Matlab Vectorising matrix subtraction using bsxfun

Question

I have an array Y=rand(1000,28); I want to find the columnwise difference thus:

[~ c]=size(Y);

for i=1:c-1
Y(:,i)-Y(:,i+1)
end

Can I do this using bsxfun?

Answer 1

You can also do this using the function diff:

dY = -diff(Y, [], 2)

The [] and 2 tell diff to operate along the second dimension of Y, as specified in your question.

Note that this is actually faster, because diff is a built-in function:

>> Y = rand(100, 10000);
>> tic; for n = 1:1000; dY = -diff(Y, [] , 2); end; toc
Elapsed time is 5.453160 seconds.
>> tic; for n = 1:1000; dY = Y(:,1:end-1) - Y(:,2:end); end; toc
Elapsed time is 11.383666 seconds.

Edit: It has been suggested to use the timeit function to calculate these timings more accurately; the results are:

>> timeit(@()-diff(Y, [] , 2))

ans =

    0.0071

>> timeit(@()Y(:,1:end-1) - Y(:,2:end))

ans =

    0.0119

Also, putting these in an m-file appears not to make a difference in this case.

Answer 2

Don't use bsxfun. Do it the easy way:

dY = Y(:,1:end-1) - Y(:,2:end)