index must be a positive integer or logical problem

Hi I can't see the problem in this code, would someone please help

 R=3.5;
  th = 0;
  for i = 1:0.01:180;
      SP(i) = (pi/2)*sin(th)*(1+((cos(th))/(R^2-(sin(th))^2)^0.5));
      th = th+0.01;
  end
  plot(th,SP)

If I change i to be 1:180 and th to be th=th+1 it works, however as soon as I put in the incremental change it gives me this error

Also the plot is wrong, giving me a straight line at one point instead of for the full range when it should be a curve with all positive values.

Could someone please explain/help

Thank you very much Tom

Answer 1

[the cyclist]

When you are looping with i=1:0.01:180, the second run through the loop has i=1.01. You are then trying to write into the "1.01"th element of the vector SP. That is what is causing the error.

EDIT:

You can do this without a for loop, by the way. (Also incorporated Paulo's correction for using degrees instead of radians for the input arguments.)

th = 0:0.01:180;
SP = (pi/2).*sind(th).*(1+((cosd(th))./(R^2-(sind(th)).^2).^0.5));
plot(th,SP)

[Paulo Silva]

Just a guess, you want to work with degrees not radians so the code might be

R=3.5;

th = 0;

n=1;

for i = 1:0.01:180;

SP(n) = (pi/2)*sind(th)*(1+((cosd(th))/(R^2-(sind(th))^2)^0.5));

th = th+0.01;

n=n+1;

end

plot(0:0.01:th,SP)

[the cyclist]

The reason that your plotting gave a straight line is that in your plotting command, you are plotting only the FINAL value of th (180), against all the values of SP, rather than 0:180 against SP.

[the cyclist]

+1 to Paulo's comment. sin() and cos() assume radian inputs.

[Paulo Silva]

+1 to you for the explanations and for removing the loop, I was away too many weeks, my MATLAB skills are returning slowly :)