Using logical indexing to modify non-negative elements of a matrix

I need to pass a matrix of radii to the function areaCircle and have it output a matrix the same size of the input with the respective areas in the entries with non-negative radius and -1 in the entries with negative radius as error signaling.

I am trying this

function area = areaCircle(r)
nim = r < 0  %negative-index matrix
r(nim) = -1  % sets the negative entries of the r matrix to -1
area = zeros(size(r));

area(nim)= pi.*r.*r ;% ? this doesn't work
end

How can I achieve it?

[madhan ravi]

What does non-begative mean?

[Juan Carlos Vega Oliver]

just a typo, I meant non-negative. Thanks

Answer 1

[Alan Stevens]

Try

function area = areaCircle(r)
nim = r < 0;  %negative-index matrix
area = zeros(size(r));

area= pi.*r.*r ;
area(nim) = -1;  % sets the negative entries of the area matrix to -1

end

[Juan Carlos Vega Oliver]

Thanks, that worked

[Juan Carlos Vega Oliver]

Can logical indexing be used to modify the matrix before the operation is done, as I was trying to do?. This might not work if instead of raising to a power I had to take log or or do a square root

[Alan Stevens]

You could do

r(nim) = NaN;

before the area/log/power etc operation.