# Algorithm Library | C++ Magicians STL Algorithm

For all those who aspire to excel in competitive programming, only having a knowledge about containers of STL is of less use till one is not aware what all STL has to offer.
STL has an ocean of algorithms, for all < algorithm > library functions : Refer here.
Some of the most used algorithms on vectors and most useful one’s in Competitive Programming are mentioned as follows :

Non-Manipulating Algorithms

1. sort(first_iterator, last_iterator) – To sort the given vector.
2. sort(first_iterator, last_iterator, greater<int>()) – To sort the given container/vector in descending order
3. reverse(first_iterator, last_iterator) – To reverse a vector. ( if ascending -> descending  OR  if descending -> ascending)
4. *max_element (first_iterator, last_iterator) – To find the maximum element of a vector.
5. *min_element (first_iterator, last_iterator) – To find the minimum element of a vector.
6. accumulate(first_iterator, last_iterator, initial value of sum) – Does the summation of vector elements

## CPP

 `// A C++ program to demonstrate working of sort(),` `// reverse()` `#include ` `#include ` `#include ` `#include //For accumulate operation` `using` `namespace` `std;`   `int` `main()` `{` `    ``// Initializing vector with array values` `    ``int` `arr[] = {10, 20, 5, 23 ,42 , 15};` `    ``int` `n = ``sizeof``(arr)/``sizeof``(arr);` `    ``vector<``int``> vect(arr, arr+n);`   `    ``cout << ``"Vector is: "``;` `    ``for` `(``int` `i=0; i());` `  `  `    ``cout << ``"\nVector after sorting in Descending order is: "``;` `    ``for` `(``int` `i=0; i

Output

```Vector is: 10 20 5 23 42 15
Vector after sorting is: 5 10 15 20 23 42
Vector after sorting in Descending order is: 42 23 20 15 10 5
Vector after reversing is: 5 10 15 20 23 42
Maximum element of vector is: 42
Minimum element of vector is: 5
The summation of vector elements is: 115```

6.count(first_iterator, last_iterator,x) – To count the occurrences of x in vector.

7. find(first_iterator, last_iterator, x) – Returns an iterator to the first occurrence of x in vector and points to last address of vector ((name_of_vector).end()) if element is not present in vector.

## CPP

 `// C++ program to demonstrate working of count()` `// and find()` `#include ` `#include ` `#include ` `using` `namespace` `std;`   `int` `main()` `{` `    ``// Initializing vector with array values` `    ``int` `arr[] = {10, 20, 5, 23 ,42, 20, 15};` `    ``int` `n = ``sizeof``(arr)/``sizeof``(arr);` `    ``vector<``int``> vect(arr, arr+n);`   `    ``cout << ``"Occurrences of 20 in vector : "``;`   `    ``// Counts the occurrences of 20 from 1st to` `    ``// last element` `    ``cout << count(vect.begin(), vect.end(), 20);`   `    ``// find() returns iterator to last address if` `    ``// element not present` `    ``find(vect.begin(), vect.end(),5) != vect.end()?` `                         ``cout << ``"\nElement found"``:` `                     ``cout << ``"\nElement not found"``;`   `    ``return` `0;` `}`

Output

```Occurrences of 20 in vector : 2
Element found```

8. binary_search(first_iterator, last_iterator, x) – Tests whether x exists in sorted vector or not.

9. lower_bound(first_iterator, last_iterator, x) – returns an iterator pointing to the first element in the range [first,last) which         has a value not less than ‘x’.

10. upper_bound(first_iterator, last_iterator, x) – returns an iterator pointing to the first element in the range [first,last)                  which has a value greater than ‘x’.

## C++

 `// C++ program to demonstrate working of lower_bound()` `// and upper_bound().` `#include ` `#include ` `#include ` `using` `namespace` `std;`   `int` `main()` `{` `    ``// Initializing vector with array values` `    ``int` `arr[] = {5, 10, 15, 20, 20, 23, 42, 45};` `    ``int` `n = ``sizeof``(arr)/``sizeof``(arr);` `    ``vector<``int``> vect(arr, arr+n);`   `    ``// Sort the array to make sure that lower_bound()` `    ``// and upper_bound() work.` `    ``sort(vect.begin(), vect.end());`   `    ``// Returns the first occurrence of 20` `    ``auto` `q = lower_bound(vect.begin(), vect.end(), 20);`   `    ``// Returns the last occurrence of 20` `    ``auto` `p = upper_bound(vect.begin(), vect.end(), 20);`   `    ``cout << ``"The lower bound is at position: "``;` `    ``cout << q-vect.begin() << endl;`   `    ``cout << ``"The upper bound is at position: "``;` `    ``cout << p-vect.begin() << endl;`   `    ``return` `0;` `}`

Output

```The lower bound is at position: 3
The upper bound is at position: 5
```

Some Manipulating Algorithms

1. arr.erase(position to be deleted) – This erases selected element in vector and shifts and resizes the vector elements accordingly.
2. arr.erase(unique(arr.begin(),arr.end()),arr.end()) – This erases the duplicate occurrences in sorted vector in a single line.

## C++

 `// C++ program to demonstrate working` `// of erase` `#include ` `#include ` `#include ` `#include ` `using` `namespace` `std;`   `int` `main()` `{` `    ``// Initializing vector with array values` `    ``int` `arr[] = { 5, 10, 15, 20, 20, 23, 42, 45 };` `    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr);` `    ``vector<``int``> vect(arr, arr + n);`   `    ``cout << ``"Given Vector is:\n"``;` `    ``for` `(``int` `i = 0; i < n; i++)` `        ``cout << vect[i] << ``" "``;`   `    ``vect.erase(find(vect.begin(),vect.end(),10));` `    ``cout << ``"\nVector after erasing element:\n"``;` `    ``for` `(``int` `i = 0; i < vect.size(); i++)` `        ``cout << vect[i] << ``" "``;`   `    ``vect.erase(unique(vect.begin(), vect.end()),` `               ``vect.end());` `    ``cout << ``"\nVector after removing duplicates:\n"``;` `    ``for` `(``int` `i = 0; i < vect.size(); i++)` `        ``cout << vect[i] << ``" "``;`   `    ``return` `0;` `}`

Output

```Given Vector is:
5 10 15 20 20 23 42 45
Vector after erasing element:
5 15 20 20 23 42 45
Vector after removing duplicates:
5 15 20 23 42 45 ```

3. next_permutation(first_iterator, last_iterator) – This modified the vector to its next permutation.

4. prev_permutation(first_iterator, last_iterator) – This modified the vector to its previous permutation.

## CPP

 `// C++ program to demonstrate working ` `// of next_permutation()` `// and prev_permutation()` `#include ` `#include ` `#include ` `using` `namespace` `std;`   `int` `main()` `{` `    ``// Initializing vector with array values` `    ``int` `arr[] = {5, 10, 15, 20, 20, 23, 42, 45};` `    ``int` `n = ``sizeof``(arr)/``sizeof``(arr);` `    ``vector<``int``> vect(arr, arr+n);`   `    ``cout << ``"Given Vector is:\n"``;` `    ``for` `(``int` `i=0; i

Output

```Given Vector is:
5 10 15 20 20 23 42 45
Vector after performing next permutation:
5 10 15 20 20 23 45 42
Vector after performing prev permutation:
5 10 15 20 20 23 42 45 ```

5. distance(first_iterator,desired_position) – It returns the distance of desired position from the first iterator.This function               is very useful while finding the index.

## CPP

 `// C++ program to demonstrate working of distance()` `#include ` `#include ` `#include ` `using` `namespace` `std;`   `int` `main()` `{` `    ``// Initializing vector with array values` `    ``int` `arr[] = {5, 10, 15, 20, 20, 23, 42, 45};` `    ``int` `n = ``sizeof``(arr)/``sizeof``(arr);` `    ``vector<``int``> vect(arr, arr+n);`   `    ``// Return distance of first to maximum element` `    ``cout << ``"Distance between first to max element: "``; ` `    ``cout << distance(vect.begin(),` `                     ``max_element(vect.begin(), vect.end()));` `    ``return` `0;` `}`

Output

`Distance between first to max element: 7`

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