maps turn into multimaps

Do maps convert to multimaps if the same key is inserted more than once?

On 2008-08-26 21:44, brad wrote:>
Of course not, if they did they would be multimaps and not maps.
>

Erik Wikström wrote:>
I have a map that behaves as if it were a multimap. Two keys are the
same. Compiles and runs as expected.

brad wrote:>>>
Wait... perhaps I misundersatnd maps. Is it unique k,v pairs or just
unique keys? If it's the former, then that would make sense. I was under
the impression it was the latter.

brad wrote:>
Wait... perhaps I misundersatnd maps. Is it unique k,v pairs or just
unique keys? If it's the former, then that would make sense. I was under
the impression it was the latter.

It is unique key, of course. You most likely have a bug in the
comparison function. Show some code if you want more help.
>
/Peter

It is unique key, of course. You most likely have a bug in the
comparison function. Show some code if you want more help.
>
/Peter

peter koch wrote:>>
It's a test for credit card pre-validation. I added 13 digit visa cards
to the test in addition to the more common 16 digit visa. The prefix for
all visas is "4" (which I use as the key). See below. As I said, it
works, I was just trying to better understand *why* it works :)
>
struct card_info
* *{
* *std::string card_name;
* *int card_length;
* *};
>
* typedef std::map<std::string, card_infocMap;
* cMap card_map;
>
* *// VISA info
* *card_info v;
* *v.card_name = "Visa";
* *v.card_length = 16;
* *card_map.insert(std::pair<std::string, card_info>("4", v));
>
* *// VISA13 info
* *card_info v13;
* *v13.card_name = "Visa";
* *v13.card_length = 13;
* *card_map.insert(std::pair<std::string, card_info>("4", v13));

You do not validate that the insert succeeds - this is most likey your
problem.
>
/Peter

peter koch wrote:
>>
It's a test for credit card pre-validation. I added 13 digit visa cards
to the test in addition to the more common 16 digit visa. The prefix for
all visas is "4" (which I use as the key). See below. As I said, it
works, I was just trying to better understand *why* it works :)
>
struct card_info
{
std::string card_name;
int card_length;
};
>
>
typedef std::map<std::string, card_infocMap;
cMap card_map;
>
// VISA info
card_info v;
v.card_name = "Visa";
v.card_length = 16;
card_map.insert(std::pair<std::string, card_info>("4", v));
>
// VISA13 info
card_info v13;
v13.card_name = "Visa";
v13.card_length = 13;
card_map.insert(std::pair<std::string, card_info>("4", v13));
>
This insert works at compile time and there are no erros at run time, I
can pre-validate either 16 or 13 digit visa cards based on the above info.

peter koch wrote:>
I might also note that this reference:
>
http://www.cppreference.com/cppmap/index.html
>
Indicates unique "key/value pairs". I was surprised to read that. It's
incorrect... if indeed we are talking about unique keys:
>
"C++ Maps are sorted associative containers that contain unique
key/value pairs."

and then I think your code above is the equivalence of:
>
card_map["4"] = card_info("Visa",16);
card_map["4"] = card_info("Visa",13);
>
This ends up with card_map having one entry with
>
key == std::string("4") and value == card_info("Visa",13)
>
>
Is this what you want? If so, then why bother with the first insertion?
>
Did I misunderstand?

I might also note that this reference:
>
http://www.cppreference.com/cppmap/index.html
>
Indicates unique "key/value pairs". I was surprised to read that. It's
incorrect... if indeed we are talking about unique keys:

On Aug 27, 8:33 am, brad <byte8b...@gmail.comwrote:
>>>>
cppreference.com is well known for containing
completely wrong statements like this. I'd
avoid the site entirely if I were you.

On Aug 27, 4:43Â*am, Old Wolf <oldw...@inspire.net.nzwrote:>
perhaps the adjective unique refers to key, not to key/value pairs.
it's the problem of ambiguous grammars.