Piecewise Function with a Vector

I am writing a matlab about my work.

There is a piecewise function in my code.

somelike:

sqrt(2*(1-P/3)) when P<0;

sqrt(4-2*(1-P/2)) when P>0

P is a vertor, P = 1.4-2/5*f + something;

f is 0:0.001:20

there is my question

I don't quite get how to do a piecewise function. Because nomarlly piecewise function look likes:

f(x)=1 when x>0
f(x)=2 when x<0

x is not a vector.

In my work, the P is a vector. I can not do 1.4-2/5*f + something > 0 or < 0

Therefore please help me and give me the idea of how to do it

Kind Regards

Answer 1

[Walter Roberson]

function r = f(P)
  r = NaN(size(P));
  r(P<0) = sqrt(2*(1-P(P<0)/3));
  r(P>0) = sqrt(4-2*(1-P(P>0)/2));
end

Notice this will give you NaN at the locations in which P is exactly 0, as you did not define the result for that case.

[Xiaochen]

but my p is still a vector which P = 1.4-f.*(something/something) + something.

Could I still use P < 0 or P >= 0 in the function?

Thanks a lot

[Walter Roberson]

Yes, the above code is vectorized, suitable for P being a vector.

If you change the > to >= as in your comment, then you can also change the NaN(size(P)) to zeros(size(P)) which will be more efficient.