Create a subvector containing specific elements of a vector
1 answer
You can express this as a transformation, for example:
#include <valarray>
#include <iostream>
#include <iterator>
#include <algorithm>
#include <vector>
template <typename T>
void pick(std::vector<T>& result, const std::vector<T>& in, const std::vector<typename std::vector<T>::size_type>& s) {
result.reserve(s.size());
std::transform(s.begin(), s.end(), std::back_inserter(result),
[&in](typename std::vector<T>::size_type idx) {
return in.at(idx);
});
}
int main() {
const std::vector<int> arr={0,1,2,3,4,5,6,7,8,9,10};
std::vector<int> result;
pick(result, arr, {3,4,7,9});
}
I used lambda, but you can also use std::bind
or (now deprecated) std::bind2nd
for this.
The C ++ 11 example std::bind
does pick
:
template <typename T>
void pick(std::vector<T>& result, const std::vector<T>& in, const std::vector<typename std::vector<T>::size_type>& s) {
result.reserve(s.size());
std::transform(s.begin(), s.end(), std::back_inserter(result),
std::bind(static_cast<const T& (std::vector<T>::*)(typename std::vector<T>::size_type) const>(&std::vector<T>::at), in, std::placeholders::_1));
}
This is seriously ugly, though due to the need to discard the member function pointer to resolve overload at
(const vs non-const versions).
+3
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