std::prev in C++

std::prev returns an iterator pointing to the element after being advanced by certain number of positions in the reverse direction. It is defined inside the header file iterator. It returns a copy of the argument advanced by the specified amount in the backward direction. If it is a random-access iterator, the function uses just once operator + or operator – for advancing. Otherwise, the function uses repeatedly the increase or decrease operator (operator ++ or operator – -) on the copied iterator until n elements have been advanced. Syntax:

BidirectionalIterator prev (BidirectionalIterator it,
       typename iterator_traits::difference_type n = 1);
it: Iterator to the base position.
difference_type: It is the numerical type that represents 
distances between iterators of the BidirectionalIterator type.
n: Total no. of positions by which the
iterator has to be advanced. In the syntax, n is assigned
a default value 1 so it will atleast advance by 1 position.

Returns: It returns an iterator to the element 
n positions before it.

CPP

// C++ program to demonstrate std::next
#include <iostream>
#include <iterator>
#include <deque>
#include <algorithm>
using namespace std;
int main()
{
    // Declaring first container
    deque<int> v1 = { 1, 2, 3, 4, 5, 6, 7 };

    // Declaring another container
    deque<int> v2 = { 8, 9, 10 };

    // Declaring an iterator
    deque<int>::iterator i1;

    // i1 points to 1
    i1 = v1.begin();

    // Declaring another iterator to store return
    // value and using std::next
    deque<int>::iterator i2;
    i2 = std::next(v1.end(), -3);

    // Using std::copy
    std::copy(i1, i2, std::back_inserter(v2));
    // Remember, i1 stills points to 1
    // and i2 points to 5
    // v2 now contains 8 9 10 1 2 3 4

    // Displaying v1 and v2
    cout << "v1 = ";

    int i;
    for (i = 0; i < 7; ++i) {
        cout << v1[i] << " ";
    }

    cout << "\nv2 = ";
    for (i = 0; i < 7; ++i) {
        cout << v2[i] << " ";
    }

    return 0;
}

Output:

v1 = 1 2 3 4 5 6 7
v2 = 8 9 10 1 2 3 4

How can it be helpful ?

• Moving iterator in Lists: Since, lists support bidirectional iterators, which can be incremented only by using ++ and – – operator. So, if we want to advance the iterator by more than one position, then std::next and if we want to decrement the iterator, then std::prev can be extremely useful. 

• Output:

v1 = 1 2 3 7 8 9
v2 = 4 5 6 1 2 3 

• Explanation: Here, just look how if we want copy only a selected portion of the list, then we can make use of std::prev, as otherwise we cannot use any +=, -= operators with bidirectional iterators supported by lists. So, we used std::prev and directly moved the iterator backwards by three positions from the end.

Can we use std::next in place of std::prev ?

One common query that may arise with std::prev is that can std::next be also used with a negative argument to move the iterator in backward direction. Well, the answer is yes. 

CPP

// C++ program to demonstrate std::next
#include <iostream>
#include <iterator>
#include <deque>
#include <algorithm>
using namespace std;
int main()
{
    // Declaring first container
    deque<int> v1 = { 1, 2, 3, 4, 5, 6, 7 };

    // Declaring another container
    deque<int> v2 = { 8, 9, 10 };

    // Declaring an iterator
    deque<int>::iterator i1;

    // i1 points to 1
    i1 = v1.begin();

    // Declaring another iterator to store return
    // value and using std::next
    deque<int>::iterator i2;
    i2 = std::next(v1.end(), -3);

    // Using std::copy
    std::copy(i1, i2, std::back_inserter(v2));
    // Remember, i1 stills points to 1
    // and i2 points to 5
    // v2 now contains 8 9 10 1 2 3 4

    // Displaying v1 and v2
    cout << "v1 = ";

    int i;
    for (i = 0; i < 7; ++i) {
        cout << v1[i] << " ";
    }

    cout << "\nv2 = ";
    for (i = 0; i < 7; ++i) {
        cout << v2[i] << " ";
    }

    return 0;
}

Output:

v1 = 1 2 3 4 5 6 7
v2 = 8 9 10 1 2 3 4

Explanation: So, we have just used std::next in place of std::prev and changed the second argument from 3 to -3 here and it still serves the same purpose. We can use std::next also, but there are two things that needs to be kept in mind:

• Since, std::next in their syntax have an argument as forward iterator, so if we want to use negative no. for advancing that iterator, then it should be at least a bidirectional iterator.
• Although, std::next can also be used, but std::prev() would be more readable when the intent is specifically to move backwards.