Using a 2D Array in Function: Error - c

Question

I tried to set values in 2D array and Print it with Function.
but I got trouble to send the Array to the Function.
#include <stdio.h>
void SetArr(int (*arr)[], int n);
void PrintfArr(int (*arr)[], int n);
int main()
{
int arr[100][100];
for(int i=0; i<100; i++)
for(int j=0; i<100; j++)
arr[i][j]=0; //initiallize Array
int size;
printf("input Size : "); scanf("%d", &size);
SetArr(arr, size);
PrintfArr(arr, size);
return 0;
}
void SetArr(int (*arr)[], int n)
{
int i, j;
for(i=0; i<n; i++)
{
for(j=0; j<n; j++)
arr[i][j]=i+j; // Error! "Subscript of Pointer to imcomplete~
}
}
void PrintArr(int (*arr)[], int n)
{
int i, j;
for(i=0; i<n; i++)
{
for(j=0; j<n; j++)
printf("%d", arr[i][i]); // Error! "Subscript of Pointer to imcomplete~
}
printf("\n");
}
As you see, both of functions got same problem while using "arr[][]"

Answer 1

In the declaration of the function, array must be mentioned with its size (length). This is what the compiler means when it says "incomplete type": it needs the size to understand how to access its elements.
In order to make size available to the compiler, it should go first in the function's declaration:
void SetArr(int n, int (*arr)[n])
Here arr is technically a "pointer to an array of n elements", but in C it's customary to treat a pointer as an array, so arr can be treated as "an array of arrays of n elements", which is the same as a 2-D array.
You might want to mention both dimensions of the array:
void SetArr(int n, int arr[n][n])
This is equivalent to the first declaration, but seems clearer.

Answer 2

Declare the functions like
void SetArr( int ( *arr )[100], int n );
void PrintfArr( int ( *arr )[100], int n );
You may not declare the functions as it is shown in the answer that you marked as the best
Consider the following code where instead of 100 I am using 3 for the array dimensions.
If you will print the array in function PrintArr and in main you will get different results!
#include <stdio.h>
void SetArr( size_t n, int (*arr)[n] )
void PrintArr( size_t n, int (*arr)[n] )
int main(void)
{
int arr[3][3];
size_t n = 2;
SetArr( n, arr );
PrintArr( n, arr );
printf( "\n" );
for ( size_t i = 0; i < n; i++ )
{
for ( size_t j = 0; j < n; j++ ) printf( "%2d", arr[i][j] );
printf( "\n" );
}
return 0;
}
void SetArr( size_t n, int (*arr)[n] )
{
for ( size_t i = 0; i < n; i++ )
{
for ( size_t j = 0; j < n; j++ ) arr[i][j] = i * n + j;
}
}
void PrintArr( size_t n, int (*arr)[n] )
{
for ( size_t i = 0; i < n; i++ )
{
for ( size_t j = 0; j < n; j++ ) printf( "%2d", arr[i][j] );
printf( "\n" );
}
}
The program output is
0 1
2 3
0 1
3 1
As you see the first output does not coincide with the second output. You would get the correct result if the functions were declared as I pointed in the beginning of the post. For example
#include <stdio.h>
void SetArr( int (*arr)[3], size_t n );
void PrintArr( int (*arr)[3], size_t n );
int main(void)
{
int arr[3][3];
size_t n = 2;
SetArr( arr, n );
PrintArr( arr, n );
printf( "\n" );
for ( size_t i = 0; i < n; i++ )
{
for ( size_t j = 0; j < n; j++ ) printf( "%2d", arr[i][j] );
printf( "\n" );
}
return 0;
}
void SetArr( int (*arr)[3], size_t n )
{
for ( size_t i = 0; i < n; i++ )
{
for ( size_t j = 0; j < n; j++ ) arr[i][j] = i * n + j;
}
}
void PrintArr( int (*arr)[3], size_t n )
{
for ( size_t i = 0; i < n; i++ )
{
for ( size_t j = 0; j < n; j++ ) printf( "%2d", arr[i][j] );
printf( "\n" );
}
}
The program output is
0 1
2 3
0 1
2 3
As you can see in this case the both outputs coincide.