How to properly compare tensors?

Hi! I have the following function:

function toint = toint(p, r, R, dRdt, mu)
c = 29979245800; %cm/s
gamma = 1/sqrt(2*(1 - (dRdt/c).^2));
limitmin = R - deltR(R, dRdt);
limitmax = R;
minsize = size(limitmin)
maxsize = size(limitmax)
if (r > limitmin) & (r < limitmax)
    toint = r.^2./((gamma.^(p-1)).*(1-getbetastar(dRdt/c).*mu).^((p+3)/2))
else
   toint = 0
end
end

where r, R and limitmin are all 3D tensors. Now, I know they have the same size and the condition doesn't result with an error. But instead of getting a 3D tensor as the result of this function, I just get the one dimensional zero. This leads me to believe that the if condition checks if all corresponding elements of the tensors follow the condition - I don't want that, I want to get a 3D tensor containing values of zero/the value it calculated for the exprassion. What should I do? Thank you in advance!

Answer 1

[Dyuman Joshi]

Use logical indexing.

function toint = toint(p, r, R, dRdt, mu)
c = 29979245800; %cm/s
gamma = 1/sqrt(2*(1 - (dRdt/c).^2));
limitmin = R - deltR(R, dRdt);
limitmax = R;
minsize = size(limitmin)
maxsize = size(limitmax)

%Preallocate the array toint as zeros, so that you only have to assign
%values to indices corresponding to the condition
toint = zeros(size(r));

%Indices corresponding to the condition
idx = (r > limitmin) & (r < limitmax);

%Assuming r is the only non-scalar array here, and rest of the variables are scalars
%If any other variables are non-scalar, use the index idx for them as well
toint = r(idx).^2./((gamma.^(p-1)).*(1-getbetastar(dRdt/c).*mu).^((p+3)/2));
end

[Noya Linder]

The result is a vector and not a tensor, but just specifiying toint(idx) = ... in the last line of your answer fixes that right up. Thank you so much! Have a great morning/evening/whatever it is wherever you are

[Dyuman Joshi]

Yes, you are correct. Idk how I missed it, it must be the time for my evening coffee haha.

Have a good day as well!