Using return values of functions without storing them?

Is there an elegant way to save variables by combining commands? Example: Eigenvalues of a 2x2-Matrix, b= [2 4; 9 7]. Compute the eigenvalues using eig(b) which will return 'ans = [-2; 11]'. If I wanted to know only the second eigenvalue and use that in a further reference, I'd have to store the answer of 'eig' - is there a way around this? I'm looking for something like 'z=pi * eig(b)(2,1);' (which doesn't work, because eig(b) is not recognized as a vector) instead of 'q=eig(b); z=pi * q(2,1);' Can anyone help with this?

Answer 1

[Teja Muppirala]

As has been noted above, what you are trying to do is not possible. But even when you nest functions inside of each other, MATLAB still has to implicitly create temporary variables in memory.

This will probably surprise you (the second loop is significantly faster):

A = rand(1000);

tic
for n = 1:100
    B1 = sum(A.^2);
end
toc

tic
for k = 1:100
    tmp = A.^2;
    B2 = sum(tmp);
end
toc

isequal(B1,B2)

So the point is, don't worry about creating temporary variables.

Answer 2

[Sean de Wolski]

save2 = @(x)x(2,1);
z = pi*save2(eig(b));

Does what you want but is probably not faster.

[Daniel Shub]

and I don't think it qualifies as elegant.

[Titus Edelhofer]

Hi Alexander,

no, currently this is not possible. But be assured, MATLAB has plenty of variables ... ;-)

[Sean de Wolski]

@Daniel: I think it's elegant if you're going to use it multiple times; for one or two times - yes; looks kinda lame-o.

[Walter Roberson]

There is also a way to do it using subsref(), but it is less elegant than Sean's method.