c++ - 将复 vector 展平并将其恢复为双 vector 并返回

Question

我有一个包含复数值的 vector (定义为 std::vector<std::complex<double>> 或 arma::cx_vec )，并希望将它们转换为包含两倍大小的双值的 vector 。之后我想再次将它们转换回来。目前我使用两个循环(这里从双 vector 到复 vector 再返回):

//get x and dx as vectors containing real values, with a size of 2 * dim
arma::cx_colvec local_x(dim), local_dx(dim);
for(size_t i = 0; i < x.size(); i += 2) {
    local_x(i / 2) = std::complex<double>(x(i), x(i + 1));
}
//Do something with local_x and local_dx
for(size_t i = 0; i < 2 * dim; i += 2) {
    dx(i) = local_dx(i / 2).real();
    dx(i + 1) = local_dx(i / 2).imag();
}
//Send dx back

我可以想象这可能会相当慢。因此，是否还有其他可能性将这些 vector 从复数 vector reshape 为双倍 vector 并返回？理想情况下涉及迭代器(这样我可以使用诸如 transform() 之类的方法)而不是对某个大小进行循环。

这个问题的背景是:我有复杂的输入数据，必须将其放入函数中 A我无法修改它，但它再次调用用户提供的函数(称为 U )。该函数不支持复杂数据类型，仅支持实数类型。因此，我的目的是在将 vector 放入 A 之前将其展平。 ，将其展开 U ，对其进行计算，重新展平并再次发送回来。

Answer 1

std::complex<double>是 explicitly被称为可被视为 double[2] 的东西

Array-oriented access

For any object z of type complex<T>, reinterpret_cast<T(&)[2]>(z)[0] is the real part of z and reinterpret_cast<T(&)[2]>(z)[1] is the imaginary part of z.

For any pointer to an element of an array of complex<T> named p and any valid array index i, reinterpret_cast<T*>(p)[2*i] is the real part of the complex number p[i], and reinterpret_cast<T*>(p)[2*i + 1] is the imaginary part of the complex number p[i]

The intent of this requirement is to preserve binary compatibility between the C++ library complex number types and the C language complex number types (and arrays thereof), which have an identical object representation requirement.

所以你可以使用std::vector::data()获得complex<double> * ，并将其重新解释为 double *元素数量是原来的两倍。