如何在vector c ++中将double转换为int？

Question

I know the cast from the first array.

std::vector<int> v_int;
std::vector<float> v_float(v_int.begin(), v_int.end());

but how do I convert second array?

vector<int> aCol(4);
vector<vector<int>> a(2, aCol);
vector<vector<double>> b(a.begin(), a.end());// ???

What should I do in this case?

Answer 1

For the compiler vector<int> and vector<double> are completely unrelated types, not implicitly convertible one to another. What you can do is something like:

for(auto&& elem: a)
    b.push_back(std::vector<double>(elem.begin(), elem.end()));

Answer 2

vector<vector<double>> b;
b.reserve(a.size());
for (const auto& elem : a) {
  b.emplace_back(elem.begin(), elem.end());
}

Answer 3

The reason your vector-of-vectors construction is not compiling is because

vector<vector<double>> b(a.begin(), a.end())

is range constructing b with iterators whose element type is vector<int> . This will attempt to construct b 's internal std::vector<double> s with std::vector<int> s, which is no surprise that it shouldn't compile.

Comparing this to your first example,

std::vector<float> v_float(v_int.begin(), v_int.end());

is range constructing v_float from iterators whose element type is int . This works because a conversion from an int to a float is allowed, but not from std::vector<int> to std::vector<double> (or std::vector<float> ).

You may want to instead look at looping through each element of a and pushing back a vector constructed from a 's begin() and end()