3*3 matrix in loop - calculation give NaN, why ?

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N
N 2014-2-18
评论： Mischa Kim 2014-2-18
I have a problem with calculating some data out a matrix that is in a loop. Below you can see the code. With this I can calculate a 3*3 matrix 202 times, which is relor. When I just let matlab show me this ( so no ; at the end), I can see that I get 202 times a 3*3 matrix. So this is good. However then I want to do some extra calculations on every matrix (KneeAngle), when I do this however I suddenly get NaN for every calculation of the KneeAngle. I cannot figure out why since I know that relor has data in it, matlab has shown me this already.
for i = 1:202;
origin2=(LE(:,i)+ME(:,i))/2;
Y2=(FH(:,i)-origin2)/norm(FH(:,i)-origin2);
ztemp2=(FH(:,i)-ME(:,i))/norm(FH(:,i)-ME(:,i));
x2=cross(Y2,ztemp2);
X2=x2/norm(x2);
z2=cross(X2,Y2);
Z2=z2/norm(z2);
R2=[X2 Y2 Z2];
midpoint=(LM(:,i)+MM(:,i))/2;
mid=(MLP(:,i)+MMP(:,i))/2;
Y3=(mid-midpoint)/norm(mid-midpoint);
ztemp3=(LM(:,i)-MM(:,i))/norm(LM(:,i)-MM(:,i));
x3=cross(Y3,ztemp3);
X3=x3/norm(x3);
z3=cross(X3,Y3);
Z3=z3/norm(z3);
R3=[X3 Y3 Z3];
relor(:,:,i)=R2\R3
KneeAngle(i:1) = asin(relor(3,2)); % Alpha
KneeAngle(i:2) = -atan((relor(3,1))/(relor(3,3))); % Beta
KneeAngle(i:3) = -atan((relor(1,2))/(relor(2,2))); % Gamma
plot(KneeAngle)
end
If I just give in the code below I do get a result ! but when I use i instead of a real number, its back to NaN. Can anybody help !?
r1=relor(:,:,15)
alpha=asin(r1(3,2))

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回答（1 个）

Mischa Kim
Mischa Kim 2014-2-18
编辑：Mischa Kim 2014-2-18
This can happen for a range of reasons. In your case I would suspect that some of the values in relor are zero. Dividing zero by zero will result in NaN.
One way to test if there are zeros in a matrix is
isequal(relor,find(relor))
which will result in a logic 0, if the matrix contains zeros.
  2 个评论
N
N 2014-2-18
编辑：N 2014-2-18
okay some are indeed zer0. But relor is done 202 times, some times its Nan other times it has data. So when I calculate the KneeAngle shouldn't I then also get a line of numbers that are sometimes NaN and other times give data ?
Mischa Kim
Mischa Kim 2014-2-18
What exactly goes on and why is hard to say from a distance. Can you attach your data and complete code?

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