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David Luff schrieb:
> Hi folks,
>
> I've run into a tricky problem when using stl map, and am hoping someone
> might be able to point me on the right direction.
>
> I have a map of airports, indexed by string, which is the ICAO code:
>
> map<string, ARP*> apt_map;
>
> Now, I want to emulate the 'search ahead' function of GPS code entry, so
> that, for instance, entering "KC" will cause "KCAD" to be displayed - the
> first airport in the database starting with "KC". To do this I use the
> lower_bound function, for both "KC" and "KD". If the returned iterators
> don't match, then there is a valid match for "KC".
>
> map<string, ARP*>::iterator it1, it2;
> it1 = apt_map.lower_bound("KC");
> it2 = apt_map.lower_bound("KD");
>
> return(it1 == it2 ? NULL : it1->second);
>
> So far, so good. Now, the problem is that the KLN89 (and probably most/all
> GPS units) regards A->Z as coming before 0->9, whereas the standard string
> compare function regards 0->9 as coming before A->Z. So in this instance I
> might get "KC52" displayed instead of "KCAD" (there isn't really a KC52, but
> there are many examples outside the US where this bites).
>
> Now, I can get round this by using a comparison of lower_bound tests for
> "KC", "KCA" and "KD", and it works. Unfortunately I then have to check for
> non-alpha chars down to the end of the returned string, and re-test any with
> 'A' substituted in place. It's effective, but really ugly! I had to think
> quite hard about code I only wrote a week ago to compose the last sentence,
> and that's always a bad sign. What I'm sure I ought to be able to do is to
> define a custom comparison operator that performs a string comparison where
> numbers are considered to come after letters, and for good measure to do a
> case-insensitive match on the letters. I want to be able to do something
> like:
>
> it1 = apt_map.lower_bound("KC", gps_less);
>
> with all the ugly bits tucked away in gps_less which I can get working and
> them forget about. Unfortunately, I can't find any good example in any of my
> books to do this, nor on the internet, and everything I try is giving me
> swathes of typical gruesome-to-mentally-parse stl error messages. If anyone
> has an example of how to do this, or any pointers to one, I'd be most
> grateful.
"The C++ Programming language 3rd ed." tells me:
Basically you've got 2 options:
1 - create a class with the "<" operator
class IACOcode
{
string the_code;
}
bool operator<( const IACOcode& a, const IACOcode& b )
{
return your ordering;
}
map<IACOcode, ARP*> apt_map;
2 - create a custom sort order
class IACOcode_compare
{
public:
bool operator()( const string& x, const string& y) const
{
return your ordering;
}
}
map<string, ARP*, IACOcode_compare> apt_map;
Then you'll automatically get the desired result with your described lookup.
CU,
Christian
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