Argument of type int(*)[] is incompatible with parameter of type "int**"

Question

I was creating a function that would give me random values of a matrix. I know how to make it in the main function but I want a separate one. I also tried with void and the same thing happened, and I keep getting the same error. I am beginner and I understand that the issue is something to do with pointers but I still don't know what should I do to make it work and fix it. I already Googled it but I can't make this work.

#include<stdlib.h>
#include<iostream> 
#include<time.h>

int row, coll;

int random(int *mat[])
{
    srand(time(0));
    for(int i=0; i<row; i++)
    {
        for(int j=0; j<coll; j++)
        {
            mat[i][j] = rand()%10;
        }
    }

}

main()
{
    std::cin >> row >> coll;
    int mat[row][coll];
    random(mat);
} 

Answer 1

You are trying to mix features of two different languages in one program.

The presented program is not a valid C++ program nor a valid C program.

For example variable length arrays as in your program

std::cin >> row >> coll;
int mat[row][coll];

is not a standard C++ feature.

The function main shall have the return type int.

Your function random

int random(int *mat[])

has the return type int but returns nothing.

The argument has the type (if to assume that variable length arrays are supported)

int ( * )[coll]

but the function parameter type is

int **.

If you are going to write a C++ program then instead of a variable length array use the standard container std::vector<std::vector<int>>.

For example

std::cin >> row >> coll;

std::vector<std::vector<int>> mat( row, std::vector<int>( coll ) );

Otherwise write a C program that can look like

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

void random( size_t row, size_t col, int mat[row][col], int limit )
{
    srand( ( unsigned int )time( NULL ) );
    
    for ( size_t i = 0; i < row; i++ )
    {
        for ( size_t j = 0; j < col; j++ )
        {
            mat[i][j] = rand() % limit;
        }
    }
}

int main( void ) 
{
    size_t row, col;
    
    scanf( "%zu %zu", &row, &col );
    
    int mat[row][col];
    
    random( row, col, mat, 10 );
    
    return 0;
}

Answer 2

I wrote a simple struct of matrix for you to carry the data around functions. Also, a output overload to make it easier to demo. Please look over it, and don't hesitate to ask, if you need more elaborations.

#include<stdlib.h>
#include<iostream>
#include<time.h>
#include <iomanip>
struct Matrix
{
    int *arr;
    const int row, col;
    Matrix() = delete;
    Matrix(const int i, const int j): row(i), col(j)
     {
       arr = new int [i*j];
     }
   ~Matrix() { delete [] arr; }
   int operator()(const int i, const int j) const {return arr[i*col+j];}
   int&operator()(const int i, const int j) {return arr[i*col+j];}
};
std::ostream& operator<<(std::ostream&osm, const Matrix&M)
{
    for (int i=0; i<M.row; i++) {
        for (int j=0; j< M.col; j++)  osm << std::setw(8) << M(i,j);
        osm << std::endl;
    }
    return osm;
}
void random(Matrix& amtx)
{
   srand(time(0));
   for(int i=0; i<amtx.row; i++)
    {
       for(int j=0; j<amtx.col; j++)
        {
           amtx(i,j) = rand()%10;
        }
    }
}
int main()
{
    int r, c;
    std::cout << "Input row and column : ";
    std::cin >> r >> c;
    Matrix mtx(r, c);
    random(mtx);
    std::cout << mtx;
   return 0;
}

and the test run:

Input row and column :  5  5
      6       3       1       9       3
      6       5       1       4       1
      1       2       7       5       5
      9       3       2       8       1
      7       4       8       3       5