Question

I think I’m lacking some basic understanding of assignment in C/C++ here! I have a function that computes the set union between two string vectors. The reason I do this is because the algorithm library’s function set_union requires that both vectors are sorted first and if I do it the following way then I can’t forget to sort:

vector<string> SetOperations::my_set_union(vector<string> set1,
                                            vector<string> set2) {
    sort(set1.begin(), set1.end());
    sort(set2.begin(), set2.end());

    vector<string> v;
    set_union(set1.begin(), set1.end(), set2.begin(), set2.end(), back_inserter(v));
    return v;
}

I then do the following:

vector<string> vec = set_ops.my_set_union(vec1, vec2);

where vec1 and vec2 are string vectors containing a single "a" and "a" each and set_ops is an instantiation of a class that I have these set operations in (like the one above). They both definitely have these elements – I have printed the two vectors out.

For some (simple?) reason, vec ends up having a single element of "a" instead of two elements ("a" and "a").

Any ideas what I’m doing wrong? Am I meant to a copy function or something?

Thank you :).

Answer 1

You have to use merge instead of set_union. set_union will eliminate similar enteries.

see merge and set_union refrences.