2D array as function parameter

[killer7]

Hi
I was trying to calculate the determinant of a 2X2 matrix. but the problem is passing in a 2D array. I know you need to give number of columns but in that it only declares 1D array. Follwing should make more sense.

double det(double M[][5],int i,int j)
{
	double det = 0;
	if(i == j == 2)
	{
		det = M[i-2][j-2] * M[i-1][j-1] - M[i-1][j-2] * M[i-2][j-1];
		return det;
	}
}

I am calling this function by:

calcdet = det(Matrix,2,2);

where Matrix is 2D array with values.

Any help would be really appreciated.

[Kernel Sanders]

Declare it as a 'double **' instead of a 'double [][]'

The syntax for accessing the elements is identical

I would also suggest returning -1 or something similar if the 'i==j==2' fails instead of letting the function terminate without a return (it will give junk back to the calling function)

[killer7]

thanks for the reply i tried changing that but now am not getting the right answer. its always giving me -1 (else condition answer). here is my function:

double det(double **M,int i,int j)
{
	double det = 0;
	if(i == j == 2)
	{
		det = M[i-2][j-2] * M[i-1][j-1] - M[i-1][j-2] * M[i-2][j-1];
		return det;
	}
	else 
		return -1;
}

and my main:

int main()
{
	int matsize,i=0,j=0;
	double calcdet=0;
	double Matrix [MAXSIZE][MAXSIZE]={{0}}; 
	FILE* file = fopen("matrix.txt","r");
	fscanf(file,"%d",&matsize);
	for (i=0;i<matsize;i++)
	{
		for (j=0;j<matsize;j++)
		{
			fscanf(file,"%lf",&Matrix[i][j]);
		}
		j = 0;
	}
	fclose(file);
	calcdet = det(&Matrix,2,2);
	printf("%lf\n",calcdet);	
	
	system("PAUSE");
	return 0;
}

I dont know what am doing wrong so if you can figure out my mistake i will be really thankful to you.

[tabstop]

You very carefully avoided any mention of a problem. Fortunately I can guess that you think "i == j == 2" means something, when in fact it does not. (Well, it does; I forget whether == associates l-to-r or r-to-l; if it associates r-to-l, it means "i==1 && j==2", if it associates l-to-r, it means "false".)

Note also that if you declare the array as double ** in your function prototype, you will not be able to use [bracket][notation] to access elements.

[killer7]

okie i changed my if condition. Now if i cant use bracket notation then how can i access elements. initially i was trying to use M[][5] but that did not work either. any suggestions?

[tabstop]

Well, M[][5] does work, assuming MAXSIZE == 5. If not, it won't work, and it's not supposed to. (And even if it is, you should say M[][MAXSIZE] anyway.)

Edit: Oh, and don't pass Matrix as &Matrix -- you had that right in the first post.

[killer7]

thank you very much tabstop. works perfectly. i guess the whole problem was "i==j==2". good observation because it was tricky
thank you again

[matsp]

Declare it as a 'double **' instead of a 'double [][]'

Tabstop hinted at it: This is actually incorrect - for arrays that are not dynamically allocated [1], using double[][SIZE] is the correct way. The double ** form assumes that the argument is an array of pointers [if we keep the rest of the code in the function the same], and that the first pointer points to an array of double.

A two dimensional array when passed to a function is not turned into an array of pointers to arrays of elements by the compiler. The compiler does the same thing for two dimensional arrays (or n-dimensional) as it does for single dimension arrays: It passes the address of the first element. In the case of n-dinensional arrays, all dimensions except the first one must be given so that the compiler can determine how to find each dimension from the index - because essentially, the array is just a flat piece of memory to the function being called.


[1] It doesn't HAVE to be dynamically allocated, but it does have to be an array of pointers to double that is passed in, and those pointers should point to a suitable array of doubles.

--
Mats