Passing two-dimensional array to a function by refrence (C Programming) [duplicate] - c

Question

This question already has answers here:
Manipulate multidimensional array in a function
(4 answers)
Closed 8 years ago.
I'm learning pointers, and gotten stuck for an hour now, with this code,
#include <stdio.h>
int determinant(int **mat) /* int mat[3][3] works fine.. int *mat[3] doesn't.. neither does int *mat[] */
{
int det;
int a=*(*(mat+0)+0); // printf("\n%d",a);
int b=*(*(mat+0)+1); // printf("\n%d",b);
int c=*(*(mat+0)+2); // printf("\n%d",c);
int d=*(*(mat+1)+0); // printf("\n%d",d);
int e=*(*(mat+1)+1); // printf("\n%d",e);
int f=*(*(mat+1)+2); // printf("\n%d",f);
int g=*(*(mat+2)+0); // printf("\n%d",g);
int h=*(*(mat+2)+1); // printf("\n%d",h);
int i=*(*(mat+2)+2); // printf("\n%d",i);
det = a*(e*i-h*f) - b*(d*i-g*f) + c*(d*h-e*g);
return det;
}
int main()
{
int mat[3][3];
int i,j;
printf("Enter the 3 X 3 matrix:\n\n");
for (i=0;i<3;i++)
{
for (j=0;j<3;j++)
{
scanf("%d",*(mat+i)+j);
}
}
printf("\nThe determinant of the given 3 X 3 matrix is %d",determinant(mat));
return 0;
}
I don't think anything is wrong with the function call. Maybe the problem is while accepting the arguments. Idk, isn't mat a pointer to an 1-dimensional array, which would again be a pointer to the array element, making mat a pointer to a pointer?
When I print some text at places (just to check), i find that the execution goes till after int det in the function, and the program crashes in the next step.
mat [3][3] works well, but i wanna use some * there, because as i said, i'm 'learning'..
Please help!
Thanks :)

Answer 1

The correct prototype for your function is
int determinant(int mat[][3]);
or
int determinant(int (*mat)[3]);
(both are equivalent because of a special rule for arrays as function arguments)
Then you can simply access your matrix elements with something like mat[i][j].

Answer 2

This is because 2 dimensional array and pointer to pointer are not same.
No matter how much dimension does an array have, its 1 dimensional in actual memory. So we can access it serially.
#include <stdio.h>
#include <conio.h>
int determinant(int *matrix1stMember)
{
int a, b, c, d, e, f, g, h, i;
a = *(matrix1stMember + 0);
b = *(matrix1stMember + 1);
c = *(matrix1stMember + 2);
d = *(matrix1stMember + 3);
e = *(matrix1stMember + 4);
f = *(matrix1stMember + 5);
g = *(matrix1stMember + 6);
h = *(matrix1stMember + 7);
i = *(matrix1stMember + 8);
return ( a*(e*i-h*f) - b*(d*i-g*f) + c*(d*h-e*g) );
}
int main()
{
int matrix[3][3]; // int matrix[y][x]; not [x][y]
int i, j;
printf("\nEnter 3x3 Matrix : ");
for(j = 0; j < 3; j++)
{
for(i = 0; i < 3; i++)
{
scanf("%d", &matrix[j][i]);
}
}
// call function determinant(int*) using first member of array
printf("\nDeterminant = %d", determinant(&matrix[0][0]));
getch();
return 0;
}
If we have to access via row and column then we can do following
data = *(_1stMemberofArray + rowIndex*totalColumn + columnIndex);
For Example,
data = matrix[2][1];
where datatype of matrix is
int matrix[3][3];
is identical to.
data = *(matrixPointer + 2*3 + 1);
where 3 is total column 2 is row(vertical or y) and 1 is column(horizontal or x).
and datatype of matrixPointer is,
int* matrixPointer;
and it should point to first member of matrix;

Answer 3

2D array dont decay to pointer to pointer. You can decay them to pointers so your code should look like
int determinant(int *mat) {
int det;
int a=*((mat+0)+0); // printf("\n%d",a);
int b=*((mat+0)+1); // printf("\n%d",b);
int c=*((mat+0)+2); // printf("\n%d",c);
int d=*((mat+1*3)+0); // printf("\n%d",d);
int e=*((mat+1*3)+1); // printf("\n%d",e);
int f=*((mat+1*3)+2); // printf("\n%d",f);
int g=*((mat+2*3)+0); // printf("\n%d",g);
int h=*((mat+2*3)+1); // printf("\n%d",h);
int i=*((mat+2*3)+2); // printf("\n%d",i);
det = a*(e*i-h*f) - b*(d*i-g*f) + c*(d*h-e*g);
return det;
}
The above code is just for illustration, showing how 2-D array decays to 1-D array.
When you try to access the array using braces like a[2][1] then compiler does is unfolding for you. By unfolding I mean, the multiplication by sizeof(type) (as shown above multiply by 3). So if you decaying to 1-D you have to do it yourself.
One more thing to add, always pass the size of the dimension to the function who is has to tread the 1-D array as 2-D. like
int determinant(int *mat, int cols, rows);
Edit 1:
Just to add that #JensGustedt ans is also ok if you want to keep the arrays intact across function calls.

Answer 4

The correct signature for the function would be
int determinant(int mat[][3])
or
int determinant(int (*mat)[3])
In the context of a function parameter declaration, T a[] and T *a are exactly equivalent.
With either option, you can subscript mat normally in the function as you would in main:
int a = mat[0][0];
int b = mat[0][1];
...
Since a subscript operation implicitly dereferences the pointer (a[i] == *(a + i)),
you don't have to do the explicit dereference dance, making your code easier to read and understand (and potentially faster; I've seen some compilers generate more instructions for *(*(a + i) + j) than a[i][j], but don't rely on that being true everywhere).
Remember that when an expression of "N-element array of T" appears in most contexts, it is converted to an expression of type "pointer to T" and its value is the address of the first element in the array. Since the expression mat in the call to printf has type "3-element array of 3-element arrays of int", it is replaced with an expression of type "pointer to 3-element array of int".

Answer 5

If we pass a multidimensional array to a function:
int a2[5][7];
func(a2);
We can not declare that function as accepting a pointer-to-pointer
func(int **a) /* WRONG */
{
...
}
The function ends up receiving a pointer-to-an-array, not a pointer-to-a-pointer.