Understanding error messages in Rcpp code with structures and functions

Question

I'm getting a couple of error messages when I run the following code that I do not understand -

expected initializer before 'add_f'

no matching function for call to 'try_grid::try_grid(Rcpp::NumericMatrix&, Rcpp:: NumericMatrix&, int&)

Can someone explain what I am doing incorrectly?

cpp file:

#include <Rcpp.h>

using namespace Rcpp;

//Declaring a structure
struct try_grid
{
    NumericMatrix x;
    NumericMatrix y;
    int size;
}

// Declaring function
Void add_f(const try_grid&);

// [[Rcpp::export]]
NumericMatrix plussomething(NumericMatrix samp, NumericMatrix tramp, int size)
{
    // Declaring and defining output matrix
    NumericMatrix sol(size, size);

    // Initialising members of the structure
    struct* try_grid m = {samp, tramp, size};

    // operation
    try_grid add_f(m);


    // Creating output
    for(int i=0; i<size; i++)
    {
        for(int j=0; j<size; j++)
        {
            sol(i,j) = m.x(i,j) + m.y(i, j);
        }    
    }
    
    return(sol);
}

// Defining function
try_grid add_f(NumericMatrix x, NumericMatrix y, int size)
{
   for(int i=0; i<size; i++)
    {
        for(int j=0; j<size; j++)
        {
            if(x(i,j) == 0 && y(i,j) != 0)
            {
                x(i,j) = y(i,j);
            }

            if(y(i,j) == 0 && x(i,j) != 0)
            {
                y(i,j) = x(i,j);
            }
        }    
    }

    return try_grid(x, y, size);
}

The R code:

#Creating a matrix
samp <- matrix(0, 5, 5)
samp[3,3] <- 1

tramp <- matrix(0, 5, 5)
tramp[2,2] <- 2

sss <- 5

#Accessing the Rcpp file
sourceCpp('rcppfunction.cpp')

Answer 1

You wrote Void with a capital V. That does not exist as a keyword -- use void. And the error message

expected initializer before 'add_f'

hints at that. You are also missing a semicolon after the struct declaring, use a wrong structure to initialize your try_f object (you cannot just 'assign a to a pointer') and have no constructor for the try_grid object you try to return.

In short, you need to rework your code a little more. Maybe try in smaller steps. Below is a minimally repaited version.

Code

#include <Rcpp.h>

using namespace Rcpp;

//Declaring a structure -- with a constructor
struct try_grid {
    try_grid(NumericMatrix x_, NumericMatrix y_, int size_) : x(x_), y(y_), size(size_) {};
    NumericMatrix x;
    NumericMatrix y;
    int size;
};

// Declaring function
void add_f(const try_grid&);

// [[Rcpp::export]]
NumericMatrix plussomething(NumericMatrix samp, NumericMatrix tramp, int size) {
    // Declaring and defining output matrix
    NumericMatrix sol(size, size);

    // Initialising members of the structure
    try_grid m{samp, tramp, size};

    // operation
    try_grid add_f(m);

    // Creating output
    for(int i=0; i<size; i++)  {
        for(int j=0; j<size; j++) {
            sol(i,j) = m.x(i,j) + m.y(i, j);
        }
    }

    return(sol);
}

// Defining function
try_grid add_f(NumericMatrix x, NumericMatrix y, int size) {
   for(int i=0; i<size; i++) {
        for(int j=0; j<size; j++) {
            if(x(i,j) == 0 && y(i,j) != 0) {
                x(i,j) = y(i,j);
            }

            if(y(i,j) == 0 && x(i,j) != 0) {
                y(i,j) = x(i,j);
            }
        }
    }

    return try_grid(x, y, size);
}

/*** R
#Creating a matrix
samp <- matrix(0, 5, 5)
samp[3,3] <- 1

tramp <- matrix(0, 5, 5)
tramp[2,2] <- 2

sss <- 5

plussomething(samp, tramp, sss)
*/

Output

> Rcpp::sourceCpp("answer.cpp")

> #Creating a matrix
> samp <- matrix(0, 5, 5)

> samp[3,3] <- 1

> tramp <- matrix(0, 5, 5)

> tramp[2,2] <- 2

> sss <- 5

> plussomething(samp, tramp, sss)
     [,1] [,2] [,3] [,4] [,5]
[1,]    0    0    0    0    0
[2,]    0    2    0    0    0
[3,]    0    0    1    0    0
[4,]    0    0    0    0    0
[5,]    0    0    0    0    0
> 

Edit: You can also use a struct instead of a class but you need the constructor to instantiate.