On Feb 23, 8:40 am, Marcin Gil <marcin....@NOSPAMgmail.com> wrote:
Chris Roth wrote:
vector<double> v1;
vector<double> v2;
What is the best way to add v1 to v2? (v2 = v1+v2 that is)? Simply
iterate through, or can an algorithm like transform be used? I have
used transform to add a single value to a vector, but not two
vectors together.
Single value: vector::push_back should be ok :)
To join two vectors you could use
vector::insert(), like here
#include <iostream>
#include <vector>
#include <iterator>
using namespace std;
int main()
{
vector<int> v1;
vector<int> v2;
v1.push_back(5);
v2.push_back(6);
v2.push_back(7);
v1.insert(v1.end(), v2.begin(), v2.end());
copy(v1.begin(), v1.end(), ostream_iterator<int>(cout, " "));
return 0;
}- Hide quoted text -
- Show quoted text -
I wonder how transform works, since it seems to be more
efficient(faster) than vector insert?
#include <iostream>
#include <vector>
#include <iterator>
using namespace std;
int main(int argc, char *argv[])
{
if (argc < 2) {
cout << "invalid number of args: " << argc << endl;
exit(1);
}
vector<int> v1;
vector<int> v2;
v1.push_back(5);
v2.push_back(6);
v2.push_back(7);
for (int i = 0; i < atoi(argv[1]); i++) {
// Alternative 1
//v1.insert(v1.end(), v2.begin(), v2.end());
// Alternative 2
v2.resize(v1.size());
std::transform(v1.begin(), v1.end(), v2.begin(), v2.begin(),
plus<int>());
}
return 0;
}
Alternative 1
time a.out 100000
real 0m0.52s
user 0m0.48s
sys 0m0.01s
Alternative 2
time a.out 100000
real 0m0.35s
user 0m0.30s
sys 0m0.02s
(or some such number). In 'alternative 2', 'resize' is executed
reallocation) and transform only changes 1 value (yes, 100000 times).