Write a function to calculate the area of a circle

Hi everyone,

I need to write a function named areaCircle to calculate the area of a circle with the ff conditions: 1. The function should take one input that is the radius of the circle. 2. The function should work if the input is a scalar, vector, or matrix. 3. The function should return, one output, the same size as the input, that contains the area of a circle for each corresponding element. 4. If a negative radius passed as input, the function should return the value -1 to indicate the error.

I already have my code for the function that satisfies 1 to 3 except for 4. My code is as shown below:

function area = areaCircle(r)
if any(r<0)
  A = r % array of random integers
  negIndices = A < 0;
  B = A; % copy A into new array B
  B(negIndices) = -1  % replace the negative values in B with -1
  area = B(negIndices);
else 
  area = pi*r.^2;
end
end

And the test inputs area:

r1 = 2;
area1 = areaCircle(r1)
r2 = [2 5; 0.5 1];
area2 = areaCircle(r2)
r3 = [1 1.5 3 -4];
area3 = areaCircle(r3)

area3 must give an array of values where the last element must be -1 to show an error. It must not be used for the calculation of the area. Thanks in advance!

[Jyothsna Chowdary]

hey can u tell me where to put the test inputs . i am not getting any output for it

[Jose Raulito De Ocampo]

If you don't mind me asking, what does "negIndices" mean?

Answer 1

[Adam]

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.

[Mr. NailGuy]

Thanks! This solved my problem. I didn't knew that you can put the area calculation and masking in the if else loop as I was thinking the area = pi*r.^2 would be calculated first then B would no longer be able to mask the array and replace the negative radius.

[Adam]

The method I gave is a little inefficient in relative terms, since you are doing the area calculation for elements which you will later throw away, but since it is such a trivial calculation the time it takes to run will be negligible so it didn't seem worth creating a mask first and only calculating the area of those values. This could be done though, of course, and would be sensible if you were doing something similar where the calculation of each result takes a long time.

[Mr. NailGuy]

Yeah, that algorithm you mentioned was the one I was thinking, but since this worked even its not optimal, it suffice for me :)

[SAMARTH SHAH]

what exactly does area(B)=" some number" do? like B= 0 0 0 1 then what does -1 assigning do

[Image Analyst]

B is a binary image. If you use it as a logical index to an array, the operation will happen to those locations where B is true (1). So when B is a binary image of a circle, then setting area(B) to -1 will make the image of "area" have a value of -1 for every pixel in the circle defined by the "B" image.

Answer 2

[vedant mate]

You have just written a compicated code which is unecessary. You could simply do this:

function area = areaCircle(r)
area = pi*r.^2;
if any(r<0)
    B = r < 0;
    area (B) = -1; 
end
%as simple as that:)

[dheeraj tippani]

great. worked. been struggling since hours, surprised to that the answer is so simple. i didnt use 'any' in if condition. helpful. thanks man.

[ahmed samy]

plz can you explain to me why you put area(B)=-1;

if i put

if any(r>0)

B=r>0;

area(B)=1;

then the output not the same ..... thanks for help

[Stephen23]

@vedant mate: that IF you used is not required either.

Answer 3

[Garvit Amipara]

Here, with the following code you can give positive or negative- scalar or matrix input and get positive output.

function area = areacircle (r)
r = input('Enter the radius to calculate the area')
for j = size(r)
    i = 1:j;
area(i) = 2 * pi * (r(i).^2) ;
if area(i) < 0;
    B = -1 * area(i);
    area = B
end
end
end

[DGM]

The input argument r is immediately discarded, and then the user is forced to manually enter an array for no good reason.

The loop indexing is unnecessary and will be wrong for anything other than a row vector.

Contrary to the requirements (though I think it's a better choice), the code appears like it intends to return negative areas instead of a simple -1 flag. Despite that difference, area(i) is never negative, so no negative inputs will ever be represented in the output in any manner. If area(i) were ever negative, the entire output would be set to a positive scalar, regardless of the size of the input -- so it's wrong either way.

To top it all off, the calculated areas are all incorrect by a factor of 2.

If it were intended to return negative areas instead of what the problem asks for, then

function area = areacircle(r)
    area = pi*sign(r).*r.^2;
end

Answer 4

[Abdurasul Choriyorov]

function area = areaCircle(r)
for c = 1:length(r)
    if r(c)>=0
        area = pi.*r.^2;
    else
        b = r<0;
        area(b) = -1;
    end
end    
end

[DGM]

Assume all values are positive:

Consider a vector of length 100. You iterate through an unnecessary loop 100 times. Each time, you calculate all 100 outputs. The loop does nothing but waste time calculating the same thing and throwing it away 99 times.

Consider the following matrix: [1 2 3; 4 5 6]. You iterate through the loop 3 times, testing only the first three elements [1 4 2]. This is nonsense. Don't use length() unless you know what it does. As before, all intermediate results are simply discarded.

Now assume some values are negative:

The results will be incorrect unless r(max(size(r))) is negative.