vector<double>& old_P；cout<<old_P[1];输出为什么不是.0

This is a C++ data type that represents an array or sequence of double precision floating point numbers. It is defined in the standard library as part of the STL (Standard Template Library).

A vector<double> can be initialized and modified like any other vector. For example, to declare a vector<double> and add values to it, you could use the following code:

#include <vector>
#include <iostream>

int main() {
    std::vector<double> myVector;
    myVector.push_back(1.0);
    myVector.push_back(2.0);
    myVector.push_back(3.0);

    std::cout << "The vector contains " << myVector.size() << " elements: ";
    for (int i = 0; i < myVector.size(); ++i) {
        std::cout << myVector[i] << " ";
    }
    std::cout << std::endl;

    return 0;
}

This would output:

The vector contains 3 elements: 1 2 3

The vector<double> type provides a convenient way to store and manipulate sequences of floating point numbers, and is often used in scientific and mathematical applications.