[Solved]Problem of An Access Violation In A Easy Program

[rainygb]

Hello everyone

I have programed a matrix with c++ and got "an access violation (segmentation fault) raised in your program" in the constructor. It is very strange because this problem only occurs, if the matrix is larger than 10*2. A 9*2 matrix works perfectly.

Matrix::Matrix(int i, int j){
	int k=0;
	row=i;
	column=j;

	matrix=(double**) malloc(sizeof(double)*column);	
		for(int i=0;i<row;i++)
			matrix[i]=(double*)malloc(sizeof(double)*row);	

		do{
				for(j=0;j<column;j++)
	 	 			this->matrix[k][j]=0;
				k++;
		}
		while(k<row);
				
	}

Row and column are already decleared as int in header.
The error message is in line 12.

Who can help me to solve this problem, please??? Thanks!!!

[mordekai]

Dangerous Error!!

Your program also crashes with 9*2 Matrizes, but with the size of the Matrix it becomes more probable.

Solution:
The reason is tricky. The mistake is in line 11. You mixed up the rows and the columns. Change line 12 to:
this->matrix[j][k]=0;
Normally matrix[a][b] means, a is the row, b the column, but you allocated memory the way, that a is the column and b the row (lines 6 and 8). This caused all the trouble.

Explanation:
In line 6 you allocate memory, the same for every column in line 8. The program tries to allocate the next memory - if possible. This is for performance reasons. In line 12 you are accessing this memory via a pointer k, which is increased in line 13. So, if your memory is in one block, your program runs perfectly, but if it's fragmented, it crashes. This is a really dangerous mistake, because maybe your program runs 9 out of 10 times without any error, but sometimes still crashes.

By the way:
You're allocating too much space in line 6. What you mean is sizeof(double *), not sizeof(double):
matrix=(double**) malloc(sizeof(double *)*column);
And instead of the for loop in lines 7&8 i would recommend:

matrix[0]=(double *)malloc(column*row*sizeof(double));
for(int i=1;i<row;i++)
     matrix[i]=matrix[0]+i*column;

This ensures, that your Memory is not fragmented. It can save you a lot of trouble

[Zaita]

I'm going to suggest using new/delete instead of malloc/free also. As these are the C++ memory management methods.

[rainygb]

it works now with new/delete.
thank u.