if statement not working

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Bodille Blomaard
Bodille Blomaard 2018-1-8
评论： Bodille Blomaard 2018-1-8
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

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采纳的回答

Walter Roberson
Walter Roberson 2018-1-8
mask = XNew < 0;
X = a;
X(mask) = -X(mask);

更多回答（1 个）

Jan
Jan 2018-1-8
编辑：Jan 2018-1-8
"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.
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Bodille Blomaard
Bodille Blomaard 2018-1-8
Yes indeed, I want to look at the individual members. I just don't understand why it doe work for the Y.
Jan
Jan 2018-1-8
编辑：Jan 2018-1-8
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.

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