Unintended infinite loop with `imshow` - matlab

Question

for i = [I, J, K];
imshow(i);
end
I, J, K are 16-bit images.
The script keeps trying to pump out the images (but doesn't) and gets into an infinite loop.
Is there something I'm missing?

Answer 1

You can store multiple images in a matrix if they have the same sizes.
However, You should store the images in a cell if they have different sizes. This method is less messier because you don't need to worry about how to extract them later.
define a cell with the size equal to the number of images.
numImages = 3;
Images = cell(1,numImage);
Store an image I into a cell:
Images{1,1} = I;
Now go over the images and show them
for ii = 1:3
imshow(Images{1,ii});
end
Example:
I = imread('cameraman.tif');
J = imread('peppers.png');
K = imread('snowflakes.png');
Images = cell(1,3);
Images{1,1} = I;
Images{1,2} = J;
Images{1,3} = K;
for ii=1:numel(Images)
figure;imshow(Images{1,ii});
end

Answer 2

For better understanding as to which point exactly you are missing/what is happening here: Using square brackets works like a concatenation here. So the line
i = [I, J, K] % separated with commas or spaces for horzcat
i = [I; J; K] % separated with semi-colons for vertcat
does the same as horzcat or vertcat:
i = horzcat(I, J, K);
i = vertcat(I, J, K);
Let's say I, J, K are 64x64 gray valued images. A (horizontal) concatenation will create a 64x192 matrix. The for loop will go through your matrix column-wise, which means it will extract a 64x1 vector 192-times (or much more often for larger images, which might feel like "infinite"). Displaying only a vector with imshow() won't show anything.
As already pointed out, using cells is a more flexible way to store images. When using arrays you have to handle each dimension (and this only works if your images are equal in size):
sizeImage = size(I); % assume all img are same size (as I)
numImages = 3; % needed for allocating array
% init array for imgs and fill images into array: e.g. 64x64x3
imageArray = zeros([sizeImage numImages]);
imageArray(:,:,1) = I; % :,: selects all elements of a dimension
imageArray(:,:,2) = J;
imageArray(:,:,3) = K;
for n = 1:numImages % iterate over image index
figure; imshow(imageArray(:,:,n)); % n = 1, 2 ... , numImages
end % is used for position in imageArray
Using the colon : when accessing arrays/cells selects all elements of a dimension. E.g. imageArray(:,:,n) will select all elements of first and second dimension, corresponding to a 64x64 image. For RGB images an array with 3 images will be 64x64x3x3 and you'll have to use imageArray(:,:,:,n) to select all three color channels.
Note that using for i = img_array won't work, as this will give vectors again.
E.g. for img_array: 64x64x5 (five 64x64 gray-valued images), this will iterate over all but one dimensions (and assign the remaining dim to i): img_array(:,1,1), img_array(:,2,1), img_array(:,3,1), ..., img_array(:,1,2), img_array(:,2,2) ..., img_array(:,64,5) and will again produce 64*3 = 192 vectors for i.
As was already pointed out, if you have variable image sizes, using cell arrays is the way. You may want to consult: Difference between cell and matrix in matlab?