Re: Modify STL map Object
Juha Nieminen <nospam@thanks.invalid> wrote:
Daniel T. <daniel_t@earthlink.net> wrote:
Just keep in mind, every time you use op[] you are doing a
logarithmic search
So do std::map::find() and std::map::insert(). So what?
Calling find once is better than calling op[] multiple times.
And calling operator[] once is better than calling find() multiple times.
So what?
Maybe your point is that it's too easy to just call operator[] every
time you need to access an element, even when you are accessing the
same element many times in succession, whereas find() is telling more
explicitly that a "find this element in the tree" operation will be
performed, thus in a way "warning" the programmer that it's not an
extremely quick operation. operator[] resembles array indexing, which
might give the wrong impression that it's a constant-time operation.
No. My point is that find() and op[] do two different things and cannot
be interchanged like you are implying.
Ok, fair enough. Of course you should know what the operator[] is
doing, and if you want to make repeated operations to the same element,
you should use a reference (ie. "type& element = theMap[value];" and
then use 'element' for the multiple operations).
However, std::map::operator[] is still handier than find()+insert()
when what you want is automatic insertion if the element doesn't exist.
The OP wasn't calling find()+insert(), at least not in a way that could
be replaced by op[].
I don't understand. operator[] *does* add the element if it didn't
exist.
What if you don't want to add the element if it doesn't exist?
Your original point was about insert(), and how it doesn't when the
element already exists, to which I pointed out that operator[] is much
handier for that purpose.
My original point was that the OP already had an iterator pointing to
the element he wanted to change. Calling insert to try to modify that
element is useless and calling op[] to modify it is wasteful.
The only point of even bringing up op[] is if you were suggesting he
replace his original 'find' with it, which would change the operation of
the code such that "if found" would be pointless. If there were any
indication from the OP that he wanted to insert an element if not found,
then I would agree with you 100%, but there was no such indication. All
he wanted to do was change an element if it already existed. It seems
like you are trying to solve a problem the OP doesn't have.
If your reference to op[] was simply ment as a tangential point, then
fine, but it should not be used to solve the problem the OP was having.