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 <algorithm>
#include <iostream>
#include <vector>
#include <numeric> //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[0]);
    vector<int> vect(arr, arr+n);

    cout << "Vector is: ";
    for (int i=0; i<n; i++)
        cout << vect[i] << " ";

    // Sorting the Vector in Ascending order
    sort(vect.begin(), vect.end());


    cout << "\nVector after sorting is: ";
    for (int i=0; i<n; i++)
       cout << vect[i] << " ";


      // Sorting the Vector in Descending order
      sort(vect.begin(),vect.end(), greater<int>());

    cout << "\nVector after sorting in Descending order is: ";
    for (int i=0; i<n; i++)
       cout << vect[i] << " ";


    // Reversing the Vector (descending to ascending , ascending to descending)
    reverse(vect.begin(), vect.end());

    cout << "\nVector after reversing is: ";
    for (int i=0; i<n; i++)
        cout << vect[i] << " ";

    cout << "\nMaximum element of vector is: ";
    cout << *max_element(vect.begin(), vect.end());

    cout << "\nMinimum element of vector is: ";
    cout << *min_element(vect.begin(), vect.end());

    // Starting the summation from 0
    cout << "\nThe summation of vector elements is: ";
    cout << accumulate(vect.begin(), vect.end(), 0);

    return 0;
}

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 <algorithm>
#include <iostream>
#include <vector>
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[0]);
    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 <algorithm>
#include <iostream>
#include <vector>
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[0]);
    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 <algorithm>
#include <bits/stdc++.h>
#include <iostream>
#include <vector>
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[0]);
    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 <algorithm>
#include <iostream>
#include <vector>
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[0]);
    vector<int> vect(arr, arr+n);

    cout << "Given Vector is:\n";
    for (int i=0; i<n; i++)
        cout << vect[i] << " ";

    // modifies vector to its next permutation order
    next_permutation(vect.begin(), vect.end());
    cout << "\nVector after performing next permutation:\n";
    for (int i=0; i<n; i++)
        cout << vect[i] << " ";

    prev_permutation(vect.begin(), vect.end());
    cout << "\nVector after performing prev permutation:\n";
    for (int i=0; i<n; i++)
        cout << vect[i] << " ";

    return 0;
}

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 <algorithm>
#include <iostream>
#include <vector>
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[0]);
    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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