difference between declaring 2d array directly or with for loop

Question

Recently I had stuck in a problem i.e passing 2D array with pointers, the code I am trying to execute -

#include<iostream>
using namespace std;

int const m=3;
int const n=2;
int const p=3;

void input(int **a,int f, int k)
{
    int i,j;
    for(i=0;i<f;i++)
    {
        for(j=0;j<k;j++)
        {
            cin>>a[i][j];
        }
    }
    return;
}

int main()
{
    int a1[m][n];
    input(a1,m,n);
}

but getting error as -

main.cpp:24:14: error: cannot convert ‘int (*)[2]’ to ‘int**’ for argument ‘1’ to ‘void input(int**, int, int)’
  input(a1,m,n);
              ^

after that I tried this code and it runs successfully-

#include<iostream>
using namespace std;

int const m=3;
int const n=2;
int const p=3;

void input(int **a,int f, int k)
{
    int i,j;
    for(i=0;i<f;i++)
    {
        for(j=0;j<k;j++)
        {
            cin>>a[i][j];
        }
    }
}

int main()
{
    int *a1[m];
    for(int i = 0; i < m; i++)
        a1[i] = new int[n];
    input(a1,m,n);
}

but I am unable to understand why it is giving me error with a1[m][n]; as this is also declaring 2D array ?

Is there any way that I can use a1[m][n]; for declaring 2D array and passing it in function ?

You tagged the question with `C++`. Is there a need not to use it? In other words, why not just use `std::array`? — Croolman, May 25 '20 at 09:06
@B001ᛦ I think concept of pointers is same in both, didn't it ? — Pankaj Sharma, May 25 '20 at 09:06
arrays decay to pointers to their first element under certain circumstances, but they dont automatically decay any further. In the second example you have an array of pointers, so it decays to a `int**`, not in the first — 463035818_is_not_a_number, May 25 '20 at 09:11
@idclev463035818 what is `a1` if I write `a1[m][n]` ? Is it address to first element of array ? Is it a single pointer ? — Pankaj Sharma, May 25 '20 at 09:16
`a1` is a `int[m][n]`. Arrays are not pointers. Only under certain circumstances they decay to pointers. — 463035818_is_not_a_number, May 25 '20 at 09:18

score 0 · Answer 1 · answered May 25 '20 at 09:06

0

Your first version might be in C++

void input(int (&a)[m][n]);

but using std::array, you will have more intuitive syntax.

answered May 25 '20 at 09:06

Jarod42

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what If m,n are not global and want to take parameter as - `void input(int **a,m,n)` ? Is there any way to define array with `a1[m][n]` and passing it with double pointer ? – Pankaj Sharma May 25 '20 at 09:22
if `m` and `n` are compile-time constant, you can turn `input` as `template`. if `m` and `n` are runtime value, `std::vector` would be appropriate. `int [m][n]` and `int**` are really different type with different layout (even if they have similar syntax for indexing). – Jarod42 May 25 '20 at 09:26

difference between declaring 2d array directly or with for loop

1 Answers1