Raise Matrix to power C++ - c++

Question

I have written a function to raise matrix to a certain power.
But when running the code the result is the memory location not the actual values. I think the problem is with the pointers.
My code:
#include <iostream>
using namespace std;
typedef int array2d [2][2];
array2d A,B,r;
void pow(array2d* r,array2d C);
int main(){
array2d resa,resb;
A[0][0]=2;
A[0][1]=2;
A[1][0]=2;
A[1][1]=2;
B[0][0]=3;
B[0][1]=3;
B[1][0]=3;
B[1][1]=3;
r[0][0]=1;
r[0][1]=0;
r[1][0]=0;
r[1][1]=1;
pow(&resa,A);
for(int i=0;i<2;i++){
for(int j=0;j<2;j++)
{
cout<<resa[i][j]<<" ";
}
cout<<endl;
}
pow(&resb,B);
for(int i=0;i<2;i++){
for(int j=0;j<2;j++)
{
cout<<resb[i][j]<<" ";
}
cout<<endl;
}
return 0;
}
void pow(array2d* r, array2d C)
{
array2d temp;
for(int w=0;w<3;w++)
{
for(int i=0;i<2;i++){
for(int j=0;j<2;j++)
{
temp[i][j]=0;
for(int k=0;k<2;k++)
temp[i][j]+=(*r)[i][k]*C[k][j];
}}
for(int i=0;i<2;i++){
for(int j=0;j<2;j++)
{
(*r)[i][j]=temp[i][j];
}}
}
}
How can I solve this problem.
Thank You.

Answer 1

Your error has nothing to do with pointers or addresses, but your algorithms is erroneous:
pow(&resa, A);
multiplies the matrix A with the uninitialized matrix resa which can yield any number of possible results.
The quick and dirty fix would be to initialize resa and resb as identity matrices:
array2d resa = { { 1, 0 }, { 0, 1 } };
array2d resb = { { 1, 0 }, { 0, 1 } };
EDIT or slightly better: Initialize r inside of pow
//passing r by pointer is not actually necessary here,
//but I don't want to modify too much of the code
(r*)[0][0] = 1;
(r*)[1][0] = 0;
(r*)[0][1] = 0;
(r*)[1][1] = 1;
The more elegant solution would be to first multiply the parameter C with itself, store the result in r and then go on with your algorithm.
On a side note: don't use c-style arrays, if you don't need them (especially not in typedefs). Use std::array instead, which will get rid of most of the confusion regarding to parameter passing.

Answer 2

you really should avoid global when you can, and using the same name in global and local variables is even worse
also the name of your variables MATTERS, here i call the input matrix IN and output matrix OUT (ok "n" should be called power)
using namespace std;
typedef int array2d [2][2];
void display( array2d a ) ;
void pow(array2d IN ,array2d OUT , int n);
int main()
{
array2d A,B;
array2d resa,resb;
A[0][0]=2;
A[0][1]=2;
A[1][0]=2;
A[1][1]=2;
B[0][0]=3;
B[0][1]=1;
B[1][0]=2;
B[1][1]=1;
pow (A , resa , 5);
pow( B , resb, 2 ) ;
return 0;
}
and
void pow(array2d IN, array2d OUT , int n )
{
for( int i = 0 ; i < 2 ; ++ i )
for( int j = 0 ; j < 2 ; ++ j )
OUT[i][j] = ( i == j ) ;
display( OUT ) ;
array2d temp;
for(int w=0;w<n;w++)
{
for(int i=0;i<2;i++)
{
for(int j=0;j<2;j++)
{
temp[i][j]=0;
for(int k=0;k<2;k++)
{
temp[i][j]+=OUT[i][k]*IN[k][j];
}
}
}
for(int i=0;i<2;i++){
for(int j=0;j<2;j++)
{
OUT[i][j]=temp[i][j];
}
}
display( OUT ) ;
}
}
with
void display( array2d a )
{
cout << endl ;
for( int i = 0 ; i < 2 ; ++ i )
{
for( int j = 0 ; j < 2 ; ++ j )
cout << a[i][j] << " " ;
cout << endl ;
}
}

Answer 3

The first parameter of your pow function, change it to the pass by reference: void pow(array2d &r,array2d C);
Then where you call it, call it like so: pow(resa,A);
Finally, you don't need to deference r in your pow function now, so (*r)[i][j]=temp[i][j]; can be changed to r[i][j]=temp[i][j];
I think this is what you wanted to do. ( pass by reference )
( I am not in front of a pc where I can test this right now, will confirm as soon as I can, and maybe flesh out why this is better ( can read up about pointers, heap memory and pass by reference )