Algorithm Library | C++ Magicians STL Algorithm - GeeksforGeeks

21 Apr, 2022
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
sort(first_iterator, last_iterator) – To sort the given vector.reverse(first_iterator, last_iterator) – To reverse a vector.*max_element (first_iterator, last_iterator) – To find the maximum element of a vector.*min_element (first_iterator, last_iterator) – To find the minimum element of a vector.accumulate(first_iterator, last_iterator, initial value of sum) – Does the summation of vector elements
sort(first_iterator, last_iterator) – To sort the given vector.
reverse(first_iterator, last_iterator) – To reverse a vector.
*max_element (first_iterator, last_iterator) – To find the maximum element of a vector.
*min_element (first_iterator, last_iterator) – To find the minimum element of a vector.
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 operationusing 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] << " ";     // Reversing the Vector    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;}
Vector is: 10 20 5 23 42 15 
Vector after sorting is: 5 10 15 20 23 42 
Vector after reversing is: 42 23 20 15 10 5 
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 occurence 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;}
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;}
The lower bound is at position: 3
The upper bound is at position: 5
Some Manipulating Algorithms
arr.erase(position to be deleted) – This erases selected element in vector and shifts and resizes the vector elements accordingly.arr.erase(unique(arr.begin(),arr.end()),arr.end()) – This erases the duplicate occurrences in sorted vector in a single line.
arr.erase(position to be deleted) – This erases selected element in vector and shifts and resizes the vector elements accordingly.
arr.erase(unique(arr.begin(),arr.end()),arr.end()) – This erases the duplicate occurrences in sorted vector in a single line.
CPP
// C++ program to demonstrate working of erase()#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 << "Vector is :";    for (int i=0; i<n; i++)        cout << vect[i]<<" ";     // Delete second element of vector    vect.erase(vect.begin()+1);     cout << "\nVector after erasing the element: ";    for (int i=0; i<vect.size(); i++)        cout << vect[i] << " ";     // sorting to enable use of unique()    sort(vect.begin(), vect.end());     cout << "\nVector before removing duplicate "             " occurrences: ";    for (int i=0; i<vect.size(); i++)        cout << vect[i] << " ";     // Deletes the duplicate occurrences    vect.erase(unique(vect.begin(),vect.end()),vect.end());     cout << "\nVector after deleting duplicates: ";    for (int i=0; i< vect.size(); i++)        cout << vect[i] << " ";     return 0;}
Vector is :5 10 15 20 20 23 
Vector after erasing the element: 5 15 20 20 23 
Vector before removing duplicate  occurrences: 5 15 20 20 23 
Vector after deleting 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;}
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;}
Distance between first to max element: 7
More – STL ArticlesThis article is contributed by Manjeet Singh. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above
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