Sum of the external elements - matrix to function C++ - c++

Question

I need to create a program that computes the external elements of a matrix. The matrix should be set by the user.
My idea is to create a first part of the program where the user can write the elements of the matrix, after that this matrix is given to a function that sets the internal elements to 0 and than gives it back to the main() and then launch another function that computes the sum of all elements.
The problem is that I am not able to pass to the function the matrix that's declared in the main {}.
I am almost sure that I need to use pointers, however I have no idea how to use them in this situation... Could you please help?
That's the code I have written so far:
#include <iostream>
#define MAX 10
using namespace std;
int zeroMatrix (int mat [][10], int, int);
int main()
{ //ask the user the number of rows and columns of the matrix.
int rows,cols;
cout << "please insert the number of rows: ";
cin >> rows;
cout << "insert the number of columns: ";
cin >> cols;
//accepting values
int matrix [rows] [cols];
for (int i=0; i<rows; i++)
{
for (int j=0; j<cols; j++)
{
cout <<"insert the element A(" << i <<","<< j << ") of the matrix " ;
cin >> matrix[i][j];
}
}
zeroMatrix (matrix [rows][cols]);
}
//change the previous matrix into a new matrix with the internal elements = 0
int zeroMatrix (int mat [][10], int rows, int cols )
{
for (int i=1; i<rows-1; i++)
{ for (int j=1; j<cols-1 ; j++)
{
mat[i][j] = 0;
}
}
}

Answer 1

#include <iostream>
using namespace std;
void zeroMatrix (int **mat, int, int);//makes the inner elements zero
int sumExternals(int **mat, int rows, int cols);//sums the external elements without
//using zeroMatrix
void printMatrix(int **mat, int, int);//prints a matrix on the console window
int main()
{ //ask the user the number of rows and columns of the matrix.
int rows,cols;
cout << "please insert the number of rows: ";
cin >> rows;
cout << "insert the number of columns: ";
cin >> cols;
//accepting values
int **matrix = new int*[rows];
for(int i=0; i<rows; ++i)
matrix[i] = new int[cols];
for (int i=0; i<rows; i++)
{
for (int j=0; j<cols; j++)
{
cout <<"insert the element A(" << i <<","<< j << ") of the matrix " ;
cin >> matrix[i][j];
}
}
printMatrix(matrix,rows,cols); //print the matrix
cout<<sumExternals(matrix,rows,cols)<<endl; //get the sum of the external elements
//Just so you can see how to proper call the function zeroMatrix
zeroMatrix(matrix,rows,cols);
printMatrix(matrix,rows,cols);
}
//changes the previous matrix into a new matrix with the internal elements = 0
void zeroMatrix (int **mat, int rows, int cols )
{
for (int i=1; i<rows-1; i++)
for (int j=1; j<cols-1; j++)
mat[i][j] = 0;
}
//calculates the sum of the external elements of a matrix
int sumExternals(int **mat, int rows, int cols)
{
int sum = 0;
for(int j=0;j<cols;++j)
{
sum+=mat[0][j]; // adds the upper bound elements to sum
sum+=mat[rows-1][j]; // adds the lower bound elements to sum
}
//Take caution not to add elements which are already taken into account
for(int i=1;i<rows-1;++i)
{
sum+=mat[i][0]; //adds the left bound elements to sum
sum+=mat[i][cols-1]; //adds the right bound elements to sum
}
return sum;
}
void printMatrix(int **matrix, int rows, int cols)
{
for(int i=0;i<rows;++i)
{
for(int j=0;j<cols;++j)
cout<<matrix[i][j]<<" ";
cout<<endl;
}
}
I added the function sumExternals which calculates the sum of the external matrix elements but without using your zeroMatrix function.

Answer 2

Use vector instead of array. Also, there's no need for two separate functions. One to find the sum of external elements and other to set internal elements to 0. You can achieve both tasks in the same function. Refer function external_elements_sum written below.
# include <iostream>
# include <vector>
using namespace std;
int external_elements_sum(vector<vector<int> >&arr) // function that finds sum of external elements and set internal elements to 0
{
int sum=0;
for(int i=0; arr.size()>0 && i<arr[i].size();i++)
{
for(int j=0;j<arr[i].size();j++)
{
if(i==0 || i==arr.size()-1 || j==0 || j==arr[i].size()-1) // external elements
sum+=arr[i][j];
else arr[i][j]=0; // internal elements
}
}
return sum;
}
int main() {
vector< vector<int> >arr;
int i,j,number,rows=3,cols=4;
for(i=0;i<rows;i++)
{
vector<int>v;
for(j=0;j<cols;j++)
{
cin>>number;
v.push_back(number);
}
arr.push_back(v);
}
int sum = external_elements_sum(arr);
for(i=0;i<arr.size()>0 && arr[0].size();i++) // arr.size()>0 to check corner case like if row is 0
{
for(j=0;j<arr[i].size();j++)
cout<<arr[i][j]<<" ";
cout<<endl;
}
cout<<"Sum of externale elements is: "<<sum;
return 0;
}
Input:
1 2 3 4
5 6 7 8
2 5 1 4
Output:
1 2 3 4
5 0 0 8
2 5 1 4
Sum of externale elements is: 35