How do I get the previous element of a list using the same iterator?

Question

I am making a Sudoku solver and I cannot manage to get backtracking working by refering to the last element of my list (which holds the positions of the blank spaces).

The code below shows my naive initial approach to this issue:

void solve(int arr[9][9])
{
    position CurrentPos;

    list<position>::iterator j;
    for (j = blankSpaces.begin();  j != blankSpaces.end();  j++)
    {
        CurrentPos.row = j->row;
        CurrentPos.col = j->col;

        for (int i = arr[CurrentPos.row][CurrentPos.col] + 1; i < 10; i++)
        {
            if (valid(arr, CurrentPos, i))
            {
                arr[CurrentPos.row][CurrentPos.col] = i;
                visitedStack.emplace_front(CurrentPos);
            }
            if (!(valid(arr, CurrentPos, i)) && (i == 9))
            {
                j--;
            }
        }
    }

}

`CurrentPos` could be renamed as `currentPos`... just some coding standards :P — Ardent Coder, Jan 28 '20 at 08:18
Prefer `std::vector` for problems like Sudoku solver. A `std::list` is not "iteration" friendly — P0W, Jan 28 '20 at 08:23
@jkengineeringworkvideos I wrote an answer. Please check it out and accept if it answers your question, or comment if you want an explanation. — NutCracker, Jan 28 '20 at 08:25
noob question.... can vector handle a dynamic number of structures? — jk engineering work videos, Jan 28 '20 at 08:26
You absolutely have to create and provide a [mcve] as part of your question. Your problem is not that you can't decrement an iterator and this (required!) process would have shown you that. As a new user here, please also take the [tour] and read [ask]. — Ulrich Eckhardt, Jan 28 '20 at 08:38
[`std::vector`](https://en.cppreference.com/w/cpp/container/vector) can solve your problem instead of `std::List`. >The storage of the vector is handled automatically, being expanded and contracted as needed. — TruthSeeker, Jan 28 '20 at 08:46

score 1 · Accepted Answer · answered Jan 28 '20 at 08:21

1

You can get the last element of the list by doing the following:

#include <iostream>
#include <iterator>
#include <list>

int main () {
    std::list<int> mylist;
    for (int i = 0; i < 10; i++)
        mylist.push_back (i*10);

    auto last = std::prev(mylist.end());
    std::cout << "The last element is " << *last << '\n';

    return 0;
}

Or you can get the iterator before another iterator by using the same like this:

#include <iostream>
#include <iterator>
#include <list>

int main () {
    std::list<int> mylist;
    for (int i = 0; i < 10; i++)
        mylist.push_back (i*10);

    auto last = std::prev(mylist.end());
    auto prelast = std::prev(last);
    std::cout << "The last element is " << *last << '\n';
    std::cout << "The prelast element is " << *prelast << '\n';

    return 0;
}

Or just use -- as you already do:

#include <iostream>
#include <iterator>
#include <list>

int main () {
    std::list<int> mylist;
    for (int i = 0; i < 10; i++)
        mylist.push_back (i*10);

    auto last = std::prev(mylist.end());
    auto prelast = std::prev(last);
    std::cout << "The last element is " << *last << '\n';
    std::cout << "The prelast element is " << *prelast << '\n';

    prelast--;
    std::cout << "The preprelast element is " << *prelast << '\n';

    return 0;
}

answered Jan 28 '20 at 08:21

NutCracker

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Visual studio said that we can't decrement a iterator – jk engineering work videos Jan 28 '20 at 08:28
Also I wanna avoid another iterator – jk engineering work videos Jan 28 '20 at 08:28
_Visual studio said that we can decrement a iterator_ - can or cannot? If you want to avoid another iterator just reassign like this `auto it = std::prev(mylist.end()); it = std::prev(it);` That will give you the prelast iterator – NutCracker Jan 28 '20 at 08:29
it said we can't decrement a begin list iterator – jk engineering work videos Jan 28 '20 at 08:33
of course you can't when decrementing a begin list iterator is undefined behavior. Check [here](https://stackoverflow.com/questions/19417171/stl-iterator-before-stdmapbegin) – NutCracker Jan 28 '20 at 08:39
@jkengineeringworkvideos so basically you need to make sure you don't need to decrement first iterator – NutCracker Jan 28 '20 at 08:50
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/206774/discussion-between-jk-engineering-work-videos-and-nutcracker). – jk engineering work videos Jan 28 '20 at 09:06

How do I get the previous element of a list using the same iterator?

1 Answers1