传递此双指针并获取值有什么问题？

Question

I'm new/noob programmer of C++, and I've this problem. I want to pass a pointer of double to a function (which will process some data on it) and read (after the process) a fixed value of that "array". I've do this :

void ReadDoubles(double* samples, int size)
{
    for (int i=0; i < size; ++i)
    {
        *samples = i*10.1;
        samples++;
    }    
}

int main()
{
    int size = 10;
    double *values=0;
    ReadDoubles(values, size);
    cout << *(values+3);
}

BUt of course it seems I can't init the pointer that way. I think I need to init the pointer allocating 10 values? Tried:

double *values[size];

but that's not the solution. How would you do this simple task?

Answer 1

You need to allocate the array at first. Here you are

#include <iostream>

void ReadDoubles( double* samples, size_t size )
{
    for ( size_t i = 0; i < size; ++i )
    {
        *samples = i*10.1;
        samples++;
    }    
}

int main()
{
    size_t size = 10;
    double *values = new double[size];
    //               ^^^^^^^^^^^^^^^^  

    ReadDoubles( values, size );

    std::cout << *(values+3) << std::endl;

    delete []values;
}

The program output is

30.3

If you don't want to use the operator new then there are two general approaches. Either you can declare an array as for example

int main()
{
    const size_t size = 10;
    //^^^^
    double values[size];
    //     ^^^^^^^^^^^^  

    ReadDoubles( values, size );

    std::cout << *(values+3) << std::endl;
}

or you can use standard class std::vector<double> .In this case the function should be rewritten appropriately.

For example

#include <iostream>
#include <vector>

void ReadDoubles( std::vector<double> &samples, size_t size )
{
    samples.resize( size );

    for ( size_t i = 0; i < size; ++i )
    {
        samples[i] = i*10.1;
    }    
}

int main()
{
    size_t size = 10;
    std::vector<double> values;

    ReadDoubles( values, size );

    std::cout << values[3] << std::endl;
}

Answer 2

If you are not allowed to change the RealDoubles function and you must have a function return the size then the following should work:

#include <string>
#include <iostream>
#include <math.h>
#include <cmath>

using namespace std;

void ReadDoubles(double* samples,int size)
{
    for (int i=0; i < size; ++i) {
        *samples = i*10.1; 
        samples++;
    }
}

int get_size()
{
    return 10;
}

int main()
{
    int size = get_size(); // get size from function
    //double *values=0;
    double *values = new double[size] {0}; // Use new to allocate array. Optional: use {0} to init first element to 0, others default initialized to 0
    ReadDoubles(values,size);
    cout << *(values+3);
    delete[] values;
}

If you prefer to avoid new and delete then you can let a std::vector manage the container for you:

#include <string>
#include <iostream>
#include <math.h>
#include <cmath>
#include <vector>

using namespace std;

void ReadDoubles(double* samples,int size)
{
    for (int i=0; i < size; ++i) {
        *samples = i*10.1; 
        samples++;
    }
}

int get_size()
{
    return 10;
}

int main()
{
    int size = get_size(); // get size from function
    //double *values=0;
    std::vector<double> value_container(size,0); // vector will do the new and delete for us
    double *values = value_container.data();
    ReadDoubles(values,size);
    cout << *(values+3);
} // vector destructor will do delete when it goes out of scope