How to return vector c++?

Question

I need an idea which allows me to calculate/store and return the value of mathematical expressions for a given interval. In this case for example: x^2 - 7 from -5 to 5 :

My .cpp file:

#include <vector>
#include "header.hpp"

double Example::function(double min, double max, double x)
{
    std::vector<double> vector1;

    for(x=min; x<=max; x++)
    {
        result = x * x - 7;
        vector1.push_back(result);
    }
    // Here i need to return the full vector1 but how?
    // if i use a for-loop, the return will be out of scope:
    // for(int i = 0; i <= size of vector; i++)
    // {
    //     return vector1[i];
    // }
}

my .hpp file:

class Example
{
  private:
    double x, min, max;
  public:
    double function(double min, double max, double x);
};

After this i would like to store the result for the given interval in a .txt file to plot it with the external software.

#include <iostream>
#include <fstream>
#include "header.hpp"

int main()
{
    std::ofstream file1("testplot1-output.txt");
    Example test;
    for(x = 0; x <= size of vector1; x++ ) // i don't get how i can access the vector1 from the .cpp file.
    {
        file1 << x << "\t" << test.function(-5, 5) << std::endl;
    }
    file1.close();
    return 0.;
}

Answer 1

If you wish to return the vector, simply do so:

std::vector<double> Example::function(double min, double max)
{
   std::vector<double> vector1;

   for (double x = min; x <= max; x++) {
      const auto result = x * x - 7;
      vector1.push_back(result);
   }

   return vector1;
}

I have taken out that third argument — which you never provided — and replaced it with a local variable, as I believe that was your intention. The member variables also seem to be pointless. So:

class Example
{
  public:
    std::vector<double> function(double min, double max);
};

(Also, consider making min and max integers rather than floating-point values.)

Anyway, then, in the calling scope:

Example test;
const auto data = test.function(-5, 5);
for (const auto elm : data) {
    file1 << x << "\t" << elm << std::endl;
}

Functions don't have to return double .

Answer 2

You need to change the return type of your function to std::vector and then return vector1

std::vector<double> function(...){
    ...
    return vector1;
}

after that you can iterate over the returned vector in your main.cpp