Algorithm Library | C++ Magicians STL Algorithm
Last Updated :
17 Oct, 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.
- sort(first_iterator, last_iterator, greater<int>()) – To sort the given container/vector in descending order
- reverse(first_iterator, last_iterator) – To reverse a vector. ( if ascending -> descending OR if descending -> ascending)
- *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
#include <algorithm>
#include <iostream>
#include <vector>
#include <numeric> //For accumulate operation
using namespace std;
int main()
{
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] << " ";
sort(vect.begin(), vect.end());
cout << "\nVector after sorting is: ";
for (int i=0; i<n; i++)
cout << vect[i] << " ";
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] << " ";
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());
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
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
int main()
{
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 : ";
cout << count(vect.begin(), vect.end(), 20);
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++
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
int main()
{
int arr[] = {5, 10, 15, 20, 20, 23, 42, 45};
int n = sizeof(arr)/sizeof(arr[0]);
vector<int> vect(arr, arr+n);
sort(vect.begin(), vect.end());
auto q = lower_bound(vect.begin(), vect.end(), 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
- 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.
C++
#include <algorithm>
#include <bits/stdc++.h>
#include <iostream>
#include <vector>
using namespace std;
int main()
{
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
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
int main()
{
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] << " ";
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
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
int main()
{
int arr[] = {5, 10, 15, 20, 20, 23, 42, 45};
int n = sizeof(arr)/sizeof(arr[0]);
vector<int> vect(arr, arr+n);
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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