I want to compute the union between two vectors:
std::vector<pair<string,string>> p1;
std::vector<pair<string,string>> p2;
The problem is that why is the union being done on the second element instead of first? the size of the union should be equal to 4 . Is it possible to modify set_union so that the Union is done on the first element?
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
int main()
{
std::vector<pair<string,string>> p1;
std::vector<pair<string,string>> p2;
p1.push_back(make_pair("A","1"));
p1.push_back(make_pair("B","2"));
p1.push_back(make_pair("C","3"));;
p2.push_back(make_pair("A","4"));
p2.push_back(make_pair("B","5"));
p2.push_back(make_pair("C","6"));
p2.push_back(make_pair("D","7"));
//sort vectors according to first element
sort(p1.begin(), p1.end(), [](const pair<string,string>& lhs, const pair<string,string>& rhs) {return lhs.first < rhs.first; });
sort(p2.begin(), p2.end(), [](const pair<string,string>& lhs, const pair<string,string>& rhs) {return lhs.first < rhs.first; });
//initialize vectors
std::vector<pair<string,string>> v(p1.size() + p2.size());
std::vector<pair<string,string>>::iterator it;
//compute union
it=std::set_union(p1.begin(), p1.end(),p2.begin(), p2.end(), v.begin());
v.resize(it-v.begin());
//print size
//size = 4
cout << v.size() << endl;
return 0;
}
The union is done on the pair
s, with the default operator<
. ("A", "1")
is different from ("A", "4")
.
Give your comparator to std::set_union
to achieve the expected result:
it=std::set_union(p1.begin(), p1.end(),p2.begin(), p2.end(), v.begin(),
[](const pair<string,string>& lhs, const pair<string,string>& rhs) { return lhs.first < rhs.first; });
Notice 1:
You should also name your lamda instead of typing it several times.
And I would use v.reserve
and back_inserter(v)
instead allocating the vector and using v.begin()
to collect output:
(...)
//save comparator
auto comp = [](const pair<string,string>& lhs, const pair<string,string>& rhs) {return lhs.first < rhs.first; };
//sort vectors according to first element
sort(p1.begin(), p1.end(), comp);
sort(p2.begin(), p2.end(), comp);
//initialize vectors
std::vector<pair<string,string>> v;
v.reserve(p1.size() + p2.size());
//compute union
std::set_union(p1.begin(), p1.end(), p2.begin(), p2.end(), back_inserter(v), comp);
//print size
//size == 4
cout << v.size() << endl;
}
Notice 2: what you do is similar to std::map
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