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

logical indexingMATLAB

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?

Best Answer

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

Related Solutions

MATLAB: Write a function to calculate the area of a circle

It isn't obvious from point 4 if it is expected that you just output -1 if any input is negative or -1 in only that location, but since you interpret it as the latter the simplest option would seem to be to use a logical mask as you have done, but you need to just always calculate

area = pi*r.^2;

Then you can just use the mask you created as B to overwrite the areas of any negative inputs with -1.

You can simplify the mask though as simply:

B = r < 0;

then

area( B ) = -1;

In your code you are not calculating any of the areas if any input is < 0. If that is what you want then it would seem that outputting just a scalar -1 would be sensible.

MATLAB: How to use for loop for below.

This line

function area = areaCircle(r)

means the function input is r and the output is area. So you need to assign the result of your calculation to the variable area like this:

area = pi.*(r.*r);

On top of this basic calculation, you need to account for negative inputs. So your function code will look something like this:

% You put text here to explain the function purpose
function area = areaCircle(r)
area = pi.*(r.*r);
% you put more code here to account for negative values that might be in r
end

So the only thing left for you to do is add that extra code for negative r values. I.e., change all elements of area to -1 where the corresponding r value is negative.

All of the other code you show above starting with the r1=2 line is NOT part of the function code, but is how you call the function from the command line or from other code.

Put the function code in a file called areaCircle.m, and then run your other code e.g. from the command line.

Best Answer

Related Solutions

MATLAB: Write a function to calculate the area of a circle

MATLAB: How to use for loop for below.

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