C++ set union help

[Sandhya212]

Hi,

I need to find the 'length' of union between 2 vectors.
The code I have is

vector<int> v1,v2;
	vector<int> unionsetv;
	vector<int>::iterator it;
        v1.push_back(1);v1.push_back(2);v1.push_back(3);
	v2.push_back(3);v2.push_back(2);v2.push_back(4);
        it = set_union(v1.begin(),v1.end(),v2.begin(),v2.end(),back_inserter(unionsetv));
        cout << unionsetv.size()<< " ";
        cout << int(it - unionsetv.begin())<< " " ;

I could not get this to work and I have used the set_union in the same way as given in the cplusplus.com reference. If I remove the iterator 'it' then I get the entire size of the unionsetv vector (here this is 6) but this will have zero values in it (obtained during union). What I need is a union where the length of union is 4 and not 6. In any case, I did not get my code to run. Please help me identify what I have overlooked.

[Insensus]

Quote from cplusplus.com
For the function to yield the expected result, the elements in the ranges shall be already ordered according to the same strict weak ordering criterion (operator< or comp).

[Sandhya212]

Oh yes. Infact the v1 and v2 vectors are sorted. It is just that in the code I gave here I did not put the exact way v1 and v2 are populated. But they are sorted before I call the set_union function. And I still do not get the length of the union. Please advise.

[arkoenig]

If you want help with your code, you have to post the code that is having problems. You can't expect people to figure out what's wrong by looking at different code.

[Sandhya212]

Yes. Here is the simplied version of the code I use.

vector<int> v1,v2;
vector<int> unionsetv;
vector<int>::iterator it;
v1.push_back(1);v1.push_back(2);v1.push_back(3);
v2.push_back(2);v2.push_back(3);v2.push_back(4);
it = set_union(v1.begin(),v1.end(),v2.begin(),v2.end(),back_inserter(unionsetv));
cout << unionsetv.size()<< " ";
cout << int(it - unionsetv.begin())<< " " ;

If I do not use 'it', then unionsetv.size() gives 6. I hope I am clearer now. Please help.

[Insensus]

Ok I've think I figured out why your code doesn't work, but I don't think there's a way to do it exactly like you want to.

The problem is this:
The return type of union_set is the same type as given for the 5th argument, the OutputIterator.
In your case this means union_set returns a back_inserter_iterator<vector<int>> yet you want to assign this to a vector<int>::iterator.

The easiest way to fix this is by indeed removing the it = and simply using unionsetv.end() whenever you would've wanted to use it.

[arkoenig]

When I try to run your code, it fails because "it" has the wrong time. When I remove the erroneous assignment to "it" in line 6, and the statement that tries to use "it" in line 8, the code prints 4 (correctly), not 6.

So if your code is printing 6, that means that what you're running is different from what you posted. I am not going to waste any more time by trying to read your mind.

[Sandhya212]

Thank you Insensus and Arkoenig for your time. It is exactly the same code I used and no different but I am not sure why I keep getting 6 instead of 4. Meanwhile, I have done as Insensus has mentioned and now it works. :)
Thank you again.