I have a vector
vector<int>v = {1,2,3,4,5};
I'd like to repeat the elements in the vector for, say, 3 times, such that the vector becoms
v = {1,2,3,4,5, 1,2,3,4,5, 1,2,3,4,5};
EDIT: In fact, if I need to repeat the elements for many times, say 1000, obviously I have to come with something quick and light?
How do I do it?
This can be tricky. If you want to avoid creating a temporary working object you have to be careful to avoid invalidating iterators as you go. This should do it:
std::vector<int> v = {1, 2, 3, 4, 5};
// to avoid invalidating iterators, preallocate the memory
v.reserve(v.size() * 3);
// remember the end of the range to be duplicated
// (this is the iterator we don't want to invalidate)
auto end = std::end(v);
// insert two duplicates
v.insert(std::end(v), std::begin(v), end);
v.insert(std::end(v), std::begin(v), end);
for(auto i: v)
std::cout << i << '\n';
More generally you could modify this to add multiple duplicates like this:
std::vector<int> v = {1, 2, 3, 4, 5};
std::size_t const no_of_duplicates = 1000;
// to avoid invalidating iterators, preallocate the memory
v.reserve(v.size() * no_of_duplicates);
// remember the end of the range to be duplicated
// (this is the iterator we don't want to invalidate)
auto end = std::end(v);
// insert duplicates (start from one because already have the first)
for(std::size_t i = 1; i < no_of_duplicates; ++i)
v.insert(std::end(v), std::begin(v), end);
Use the insert method of vector class
v.insert(v.end(), v.begin(), v.end());
Use std::copy
std::vector<int> v = { 1 , 2, 3, 4, 5};
std::vector<int> r;
for (auto i = 0; i < 3; ++i) {
std::copy(v.begin(), v.end(), std::back_inserter(r));
}
v.swap(r);
