Hi

I'm adapting some code I've written using 2d arrays (to represent matrices) to handle large arrays such that double matrix[][] goes to double **matrix and then I'm using malloc.

It seems to work fine for part of my program up to where I have to find the matrix transpose at which point it does something I don't understand. I've taken that bit of code out and run it by itself (included below), get the same problem... which is that the input matrix is getting modified when the function is called, here's an example for a test 3 by 3:
The input is:in[0][0] = 10.000000
in[0][1] = 10.000000
in[0][2] = 10.000000
in[1][0] = 5.000000
in[1][1] = 5.000000
in[1][2] = 5.000000
in[2][0] = 3.333333
in[2][1] = 3.333333
in[2][2] = 3.333333

but after being passed to find_transpose is comes out as:in[0][0] = 10.000000
in[0][1] = 10.000000
in[0][2] = 10.000000
in[1][0] = 10.000000
in[1][1] = 5.000000
in[1][2] = 10.000000
in[2][0] = 3.333333
in[2][1] = 3.333333
in[2][2] = 10.000000

and the actual transpose output is:
out[0][0] = 10.000000
out[0][1] = 10.000000
out[0][2] = 10.000000
out[1][0] = 10.000000
out[1][1] = 5.000000
out[1][2] = 3.333333
out[2][0] = 10.000000
out[2][1] = 10.000000
out[2][2] = 10.000000

I really don't understand why!
Can anyone help?

Thanks
jbd


nt find_transpose(int n, double **a, double **b)
{
int I,j;
for (I=0; I<n; I++)
{
for (j=0; j<n; j++)
b[i][j] = a[i][j];
}
for (I=0; I<n; I++)
{
for (j=0; j<n; j++)
b[i][j] = a[j][i];
}

}


int main (void)
{

int I, j, p=3;
double **in, **out;

in = malloc(p * sizeof(int *));
out = malloc(p * sizeof(int *));

for (I=0; I<p; I++){
in[i]= malloc(p * sizeof(int *));
out[i]= malloc(p * sizeof(int *));}

for (I=0; I<p; I++)
{
for (j=0; j<p; j++)
{in[i][j]= 10./(I+1);
printf("in[%i][%i] = %f\n", I, j, in[i][j]);
}
}

find_transpose(p, in, out);

for (I=0; I<p; I++)
{
for (j=0; j<p; j++)
printf("in[%i][%i] = %f\n", I, j, in[i][j]);
}

for (I=0; I<p; I++)
{
for (j=0; j<p; j++)
printf("out[%i][%i] = %f\n", I, j, out[i][j]);
}

return 0;
}

This is what I got running your program in Visual Studio 2005

in[0][0] = 10.000000
in[0][1] = 10.000000
in[0][2] = 10.000000
in[1][0] = 5.000000
in[1][1] = 5.000000
in[1][2] = 5.000000
in[2][0] = 3.333333
in[2][1] = 3.333333
in[2][2] = 3.333333
in[0][0] = 10.000000
in[0][1] = 10.000000
in[0][2] = 10.000000
in[1][0] = 5.000000
in[1][1] = 5.000000
in[1][2] = 5.000000
in[2][0] = 3.333333
in[2][1] = 3.333333
in[2][2] = 3.333333
out[0][0] = 10.000000
out[0][1] = 5.000000
out[0][2] = 3.333333
out[1][0] = 10.000000
out[1][1] = 5.000000
out[1][2] = 3.333333
out[2][0] = 10.000000
out[2][1] = 5.000000
out[2][2] = 3.333333


I changed find_transpose() to a void function, and I casted your malloc s

Other than that I didn't do anything.

What compiler are you using?

Hi

I've tried compiling with gcc version 4.0.2 20051125 (Red Hat 4.0.2-8)
And gcc version 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2)
... both with the same result

Thanks
Jbd

You can download Visual Studio from here Visual Studio 2008 Express Editions

Why do you have 2 for loops in transpose?

No good reason, sorry should have taken that out... doesn't make any difference to the result though.

Jbd

Will try Visual Studio, thanks very much.

Jbd