In std::multiset is there a function or algorithm to erase just one sample (unicate or duplicate) if an element is found

Question

Perhaps this is a duplicate but I did not find anything searching: When erase(value) is called on std::multiset all elements with the value found are deleted. The only solution I could think of is:

std::multiset<int>::iterator hit(mySet.find(5));
if (hit!= mySet.end()) mySet.erase(hit);

This is ok but I thought there might be better. Any Ideas ?

Does this approach ensures that the given key ("5") is duplicate? — Arun, Feb 06 '12 at 22:18
@ArunSaha: No. But if its not a duplicate, I want to delete it anyhow. From the answers I got, I get the feeling that there is no better solution. Maybe the question was stupid in the first place :-P — Martin, Feb 06 '12 at 22:22
@Martin: If you want to delete it anyhow, then what is wrong with just `erase( 5 )` which erases all copies of "5"? — Arun, Feb 07 '12 at 17:02
@ArunSaha: Just clarified the title to make the purpose obvious — Martin, Aug 19 '12 at 09:55
For `multimap`: is there any guarantee on which elements `find` returns? (Order of insertion? Even after such erasure? Implementation dependent?) — P Marecki, Feb 10 '13 at 14:46
Honestly it's such an unobvious pitfall while using multiset which is not among the most frequently used classes. — Predelnik, May 19 '17 at 16:35
7 years later still wondering where `std::multiset::erase_one_of` is! — Apollys supports Monica, Oct 16 '19 at 19:49

score 47 · Answer 1 · edited Dec 15 '18 at 17:52

47

auto itr = my_multiset.find(value);
if(itr!=my_multiset.end()){
    my_multiset.erase(itr);
}

I would imagine there is a cleaner way of accomplishing the same. But this gets the job done.

edited Dec 15 '18 at 17:52

Davide Spataro

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answered Jul 12 '16 at 05:53

user2251346

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This is no different to what is in the question. – Troubadour Aug 29 '18 at 15:09
2

I agree! Doesn't make any sense. 12 other people saw something useful in the answer so I know i am not going crazy. – user2251346 Sep 04 '18 at 22:24
10

Never overlook the possibility that you're going crazy along with everyone else :) – Apollys supports Monica Oct 17 '19 at 19:44

score 44 · Answer 2 · edited Feb 08 '21 at 16:00

44

Try this one:

multiset<int> s;
s.erase(s.lower_bound(value));

As long as you can ensure that the value exists in the set. That works.

edited Feb 08 '21 at 16:00

Mooncrater

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answered Nov 20 '18 at 03:44

Strange

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It is nice that this snippet is cleaner, however it feels less intention revealing. – EarthenSky Apr 22 '21 at 18:07
1

@EarthenSky You can do s.erase(s.find(value)) – TYeung Jan 03 '23 at 16:04

varun kunchakuri · Answer 3 · 2016-07-27T05:50:40.370

4

 if(my_multiset.find(key)!=my_multiset.end())
   my_multiset.erase(my_multiset.equal_range(key).first);

This is the best way i can think of to remove a single instance in a multiset in c++

edited Jul 27 '16 at 05:50

answered Jul 27 '16 at 05:30

varun kunchakuri

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Compared to the solution i proposed in the question your code does two searches (find + equal_range) instead of one which is inefficient – Martin Aug 07 '16 at 21:50
as this is same complexity, I very like this answer. Thank you – Crystal Nov 12 '19 at 17:51

score 3 · Answer 4 · edited Jan 12 '20 at 10:03

3

This worked for me:

multi_set.erase(multi_set.find(val));

if val exists in the multi-set.

edited Jan 12 '20 at 10:03

סטנלי גרונן

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answered Jan 12 '20 at 09:39

Yukty

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If `val` does not exist in `multi_set`, `multi_set.find(val)` will return `multi_set.end()`. Passing and end iterator to `erase()` is undefined behavior. I wouldn't do this, even though it may work fine for some implementations. – Fauzan Aug 15 '23 at 14:52

score 2 · Answer 5 · edited May 23 '17 at 11:55

I would try the following.

First call equal_range() to find the range of elements that equal to the key.

If the returned range is non-empty, then erase() a range of elements (i.e. the erase() which takes two iterators) where:

the first argument is the iterator to the 2nd element in the returned range (i.e. one past .first returned) and
the second argument as the returned range pair iterator's .second one.

Edit after reading templatetypedef's (Thanks!) comment:

If one (as opposed to all) duplicate is supposed to be removed: If the pair returned by equal_range() has at least two elements, then erase() the first element by passing the the .first of the returned pair to single iterator version of the erase():

Pseudo-code:

pair<iterator, iterator> pit = mymultiset.equal_range( key );

if( distance( pit.first, pit.second ) >= 2 ) {
    mymultiset.erase( pit.first );
}

I think the question is asking about eliminating just one duplicate, not all duplicates. — templatetypedef, Feb 06 '12 at 21:53
Do have an idea whether this is faster than my solution and if yes why ? — Martin, Feb 07 '12 at 18:26

score 1 · Accepted Answer · answered Feb 20 '23 at 16:04

1

Since C++17 (see here):

mySet.extract(val);

answered Feb 20 '23 at 16:04

TMueller83

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Dipen Dadhaniya · Answer 7 · 2019-10-15T07:07:31.253

0

We can do something like this:

multiset<int>::iterator it, it1;
it = myset.find(value);
it1 = it;
it1++;
myset.erase (it, it1);

edited Oct 15 '19 at 07:07

answered Dec 24 '15 at 17:38

Dipen Dadhaniya

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Overkill. "Iterator pointing to a single element to be removed from the unordered_multiset." – Andrew May 19 '18 at 20:05

score 0 · Answer 8 · answered Aug 22 '22 at 19:41

Here is a more elegant solution using "if statement with initializer" introduced in C++17:

if(auto it = mySet.find(value); it != mySet.end())
    mySet.erase(value);

The advantage of this syntax is that the scope of the iterator it is reduced to this if statement.

score -1 · Answer 9 · answered Dec 21 '20 at 09:48

-1

 auto itr=ms.find(value);  
  while(*itr==value){
  ms.erase(value);
  itr=ms.find(value);  
  }

Try this one It will remove all the duplicates available in the multiset.

answered Dec 21 '20 at 09:48

Annkit Singh

1

score -2 · Answer 10 · edited Nov 03 '20 at 05:41

-2

In fact, the correct answer is:

my_multiset.erase(my_multiset.find(value));

edited Nov 03 '20 at 05:41

Mike Smith

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answered Sep 05 '14 at 05:18

user3075328

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If _value_ doesn't exist in the multiset, it causes _undefined behaviour_. – kien_coi_1997 Feb 23 '15 at 14:27

In std::multiset is there a function or algorithm to erase just one sample (unicate or duplicate) if an element is found

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