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.
Cheers - Dave
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