How to "vectorize" this operation?

Question

Peter Laslo 2013-4-12

关闭： MATLAB Answer Bot 2021-8-20

Hi,

I have data of sample size m of n by n matrices in an n by m*n matrix call it P.

I also have a function, call if f, that operates on a fixed vector, call it v, and n by n matrices.

I want to to create a 1 by m vector, call it d, by operating f on v and each of the n by n matrices in P.

So, say for example, n = 3 and m = 6 I would want:

d(1) = f(v,P(:,1:3)), d(2) = f(v,P(:,4:6)), . . ., d(6) = f(v,P(:,16:18))

How can I do this without making a loop?

Thanks!

Peter Laslo 2013-4-12

f is a real-valued function of the the vector v and the matrices. So I'm trying to create 1 by m vector of real-numbers.

Matt J 2013-4-13

编辑：Matt J 2013-4-13

Whether that's possible without a loop would depend on the specific form of f(), i.e., the operations that f consists of.

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Answer 1

Matt J 2013-4-12

This looks like it avoids a for-loop, but doesn't really.

 P=reshape(P,n,n,m);
 d= arrayfun(@(i) f(v,P(:,:,i))  , 1:m);

Peter Laslo 2013-4-12

编辑：Peter Laslo 2013-4-12

Thanks, I thought this would work:

a = 1:m;

d = f(v,P(:,:,a))

but the function tries to operate on P all at once (instead of the individual matrices) and I get an error.