intersection of two std::maps

[jacek.dziedzic]

Hi!

What is the canonical way of finding an intersection of two
std::maps?
i.e. I have

std::map<whatever,size_tmap1;
std::map<whatever,size_tmap2;

.... and I need an std::vector containing all the values that occur
simultaneously in both of the maps, nevermind the keys. In each
of the maps the values do not repeat.

One idea is to copy all values from map1 into a vector, and then
for each value from map2 check if it's present in the vector already,
if not -- then push it into the vector. This, of course, has ugly
complexity, so perhaps a set would do better. Is there a better
way?

I was also thinking of putting the values into a multiset and then
taking out all values with a count of 2, but there must be a simpler
way.

TIA,
- J.

[Mark P]

ja************@gmail.com wrote:

Hi!

What is the canonical way of finding an intersection of two
std::maps?

I'm not sure there is one. It's a pretty strange request since,
ignoring the keys, the map is not a very useful value container.
Pushing all of the values into a set is one possibility. I don't see
any advantage to your multiset proposal. I think what I would do is:

1. Copy all values from each map to a separate vector.
2. Sort each vector.
3. Use std::set_intersection to obtain the intersection.

If copying values is too costly you can also use vectors of pointers to
the original values in the map, but then you'll need to write your own
comparison functions for steps 2 and 3, and do some additional copying
at the end.

-Mark

i.e. I have

std::map<whatever,size_tmap1;
std::map<whatever,size_tmap2;

... and I need an std::vector containing all the values that occur
simultaneously in both of the maps, nevermind the keys. In each
of the maps the values do not repeat.

One idea is to copy all values from map1 into a vector, and then
for each value from map2 check if it's present in the vector already,
if not -- then push it into the vector. This, of course, has ugly
complexity, so perhaps a set would do better. Is there a better
way?

I was also thinking of putting the values into a multiset and then
taking out all values with a count of 2, but there must be a simpler
way.

TIA,
- J.

[jacek.dziedzic]

On Nov 5, 8:30 pm, Mark P <[e-mail]>
wrote:

You could do that as well. It's fewer lines of code, perhaps, but
conventional wisdom is that unless you need to sort "online", it's
generally more efficient to collect all values and perform a single sort
operation at the end (as I suggest) rather than maintaining a sorted
structure as items are individually added (the set approach). See Scott
Meyers "Effective STL" and do keep in mind the usual caveats about
premature optimization, but I would expect better performance with the
vector.

I see, thank you.
- J.

[jacek.dziedzic]

On 6 Lis, 04:09, "Jim Langston" <[e-mail]:

>
Also, your values are not guaranteed to be unique in the map since they are
not keyed.

Note however that in the OP I wrote:

JDIn each of the maps the values do not repeat.

I know for sure that in each of the maps there are no duplicate
values.

Copying to std::vector or std::set probably depends on what you
want to do with duplicate values in the same map. Do you want to enforce
uniqueness on each the values in each copied set/vector? If so, use set.
If not, use vector.

Vector should be faster however for copying the data into as it does not
have to build the index.

I guess I'll stick with vector.

thanks,
- J.