 71, 76,```

Note that the vector should not contain 1 element twice: In case you enter same elements, the set_union function will only consider 1 instance of the element, and default rest to 0.

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