«

The situation is that a std::list<std::set<std::string> > is being
iterated through. Upon certain criteria some sets become empty. I need
to remove the empty sets from the list.

Is it safe to iterate through a list and call list::erase( iterator )
in mid iteration?

Christopher wrote:
> The situation is that a std::list<std::set<std::string> > is being
> iterated through. Upon certain criteria some sets become empty. I need
> to remove the empty sets from the list.
>
> Is it safe to iterate through a list and call list::erase( iterator )
> in mid iteration?

Well, you can use

struct set_is_empty
{
bool operator()(const std::set& s) const { return s.empty(); }
};

std::erase(std::remove_if(l.begin(), l.end(), set_is_empty());

or else, the canonical iteration for this list is:

for (it = l.begin(); it != l.end(); )
if (it->empty())
it = l.erase(it);
else
++it;

red floyd wrote:
> Christopher wrote:
>> The situation is that a std::list<std::set<std::string> > is being
>> iterated through. Upon certain criteria some sets become empty. I need
>> to remove the empty sets from the list.
>>
>> Is it safe to iterate through a list and call list::erase( iterator )
>> in mid iteration?
>
> Well, you can use
>
> struct set_is_empty
> {
> bool operator()(const std::set& s) const { return s.empty(); }
> };
>
> std::erase(std::remove_if(l.begin(), l.end(), set_is_empty());
crap. That should be:

l.erase(std::remove_if(l.begin(), l.end(), set_is_empty()), l.end());
>
> or else, the canonical iteration for this list is:
>
> for (it = l.begin(); it != l.end(); )
> if (it->empty())
> it = l.erase(it);
> else
> ++it;

On Dec 20, 2:10 am, red floyd <no.s...@here.dude> wrote:
> Christopher wrote:
> > The situation is that a std::list<std::set<std::string> > is being
> > iterated through. Upon certain criteria some sets become empty. I need
> > to remove the empty sets from the list.

> > Is it safe to iterate through a list and call list::erase( iterator )
> > in mid iteration?

> Well, you can use

> struct set_is_empty
> {
> bool operator()(const std::set& s) const { return s.empty(); }
> };

> std::erase(std::remove_if(l.begin(), l.end(), set_is_empty());

Which could be unnecessarily expensive. In the case of
std::list, the canonical form is:

l.remove_if( set_is_empty() ) ;

However, the original poster said that sets "become" empty
during his iteration, so this can't be used.

> or else, the canonical iteration for this list is:

> for (it = l.begin(); it != l.end(); )
> if (it->empty())
> it = l.erase(it);
> else
> ++it;

Adopted to his case, you'd add braces and put the if at the end
of the loop. (Also, I'd write this with a while, rather than a
for. Something like:

std::list<...>::iterator iter = l.begin() ;
while ( iter != l.end() ) {
// processing...
if ( iter->empty() ) {
iter = l.erase( iter ) ;
} else {
++ iter ;
}
}

I'd prefer even more if that if could be replaced with a ?: on
the right side of an assignment, since the most important aspect
here is the update of the iterator, and not how it's being
updated, but I can't think of a nice way of doing this off hand.

--
James Kanze (GABI Software) email:
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

On 12ÔÂ20ÈÕ, ÏÂÎç5ʱ40·Ö, James Kanze <james.ka...@gmail.com> wrote:
> On Dec 20, 2:10 am, red floyd <no.s...@here.dude> wrote:
>
> > Christopher wrote:
> > > The situation is that a std::list<std::set<std::string> > is being
> > > iterated through. Upon certain criteria some sets become empty. I need
> > > to remove the empty sets from the list.
> > > Is it safe to iterate through a list and call list::erase( iterator )
> > > in mid iteration?
> > Well, you can use
> > struct set_is_empty
> > {
> > bool operator()(const std::set& s) const { return s.empty(); }
> > };
> > std::erase(std::remove_if(l.begin(), l.end(), set_is_empty());
>
> Which could be unnecessarily expensive. In the case of
> std::list, the canonical form is:
>
> l.remove_if( set_is_empty() ) ;
That is what I found in MSDN,
remove_if is a STL algorithm which removes all elements from the range
(First,Last) that cause the predicate to return true. It returns an
iterator equal to Last - n, where n = number of elements removed. The
last n elements of the range have undefined values. The size of the
container remains the same.
But there is a method named remove_if in std::list.
template<class Predicate>
void remove_if(
Predicate _Pred
)
Erases elements from a list for which a specified predicate is
satisfied.
I didn't know there is a method named remove_if in the std::list. Can
any one told me why there is no similar method in vector ?

Thanks in advance,

On Dec 20, 12:40 pm, "cgsp...@gmail.com" <cgsp...@gmail.com> wrote:
> On 12月20日, 下�5时40分, James Kanze <james.ka...@gmail.com> wrote:

> > On Dec 20, 2:10 am, red floyd <no.s...@here.dude> wrote:

> > > Christopher wrote:
> > > > The situation is that a std::list<std::set<std::string> > is being
> > > > iterated through. Upon certain criteria some sets become empty. I need
> > > > to remove the empty sets from the list.
> > > > Is it safe to iterate through a list and call list::erase( iterator )
> > > > in mid iteration?
> > > Well, you can use
> > > struct set_is_empty
> > > {
> > > bool operator()(const std::set& s) const { return s.empty(); }
> > > };
> > > std::erase(std::remove_if(l.begin(), l.end(), set_is_empty());

> > Which could be unnecessarily expensive. In the case of
> > std::list, the canonical form is:

> > l.remove_if( set_is_empty() ) ;

> That is what I found in MSDN,
> remove_if is a STL algorithm which removes all elements from the range
> (First,Last) that cause the predicate to return true. It returns an
> iterator equal to Last - n, where n = number of elements removed. The
> last n elements of the range have undefined values. The size of the
> container remains the same.
> But there is a method named remove_if in std::list.
> template<class Predicate>
> void remove_if(
> Predicate _Pred
> )
> Erases elements from a list for which a specified predicate is
> satisfied.
> I didn't know there is a method named remove_if in the std::list. Can
> any one told me why there is no similar method in vector ?

Because you don't need it, and it's not directly supported by
the underlying data structure.

--
James Kanze (GABI Software) email:
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34