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Albert Jin
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 Posted: Thu Jun 08, 2006 3:29 am Post subject: How to get the iterator for a value less than the key specif |
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Neither of the two methods, lower_bound and upper_bound, for std::set
does return the iterator less than the key specified. Is there a
better way to approach this?
template <typename Set>
typename Set::const_reverse_iterator less_lower_bound(Set const & set,
typename Set::const_reference key) {
Set::key_compare key_comp = set.key_comp();
Set::const_reverse_iterator i = set.rbegin();
for (; i != set.rend(); ++i) {
if (!key_comp(key, *i)) {
break;
}
}
return i;
}
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Eric Johnson
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| Posted: Fri Jun 09, 2006 9:10 am Post subject: Re: How to get the iterator for a value less than the key sp |
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Albert Jin wrote:
| Quote: | Neither of the two methods, lower_bound and upper_bound, for std::set
does return the iterator less than the key specified. Is there a
better way to approach this?
|
Hi Albert,
The problem I see with your approach is that it performs a linear
search, so it will tend to be slow on large data sets. What I have
done is to just use lower_bound and then decrement the iterator. You
do have to check against begin() before decrementing the iterator, but
it should be faster than your approach.
Something like this:
typedef std::set<SomeClass> ClassSet;
.. . .
ClassSet MySet;
.. . .
SomeClass Obj;
.. . .
ClassPtrSet::iterator iter = MySet.lower_bound(Obj);
if ( iter != MySet.begin() )
{
--iter;
}
// at this point the iterator should be "pointing" to the value less
than the key specified.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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