matlab "arrayfun" function

Consider the following function, which takes a gray image (2D matrix) as inpu开发者_如何学编程t:

function r = fun1(img)
r = sum(sum(img));

I'm thinking of using arrayfun to process a series of images(3d matrix), thus eliminating the need for a for loop:

arrayfun(@fun1, imgStack);

But arrayfun tries to treat every element of imgStack as an input to fun1, the result of the previous operation also being a 3D matrix. How can I let arrayfun know that I want to repeat fun1 only on the 3rd dimension of imgStack?

Another question, does arrayfun invoke fun1 in parallel?

In this case, you don't need arrayfun to perform your calculation, you can simply do this:

imgStack = rand( 10, 10, 4 ); % 4 10x10 images
r = sum( sum( imgStack, 1 ), 2 ); % sum along both dimensions 1 and 2

In general, lots of MATLAB operations will operate on a whole array at once, that's the usual way of avoiding loops.

MATLAB's normal "arrayfun" is not parallel. However, for GPUArrays (with Parallel Computing Toolbox), there is a parallel version of arrayfun.

On your first question: You might try accumarray for this. One suggestion

function ds = applyfun_onfirstdim(arr, h_fun)

dimvec = size(arr);
indexarr = repmat( (1:dimvec(1))', [1, dimvec(2:end)] );

ds = accumarray(indexarr(:), arr(:), [], h_fun);

This creates an auxiliary index-array of the same dimensions as the input "arr". Every slice you want to apply h_fun to gets the same index-number. In this example it is the first.