Re: problems using blitz::TinyVector as key for map

[Faheem Mitha <faheem at email dot unc dot edu>]

Hi,

Thanks for the quick reply.

On Fri, 7 Oct 2005, Peter Doerfler wrote:

> What does blitz::all() do? Looks like operator<() is defined for TinyVector.
> In that case you shouldn't need to make your own less since std::less just
> calls operator<().

The < operator operates elementwise, and returns a 'vector expression' of 
bools. The all operator takes the 'vector expression' of bools, and 
returns true if all the bools in the expression are true.

It turns out the problem is that the comparison operator as defined is not 
a 'strict weak ordering'. I looked at this after I sent out the message 
last night, and realized it could be the problem, but didn't see anything 
wrong with it at the time.

> > int main()
> > {
> >    map<blitz::TinyVector<int, 3>, int, blitz::TinyVectorless<int, 3> >
> > dict;
> >    dict[blitz::TinyVector<int, 3>(50, 50, 49)] = 0;
> >    dict[blitz::TinyVector<int, 3>(60, 60, 60)] = 1;
> >    blitz::TinyVector<int, 3> x(2, 2, 1000);
> >    blitz::TinyVector<int, 3> y(2, 2, 2);
> >    cout << (dict.find(x))->first << endl;
> >    cout << (dict.find(y))->first << endl;
> >    return 0;
> > }
>
> You are dereferencing the end of the map because neither x nor y are contained
> in dict. AFAIK dereferencing end() is undefined and could do anything. Dig
> into stl_tree.h to figure out why you might be getting the results you see --
> in case you care ;)

Right. My mistake.

> Anyway, you need to check whether your key was actually found, somewhere along
> the lines of
>
> typedef map<blitz::TinyVector<int, 3>, int, blitz::TinyVectorless<int, 3> >
> dict_type;
> typedef dict_type::iterator dict_iterator;
> dict_iterator it = dict.find(x);
> cout << (it!=dict.end() ? (*it).first : "not found") << endl;
>
> Not tested since I'm not using blitz.
>
> HTH,
> Peter
>
>
> PS: You might want to pick up a good book about using the STL like "Effective
> STL" if you'll be using it more.

I actually have the C++ Standard Library book by Josuttis. The problem 
is just that I am stupid. Do you like the "Effective STL" book?

                                                               Faheem.