MATLAB: Problem with using variable of For

findfor loopMATLAB

Hello I have problem with for. I made for trying easy program which should demonstrate my problem.
function [p,j]=test
Q_q=[-0.3,-0.6,-0.3,0,0,0,0.3,0.3,0.6];
Delta=0.3;
place=1;
j=0;
for i=-0.9:Delta:0.9
  if((length(find(Q_q==i)))==0)
      j=j+1;
  end
  p(place,1)=(length(find(Q_q==i)));
  p(place,2)=i;
  if(p(place)>0)
      place=place+1;
  end
end
end
My problem is that length(find(Q_q==i)) doesn´t work. I mean if will write just
length(find(Q_q==-0,3))
answer is 2 but if I am going step by step all the way in the loop in moment where i=-0.3 then length(find(Q_q==i)) is still zero.
Can somebody tell me how where is the problem? And how to correct it?

Best Answer

NEVER test for exact equality of floating point numbers. Because, for example, it is impossible to store the number 0.3 EXACTLY as a floating point number. Remember that numbers are stored in a BINARY form. So just as you cannot write 1/3 exactly as a decimal, you cannot write the fraction 3/10 exactly in binary.
Use a tolerance. Something like
abs(Q_q - -0.3) < tol
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