Returning Multi dimension array

I am a beginner of C language. I am confused, how to return a multi dimentional array from function!

Code:

 #include<stdio.h>
#include<conio.h>
#define n 2

int * add(int [n][n],int [n][n]);

main()
{
    int c[n][n];
    int a[n][n]={ {1,2},{3,4} };
    int b[n][n]={ {4,5},{13,4} };
    int i,j;

    c=multi(a[n][n],b[n][n]);

    for(i=0;i<n;i++)
    {
        printf("\n");
        for(j=0;j<n;j++)
        printf("%5d",c[i][j]);
    }
}

int * add(int a[n][n],int b[n][n])
{
    int c[n][n];
    int i,j;

    for(i=0;i<n;i++)
    for(j=0;j<n;j++)
    c[i][j]=a[i][j]+b[i][j];

    return(c);
}

Returning the array name c is fine as long as the caller can handle a pointer to an array of integers.

You can also have the user pass in a pointer to an array as one of the parameters, and then your function uses that pointer to write the result. It seems counter-intuitive, but that way the caller is entirely in control of allocating and deallocating memory, etc... This is quite common in libraries, because it leaves the return value open for error codes, etc...

No one bothered to mention that c is not an int *, and that you can't return a pointer to a local variable here I see.


Quzah.

No one bothered to mention that a 2D array does not decay to a pointer either.
The arrays you pass into the function are already passed by address. Use that to your advantage. Use it to pass in an array to fill up so the caller can acquire the information.

Originally Posted by itCbitC

Returning the array name c is fine as long as the caller can handle a pointer to an array of integers.

Yes i have tried it this way, but it is still not working and giving an error as

can not convert int to int *

my code was like this
[code]

int *c;
.
.
.
.
c=add(a[][],b[][]);

First off...
c=add(a[][],b[][]);
...is not a valid function call.
It should be...
c=add(a,b);
...to pass the 2D arrays to the function.

Secondly, as we've told you: a 2D array isn't a pointer. You cannot assign a 2D array to mere pointer. Don't attempt it.

Originally Posted by Elysia

No one bothered to mention that a 2D array does not decay to a pointer either.
The arrays you pass into the function are already passed by address. Use that to your advantage. Use it to pass in an array to fill up so the caller can acquire the information.

yes that's the only way i haven't tried yet.

thank for your help and support.

Originally Posted by Elysia

First off...
c=add(a[][],b[][]);
...is not a valid function call.
It should be...
c=add(a,b);
...to pass the 2D arrays to the function.

Secondly, as we've told you: a 2D array isn't a pointer. You cannot assign a 2D array to mere pointer. Don't attempt it.

You mean that as i can use name of a single dimention array as it's base address but name of two dimention array can't be used as address!
I mean in the following declaration
int array1[]

"array1" is the base address of arrray but in declaration

int array2[][]

"array2" is not!

Yes, it can. However, its type is different:

int myarr[10];
int * pArr = myarr; // OK

int myarr[5][10];
int * pArr = myarr; // Not OK
int (*pArr)[10] = myarr; // OK

Use all upper case for #define macro.
#define N 2
Or use enum.
Array decays to pointer.
2D array decays to pointer to array of element COLUMN . (i.e (*T)[COL] ).
E.g

Code:

int (*add(void))[ N ] 
{
   static int foo[ROW][N] ;
   return foo;
}

It looks like you should be using struct. Aren't you doing matrix addition?
If you still want to stick with 2D array.
void add( int a[] [N], int b[ ] [N], int result[] [ N] ) ;
looks simpler.
Read c-faq.

Originally Posted by Elysia

Yes, it can. However, its type is different:

int myarr[10];
int * pArr = myarr; // OK

int myarr[5][10];
int * pArr = myarr; // Not OK
int (*pArr)[10] = myarr; // OK

I see.....

than can't i return a two dimensional array to main in my code?

or can I by declaring it's prototype as follows

int *[n] add(int [][n],int [][n]);

You could, but then again, why would you need to?
int (*)[N] add(int [][N],int [][N]);
And as pointed out, use upper-case for macros, and preferably more than one character.
Something like SIZE would be a better choice.

Thanks a lot!

Just one more question!
why did you recommended me to use upper case macro and more than one alphabet!

To avoid name collision.
Example:

Code:

#define n 10
int main()
{
    int n[10]; // Syntax error. Why?
}