These functions return useful iterators to access vector elements in reverse order (from end to beginning)
Using vector::crend()
It’s a public member function that returns a const_reverse_iterator pointing to the element preceding the first element.
Return Value
A const_reverse_iterator to the reverse
end of the sequence.
Syntax:
const_reverse_iterator crend() const noexcept;
Time Complexity – constant O(1)
Example 1:
C++
#include <iostream>
#include<vector>
using namespace std;
int main()
{
vector<string>str{"avi","aniket","akash","ajay"};
vector<string>::const_reverse_iterator itr=str.crend()-1;
std::cout<< *itr;
return 0;
}
|
Using vector::crbegin()
It returns a const_reverse_iterator pointing to the last element in the container (i.e., its reverse beginning)..
Return Value
A const_reverse_iterator to the reverse
beginning of the sequence.
Syntax:
const_reverse_iterator crbegin() const noexcept;
Example 1 :
C++
#include <iostream>
#include<vector>
using namespace std;
int main()
{
vector<string> v{"akash","ajay","aniket","avi"};
vector<string>::const_reverse_iterator itr=v.crbegin();
cout<<*itr;
return 0;
}
|
CPP
#include <iostream>
#include <vector>
using namespace std;
int main ()
{
vector<int> vect = {10, 20, 30, 40, 50};
for (auto i = vect.crbegin(); i != vect.crend(); i++)
cout << ' ' << *i;
cout << '\n';
return 0;
}
|
Time Complexity – constant O(1)
Auxiliary Space: O(1)
Applications crend : Returns a reverse iterator to the element following the last element of the reversed container. It corresponds to the element preceding the first element of the non-reversed container. This element acts as a placeholder, attempting to access it results in undefined behavior
crbegin : Returns a reverse iterator to the first element of the reversed container. It corresponds to the last element of the non-reversed container.
Both are used in deque operations, concurrently accessing or modifying different elements is safe. The copy construction or assignment of the returned iterator is also guaranteed to never throw exception.
Let us see the differences in a tabular form -:
| | vector::crend() | vector::crbegin() |
| 1. | It is used to return a const_reverse_iterator pointing to the theoretical element preceding the first element in the container. | It is used to return const_reverse_iterator to reverse beginning |
| 2. | Its syntax is -: const_reverse_iterator crend(); | Its syntax is -: const_reverse_iterator crbegin(); |
| 3. | It does not take any parameters. | It does not take any parameters. |
| 4. | Its complexity does not changes. | Its complexity does not change. |
| 5. | Its iterator validity does not changes. | Its iterator validity does not change. |