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I'm struggling to create multi-variable output from a function: I want to return 2D array sites(16x15) and the integer number N.

I tried:

  1. std::make_tuple here
  2. std:make_pair here

My problem is that I probably do not know how to define a 2D array in the declaration of the function std::pair <int [][],int> correctly.

Piece of code named function.cpp:

#include <iostream>

std::pair <int[16][15],int> sites_diamond()
{
    int sites[16][15]={0};
    int N=0;
    for (int r=0; r<7; r++) {
        N=N+1+2*r;
        for (int c=0; c<(7-r);c++){
            sites[r][c]=0;
            sites[15-r][c]=0;
            sites[r][14-c]=0;
            sites[15-r][14-c]=0;
        }
    }
    N=2*(N+15);
    return std::make_pair(sites, N);
}

using namespace std;

int main(){
    std::pair <int[16][15], int> result = sites_diamond();
    cout << " sites \n"<<result.first<< endl;
    cout << "number \n"<<result.second<< endl;

    return 0;
}

Error I'm getting:

function.cpp: In function ‘std::pair<int [16][15], int> sites_diamond()’:
function.cpp:21:26: error: could not convert ‘std::make_pair<int (&)[16][15], int&>(sites, N)’ from ‘std::pair<int (*)[15], int>’ to ‘std::pair<int [16][15], int>’
     return std::make_pair(sites, N);

Thank you ahead for any suggestions. I work primarily in Python, but I want to rewrite a code to C++.

NutCracker
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Maria Zelenayova
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    Instead of the raw array definiton, rather use `std::array,16>`. That will make dealing with the array as value a lot easier. – πάντα ῥεῖ Dec 28 '20 at 12:37
  • I assume that the posted code isn't the real one (it's assigning zero values to the arrays multiple times), but is it correct to state that the value of N only depends by 15 and 16? – Bob__ Dec 28 '20 at 15:12
  • Please check out the answers and mark the one that solves your problem – NutCracker Dec 29 '20 at 13:26

2 Answers2

1

You can go with std::array. It is more C++-ish and you don't need to care about memory allocation/deallocation.

std::pair <std::array<std::array<int, 15>, 16>, int> sites_diamond()
{
    std::array<std::array<int, 15>, 16> sites;
    // ...
    return std::make_pair(sites, N);
}

and then the usage would be:

auto result = sites_diamond();
cout << " sites \n"  << result.first.size() << endl;
cout << " number \n" << result.second       << endl;
NutCracker
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0

As the error is quite explanatory, I will just suggest the solution. Use pointers. Define your pair like this:

std::pair<int**, int> result;

Of-course, inside your function, change how you define sites:

int **sites;
sites = new int*[16];
for (int i = 0;i < 16;i++)
    sites[i] = new int[15];

Regarding

cout << " sites \n"<<result.first<< endl;

I don’t know what are you trying to print here, it's gonna print some random address anyway.

Don’t forget to delete this dynamically allocated memory when you are done. But all in all, I would just suggest to use something like vectors (2-d vector in this case, leak-proof too) in place of C-styled arrays.

Jarvis
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