Rotating a matrix

[llorenzi]

Hi everybody.
I have a code to modify. This is the original code:

 
N=nn-n+1;  M=mm-m+1;
  for (j=1; j<=N; ++j) {
    J=j+n-1;
    for (i=1; i<=M; ++i) {
      I=i+m-1;
      
      for (jj=j,jj2=1; jj<=J; ++jj,++jj2) {
		for (ii=i,ii2=1; ii<=I; ++ii,++ii2) {
			ndx=ii-1+mm*(jj-1);
			ndx2=ii2-1+m*(jj2-1);
			if (!toFill[ndx2]) {
				err=img[ndx      ] - Ip[ndx2    ]; patchErr += err*err;
				err=img[ndx+=mmnn] - Ip[ndx2+=mn]; patchErr += err*err;
				err=img[ndx+=mmnn] - Ip[ndx2+=mn]; patchErr += err*err;
			  }
			}
		  }

      }
  }

I have to rotate the little matrix (img), so it does mean to rotate his index: ndx

For example if I have to flip it, I just write:

 
ndx=(I-ii+1)-1+mm*(jj-1);

But if I want to rotate it (+90'), how can I do?

[weaknessforcats]

Write a 3x3 matrix on paper and rotate it 90 degrees. The bottom row becomes the first column. Develop your algorithm from what you had to do with your pencil.

[jkmyoung]

Note that rotate 90 degrees could also be:
flip +
diagonal invert (eg x = y, y = x)

The direction of flipping depends on which way you're rotating.