Wow. So many answers that are close, none with all the right pieces. You need both resize (or reserve) and copy_n, along with remembering the original size. auto old_count = xx.size(); xx.resize(2 * old_count); std::copy_n(xx.begin(), old_count, xx.begin() + old_count); or auto old_count = xx.size(); xx.reserve(2 * old_count); std::copy_n(xx.begin(), old_count, std::back_inserter(xx)); When using reserve, copy_n is … Read more
STL vectors perform bounds checking when the .at() member function is called, but do not perform any checks on the [] operator. When out of bounds, the [] operator produces undefined results.
The rate at which the capacity of a vector grows is implementation dependent. Implementations almost invariably choose exponential growth, in order to meet the amortized constant time requirement for the push_back operation. What amortized constant time means and how exponential growth achieves this is interesting. Every time a vector’s capacity is grown the elements need … Read more
EDIT: using c++14, the best solution is very easy to write thanks to lambdas that can now have parameters of type auto. This is my current favorite solution std::sort(v.begin(), v.end(), [](auto &left, auto &right) { return left.second < right.second; }); ORIGINAL ANSWER: Just use a custom comparator (it’s an optional 3rd argument to std::sort) struct … Read more
You cannot store arrays in a vector or any other container. The type of the elements to be stored in a container (called the container’s value type) must be both copy constructible and assignable. Arrays are neither. You can, however, use an array class template, like the one provided by Boost, TR1, and C++0x: std::vector<std::array<double, … Read more
From C++11 onwards, you should prefer: #include <algorithm> #include <random> auto rng = std::default_random_engine {}; std::shuffle(std::begin(cards_), std::end(cards_), rng); Live example on Coliru Make sure to reuse the same instance of rng throughout multiple calls to std::shuffle if you intend to generate different permutations every time! Moreover, if you want your program to create different sequences … Read more
vector1.insert( vector1.end(), vector2.begin(), vector2.end() );
If you have a C++11 compiler, I would suggest using a range-based for-loop (see below); or else use an iterator. But you have several options, all of which I will explain in what follows. range-based for-loop (C++11) In C++11 (and later) you can use the new range-based for-loop, which looks like this: std::vector<char> path; // … Read more
If you have a C++11 compiler, I would suggest using a range-based for-loop (see below); or else use an iterator. But you have several options, all of which I will explain in what follows. range-based for-loop (C++11) In C++11 (and later) you can use the new range-based for-loop, which looks like this: std::vector<char> path; // … Read more
So how is the 10000th element stored here? The element isn’t stored in the vector. It’s ‘stored’ in a piece of memory that is unrelated to the vector. Isn’t the expected behavior here a “Segmentation fault”? No. The behaviour is undefined, so there is no behaviour to expect. But the above runs successfully. That’s a … Read more