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### #ActualBrother Bob

Posted 23 August 2012 - 04:44 PM

std::set_intersection can do that.

std::vector<int> results;
std::set_intersection(someValues.begin(), someValues.end(), someValuesToCheck.begin(), someValuesToCheck.end(), std::back_inserter(results));

someValuesToCheck = result;


That would be very useful. Unfortunately can't use it because I'm using different type of sets (someValues is a int, someValuesToCheck is a structure) and from what I've read in the documentation the back_inserter expects to compare two equal type of sets.
Thanks anyway, this will come useful in other occasions...

The function allows the two sets to be of different types, as long as each of the two types are less than comparable with itself, and that the type of the first set is less than comparable with the type of the second set (alternatively you provide a predicate to less than compare an element of the first set with an element of the second set). I should have realized that from your first post that the types were different, sorry, so I didn't mention more about that.

That is, if the first set is of type T1, and the second set is of type T2, then you need the following operators:
• bool operator<(T1, T1)
• bool operator<(T2, T2)
• bool operator<(T1, T2), or a predicate bool pred(T1, T2)
Operator 1 or 2 is the built in int operator (depending on which one is the integer of course), and the other one you have to define as a sorting criteria for your structure. Operator 3 can either be an operator or a predicate you pass a parameter to the set intersect function.

edit: And I should add now that since you probably shouldn't define an arbitrary operator < for your class for 1 or 2 above, and since that you cannot provide that operator as an explicit predicate, you probably shouldn't go this route. It requires an operator that may not be obvious and unambiguous, so you should not define it.

### #1Brother Bob

Posted 23 August 2012 - 04:38 PM

std::set_intersection can do that.

std::vector<int> results;
std::set_intersection(someValues.begin(), someValues.end(), someValuesToCheck.begin(), someValuesToCheck.end(), std::back_inserter(results));

someValuesToCheck = result;


That would be very useful. Unfortunately can't use it because I'm using different type of sets (someValues is a int, someValuesToCheck is a structure) and from what I've read in the documentation the back_inserter expects to compare two equal type of sets.
Thanks anyway, this will come useful in other occasions...

The function allows the two sets to be of different types, as long as each of the two types are less than comparable with itself, and that the type of the first set is less than comparable with the type of the second set (alternatively you provide a predicate to less than compare an element of the first set with an element of the second set). I should have realized that from your first post that the types were different, sorry, so I didn't mention more about that.

That is, if the first set is of type T1, and the second set is of type T2, then you need the following operators:
• bool operator<(T1, T1)
• bool operator<(T2, T2)
• bool operator<(T1, T2), or a predicate bool pred(T1, T2)
Operator 1 or 2 is the built in int operator (depending on which one is the integer of course), and the other one you have to define as a sorting criteria for your structure. Operator 3 can either be an operator or a predicate you pass a parameter to the set intersect function.

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