C++ multisets: removing the last element

Question

How to remove the last element from a multiset. I tried passing the reverse iterator to the erase function but that is not allowed. Any suggestions?

Duplicate of [Multiset erase last element](http://stackoverflow.com/questions/8992658/multiset-erase-last-element)? — user2802841, Oct 19 '13 at 17:36
Possible duplicate of [Multiset erase last element](https://stackoverflow.com/questions/8992658/multiset-erase-last-element) — WilliamKF, Jun 12 '19 at 18:01

awesoon · Accepted Answer · 2013-10-20T06:15:01.957

6

Every reverse_iterator has a base() function, that returns "normalized" iterator. You could use it like this:

auto e = ms.rbegin();
ms.erase((++e).base());

We must increment e before getting underlying iterator, because the base iterator refers to the element that is next to the element the reverse_iterator is pointing to.

Another solution is using std::prev function:

ms.erase(std::prev(ms.end())); // since C++11

If C++11 is not an option for you, you could implement it yourself:

// copy-pasted from cppreference
template<class BidirIt>
BidirIt prev(BidirIt it, 
             typename std::iterator_traits<BidirIt>::difference_type n = 1)
{
    std::advance(it, -n);
    return it;
}

It's more simpler, than first, but I leave two solution, because earlier I described first example incorrectly.

edited Oct 20 '13 at 06:15

answered Oct 19 '13 at 17:32

awesoon

32,469
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74
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1

Cool, I didn't know that. Alternatively, `std::prev(std::end(ms))` – chris Oct 19 '13 at 17:34
@chris, Please, look at answer again. My first explanation was incorrect. – awesoon Oct 19 '13 at 18:00
@soon i tried the second one.. it gives an error saying prev is not a member of std – ishan3243 Oct 19 '13 at 18:58
Is there any benefit to `std::prev` over `operator--` here? – 24n8 Jul 01 '21 at 21:34
@24n8 yes - it can be integrated into a more complex expression without fear of mutation or side-effects – Reinderien Sep 04 '22 at 12:29

Vlad from Moscow · Answer 2 · 2013-10-19T18:11:27.777

If you want to delete only one element then you can write

if ( !ms.empty() ) ms.erase( std::prev( ms.end() ) );

If you want to delete all elements with the given key equal to the last key then you can write

if ( !ms.empty() )
{
    auto p = ms.equal_range( *std::prev( ms.end() ) );
    ms.erase( p.first, p.second );
}

And at last if you want to delete all duplicates except one then you can write

if ( !ms.empty() )
{
    auto p = ms.equal_range( *std::prev( ms.end() ) );
    ms.erase( std::next( p.first ), p.second );
}

score 0 · Answer 3 · answered Oct 19 '13 at 17:41

0

Because multiset erase function takes iterator which is regular. So if you want to erase last element you should do like that.

//in C++11
multiset.erase(std::prev(multiset.end()));
//in C++98 
multiset.erase(--multiset.end());

answered Oct 19 '13 at 17:41

yilmazburk

907
9
17

Franken Steak · Answer 4 · 2013-10-19T21:49:09.477

0

You could try the following code:

multiset<int> mySet;

read in some integers.....

multiset<int>::reverse_iterator rit;
rit = mySet.rbegin();

mySet.erase((++rit).base());

the .rbegin() function returns a reverse iterator pointing to the end of the set, rit is then incremented since the base function is currently pointing to the value next to the value we want (credit to @soon for that part, in his answer). The erase() function then erases the element in the parameter.

Hope this helps. =)

edited Oct 19 '13 at 21:49

answered Oct 19 '13 at 17:46

Franken Steak

60
2
9

@soon I seem, to have fixed it. Thanks for pointing out my error. – Franken Steak Oct 19 '13 at 21:49

C++ multisets: removing the last element

4 Answers4