Optional logic when passing structures in c++

Question

I am new too programming and am trying to pass an array of structures and an "option" to a function. I then want the function to be able to manipulate the data within the the array of the struct.

struct coordinates{
    int x;
    int y;
    int z;
}COORD;

The option parameter is an integer that will specify which part of the structure to manipulate.

1. Manipulate x --- option = 0
2. Manipulate y --- option = 1
3. Manipulate z --- option = 2

An example of this is shown below. The function takes the array of structs and makes a 3 point moving average of the data-point specified data point. The issue I am having is that the program I am trying to build has 50+ members in each struct so writing out each if statement by hand is really tedious. What I am asking is if there is a more elegant solution???

#include <iostream>

struct coordinates{
    int x;
    int y;
    int z;
}COORD;

int main() {
    COORD points[10];

    // Initialising points
    for(int i = 0, i < 10, i++){ 
        points[i].x = 1;
        points[i].y = 2;
        points[i].z = 3;
    }
                            //(struct,startpoint,option)
    std::cout << movingaverage(&points,3,1); // Output
}

int movingaverage(COORD *data, int start_point, int option){

    if(option == 0) {
        // Assigns the "start" value of the sum value.
        sum = data[start_point]->x;
        sum = sum + data[start_point - 1]->x;
        sum = sum + data[start_point + 1]->x;
    }

    else if(option == 1) {
        // Assigns the "start" value of the sum value.
        sum = data[start_point]->y;
        sum = sum + data[start_point - 1]->y;
        sum = sum + data[start_point + 1]->y;
    }
    else if(option == 2) {
        // Assigns the "start" value of the sum value.
        sum = data[start_point]->z;
        sum = sum + data[start_point - 1]->z;
        sum = sum + data[start_point + 1]->z;
    }
    sum = sum / n;
    return sum; //Sum is the moving average
}

Answer 1

C++ does not support reflection and therefore you cannot iterate over members of a struct. The way I would do this is with a getter function.

template<class Getter>
int movingaverage(COORD *data, int start_point, const Getter &get){
    auto sum = get(data[start_point]);
    sum = sum + get(data[start_point - 1]);
    sum = sum + get(data[start_point + 1]);
    return sum;
}

std::cout << "x: " << movingaverage(&points, 3, [](const COORD &coord){return coord.x;}) << '\n';
std::cout << "y: " << movingaverage(&points, 3, [](const COORD &coord){return coord.y;}) << '\n';
std::cout << "z: " << movingaverage(&points, 3, [](const COORD &coord){return coord.z;}) << '\n';

If you or your compiler are confused about the lambda part you can just write a regular function and pass that instead.

Answer 2

Also pointer to members are a possible solution; in combination with a template function, the members can be of different type. But, yes, the syntax is something strange:

#include <iostream>
#include <vector>
using namespace std;

/// calculate the sum of a given structure member in a vector:
template <typename T, typename M>
M membersum(vector<T> array, M (T::*mptr)) {
    M sum = 0;
    for (int i=0; i<array.size(); i++) {
        sum += (array[i].*mptr);
    }
    return sum;
}

struct Point {
    Point(int x, int y, float z): x(x), y(y), z(z) {}
    int x;
    int y;
    float z;
};

int main() {
    vector<Point> points;
    points.push_back(Point(1,2,3.8));
    points.push_back(Point(1,2,4.5));
    points.push_back(Point(1,2,1.7));
    // your code goes here
    cout << "sum x: " << membersum(points, &Point::x) << endl;
    cout << "sum y: " << membersum(points, &Point::y) << endl;
    cout << "sum z: " << membersum(points, &Point::z) << endl;
    return 0;
}

This example I created on ideone.com.

Answer 3

You can try something along the lines of

class coordinates{
    private:
    int xyz[3]; //implementation detail(!)

public:
    int x() const { return xyz[0]; }
    int y() const { return xyz[1]; }
    int z() const { return xyz[2]; }

    //some relevant setters go here

    static int movingaverage(COORD *data, int start_point, int option){

        sum = data[start_point]->xyz[option];
        sum = sum + data[start_point - 1]->xyz[option];
        sum = sum + data[start_point + 1]->xyz[option];
    }
};

Answer 4

I would suggest to use a simple functor.

Class MovingAvg
{
MovingAvg(std::vector<Coordinate> *pSet);
void SetPointSet(std::vector<Coordinate> *p);

double operator()(int startPoint, int option)
{
   //calculation here. 
   double sum = 0;
   for(int i = -1; i <= 1; ++i)
   {
       switch(option)
         case 0:
           sum += pointSet[startPoint + i].x; //check for bounday conditions.
    .................

   }
   return sum / 3;
}

private:
//container
std::vector<Coordinate> *pointSet;
}

Use it as.

MovAvg mvG(pointSet);
double sum = mvG(startP, option); 

//change the private member if the point set must be changed.

Functors are pretty handy in cases like this.