lower_bound() misbehaving

[Steve555]

In the case where lower_bound() doesn't find the value, my book
(O'Reilly STL) states "...the iterator points to the position of the
first item greater than the value.

In my code I check that the value doesn't exist, but the iterator
points to it anyway!:

MovieRatingVector::iterator iter;
for(iter = mMovieRatings.begin(); iter != mMovieRatings.end(); ++iter)
{
thisMovieID = (*iter).movieID;
thisRating = (*iter).rating;
cout << thisMovieID<< " "<< thisRating<<endl;

}
Produces:
457 3
3151 1
11607 3
12155 3
12911 4

This is correct.

Then, suspecting a problem, immediately I test for movieID = 16469
(which I know ISN'T there)

MovieRating pair;
pair.movieID = 16469;
pair.rating = 0;
iter = std::lower_bound(mMovieRatings.begin(), mMovieRatings.end(),
pair);
cout<< (*iter).movieID << " "<<(*iter).rating;
Produces:
16469 3

What's happened? Has it just happened to find the value 16469 in an
adjacent block of memory?
And even if it has, why return it? How do I guard against this?
I don't know if it's just lucky coincidence, but the following worked
on the first first few thousand such checks without error: (I expected
that, per O'Reilly, it would be pointing to a value greater than my
value, NOT equal)

if((*iter).movieID == inMovieID)
return outRating;
else
return 0;

 

[Steve555]

So, in the list above, what is the first value that's greater than 16469?

It's a little misleading to say that the iterator points to the
position of the first item greater than the value, because that item
might not exist.




Your code dereferenced the end() iterator.


If you're going to look at the element, first check whether the
iterator is equal to end(). If you're going to insert a new element at
that position, just do it; no check needed.

--
  Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)

Thanks for the clear explanation Pete, I understand why it's pointing
to end() now.

Steve