if statement not working

If the Xnew is negative the answer for X should also be negative, but this is not the case. The Y does work correctly however. I included the input array, the r is constantly 13.

function [X,Y,Z] = bolcoordinaten2(Xnew,Ynew,r)
Z = r-cos(2*atan((abs(sqrt((Xnew.^2)+(Ynew.^2))))./(2*r))).*r;
a = cos(atan(Ynew./Xnew)).*r.*sin(2*atan((abs(sqrt((Xnew.^2)+(Ynew.^2))))./(2*r)));
    if Xnew < 0
        X = a*(-1);
    else
        X = a;
    end
b = sin(atan(Ynew./Xnew)).*r.*sin(2*atan((abs(sqrt((Xnew.^2)+(Ynew.^2))))./(2*r)));
    if Ynew < 0
        Y = b*(-1);
    else
        Y= b;
    end
end

Answer 1

[Walter Roberson]

mask = XNew < 0;
X = a;
X(mask) = -X(mask);

[Bodille Blomaard]

Thank you!! it worked!

Answer 2

[Jan]

 "If the Xnew is negative the answer for X should also be negative"

As code considering that a could be negative also:

    if Xnew < 0
        X = -abs(a);
    else
        X = a;
    end

Do you want that X is positive, if Xnew is positive also? Then:

    else
       X = abs(a);
    end

Alternatively without IF:

X = sign(Xnew) * abs(a);

But this would set X to 0 if Xnew is 0.

The code can be accelerated by avoiding repeated calculations:

function [X,Y,Z] = bolcoordinaten2(Xnew,Ynew,r)
c = atan(abs(sqrt(Xnew.^2 + Ynew.^2)) ./ r);
d = atan(Ynew ./ Xnew);
Z = r - cos(c) .* r;
X = cos(d) .* r .* sin(c);
if Xnew < 0
  X = -abs(X);
end
Y = sin(d) .* r .* sin(c);
if Ynew < 0
  Y = -abs(Y);
end
end

[EDITED] Ah, your inputs are vectors. I cannot run Matlab currently, such that I do not know what's in the attached MAT file. Please note that "does not work" is not useful to describe a problem. Better explain what you get and what you want instead.

For vectors use logical indexing:

X       = abs(cos(d) .* r .* sin(c));  % Or without abs()?
negX    = (Xnew < 0);
X(negX) = -X(negX);

Your code might work for Y, because of the sign of sin() in the applied range. But for other input values the shown method might fail also.

[Bodille Blomaard]

unfortunately this doesn't work also

[Walter Roberson]

I predict that your inputs are vectors or arrays rather than scalars.

 if Xnew < 0

means the same thing as

 if all(Xnew < 0)

whereas you probably only want to act upon the individual members that are negative.

[Bodille Blomaard]

Yes indeed, I want to look at the individual members. I just don't understand why it doe work for the Y.

[Jan]

Ah, the inputs are vectors. See [EDITED].

You can find out what happens if you use the debugger: https://www.mathworks.com/help/matlab/matlab_prog/debugging-process-and-features.html. Note that if Xnew < 0 is executed as if all(Xnew < 0) internally. If XNew has different signs, this will not do, what you expect.