在程序中使用for循环打印矩阵的错误

Question

I wrote the following code to test in int main()

int main() {
int *p1 = findmatrixsize("inputmatrix.txt","A");//A function gets matrix size
cout << p1[0] << endl << p1[1] << endl;
int *p2 = findmatrixsize("inputmatrix.txt","B");
cout << p2[0] << endl << p2[1] << endl;

double **C = normalmultiplication("inputmatrix.txt","A", "B"); 
cout << C[0][0] << endl;

ofstream fout;
const string s="outputmatrix.txt";
fout.open(s);
for(int i=0;i<p1[0];i++){
    for(int j =0;j<p2[1];j++){
        fout << C[i][j] << " ";
    }
    fout<< endl;
}
fout.close();

return 0;
}

Everything works except when I am executing the multiarray printing. Here is the problem:

1.If I comment out the printing part, p1[0],p1[1],p2[0],p2[1],C[i][j] gives me perfect and correct result that I want.

2.However, when I print by using normal loops it doesn't work. But, but,but, if I change the p1[0],p2[1] in the for loop to a concrete number like 3(when my matrix dimension is 3), it works again.

3.It compiles, but the for loop printing part is not written into the txt. When I changed it to the standard print cout(not only the single line, I mean the whole method), I couldn't see them in the terminal neither. But as I mentioned above, when I change p[0] p[1] to 3, it works, which means I can see them either in the txt or terminal

I have never met such weird situation in C++ before, so I hope someone can help me out!

FYI, I use Visual Studio 2010 as compiler.

Thanks for your help!

Answer 1

We don't know what findmatrixsize and normalmultiplication do.

But if they do what I think they do (and I think they return pointers to their local data), the objects referred to by p1 , p2 and C share storage.

Check if p1 == p2 .

Answer 2

The problem is in the way you get result from findmatrixsize and normalmultiplication functions: they keep the result of calculation in stack and return a pointer to it, but the stack is destroyed soon after these functions finish their work. Reimplement these functions so that the place in memory is valid outside of these functions. For example,

int p1[2];
findmatrixsize("inputmatrix.txt","A", p1)

and put the result inside of findmatrixsize into the last argument, eg.

void findmatrixsize(const char* fileName, const char* matrixName, int* result)
{
  // routine
  result[0] = someValue1;
  result[1] = someValue2;
}