How to find what the value of a variable is in a loop using another variable in the loop?

Question

Mark Kelly 2013-11-5

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/104951-how-to-find-what-the-value-of-a-variable-is-in-a-loop-using-another-variable-in-the-loop

回答： Image Analyst 2013-11-5

Hi,

Just trying to figure out how to get a value of a changing value in a loop if I have another value that is also looping. For example, here I am trying to figure out what motor speed a certain torque will give me:

   for i=1:2000
   %Increase Current through loop
   I(i)=-0.1+(i*0.1);
   %Calculate RPM with increasing Current
   Omega(i)=((Ebat-(I(i)*R))/Ke)/((2*pi)/60);
   %Calculate Torque with increasing Current
   Torque(i) = Kt * I(i);
   %This is my current attempt at getting the motor speed at a certain value of      
   %torque, however the torque is never exactly equal to TorqueReq which is a   %constant 
   if Torque(i) == TorqueReq
   x=MotorSpeed(i)
   end

So If I have a certain value for torque, how can I find the corresponding value for speed at that torque? Thanks for any help!

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Answer 1

Image Analyst 2013-11-5

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/104951-how-to-find-what-the-value-of-a-variable-is-in-a-loop-using-another-variable-in-the-loop#answer_114206

在 MATLAB Online 中打开

You need to read the FAQ, http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F, about why you should not compare floating point numbers for exact matches.

I fixed your code to check within a tolerance, as according to the FAQ:

tolerance = 0.001; % or some small number
for k=1:2000
  
  %Increase Current through loop
  I(k)=-0.1+(k*0.1);
  
  %Calculate RPM with increasing Current
  Omega(k)=((Ebat-(I(k)*R))/Ke)/((2*pi)/60);
  
  %Calculate Torque with increasing Current
  Torque(k) = Kt * I(k);
  
  %This is my current attempt at getting the motor speed.
  % TorqueReq is a constant.
  if abs(Torque(k)- TorqueReq) < tolerance
    x=MotorSpeed(k)
    break; % if you want to exit the loop.
  end
end