error in storing values

I have a code in which i process two images

for xx = 1:length(dirlist(1:2))

      x = imread([pathname, dirlist(xx).name]);  

x=rgb2gray(x);
.
.
.
.
.
.
PSNR =   20 * log10 (255 / sqrt(S));

    PSNR(:,xx)=PSNR 

end

i have calculated PSNR for two images i get different values of PSN for each image,but wen i concatanate 2 values of PSNR i get same value ,please help

for ex wen i process

PSNR =

     38.8500

PSNR =

     39.8618

PSNR =

     39.8618   39.8618

please help

Answer 1

[Wayne King]

Don't use the same variable below, use a different variable.

      PSNR =   20 * log10 (255 / sqrt(S));
      PSNRS(xx) =PSNR;