Vectorization of For loops

Hi I am trying to get the location of the pixels of an image file using a specific sequence and saving it to a specific location. How could I vectorize my for loops to get a lower run time?

 [FILENAME, EXT, USER_CANCELED] = uigetfile('*.jpg')
 image = imread(FILENAME);
 figure, imshow(FILENAME), title('Original Image')
[im_row,im_col,rgbdim]=size(image);
total_pix=im_row*im_col; %total number of pixels
tf = ceil(0.75*total_pix); %75 percent of the image  
pixprow = ceil(tf/im_row); 
pix_count=0;
for i=1:im_row
    for j=1:pixprow
     pix_count=pix_count+1;   
     rloc1(pix_count,1)=i;
     rloc1(pix_count,2)=im_col-j;

    end
end
sub=0;
sub2=0;
for 
 i=1:2:pix_count-1;
     sub=sub+1;
     rloc(i,:)=rloc1(sub,:);
     rloc(i+1,:)=rloc1(pix_count-sub2,:);
     sub2=sub2+1;
end

Answer 1

[Eric]

Honestly, your slow code is due to the fact that you don't preallocate rloc1 or rloc. You will find that simply adding

 rloc1 = zeros(im_row*pixprow,2);
 rloc = rloc1;

just before the first loop will already great improve your run time. But to answer your question, you can replace for loop 1 with:

 pix_count = im_row*pixprow;
 rloc1 = zeros(pix_count,2);
 [rloc1(:,2) rloc1(:,1)] = ind2sub([pixprow im_row],1:pix_count);
 rloc1(:,2) = im_col - rloc1(:,2);

and loop 2 with:

 rloc = zeros(size(rloc1));
 rloc(2:2:pix_count,:) = rloc1(pix_count:-1:pix_count/2+1,:);
 rloc(1:2:pix_count,:) = rloc1(1:pix_count/2,:);