MATLAB: Isempty in while loop not working

isemptywhile loop

Hello! this might be a rookies mistake but I really cant understand whats happening… I have the code below, there is a large vector and I need to find where's a value in it. if the value doesnt exist I want to keep lowering it until I find one that does.
So in this case 0.4 doesn't exist neither 0.3 nor 0.2 but 0.1 does. the problem is that matlab runs for ever with my while loop and doesnt return k=1….
alpha=[0.1 0.5 0.9 1.3 1.9];
theta=0.4;
k = find(alpha==theta);
angle=theta;
while isempty(k)
    angle=angle-0.1;
    k = find(alpha==angle);
    fprintf('The solution did not converge for %.2f so the angle %.2f is used. Extend the number of iterations to obtain solution \n',theta,angle);
end
thanks in advance!

Best Answer

"this might be a rookies mistake"
It is. The mistake occurs here:
alpha==angle
Small differences in the floating-point values mean that those values are actually different. Floating-point numbers have been explained a thousand times on this forum:
https://www.mathworks.com/matlabcentral/answers/57444-faq-why-is-0-3-0-2-0-1-not-equal-to-zero
https://www.mathworks.com/matlabcentral/answers/69-why-does-1-2-3-1-3-not-equal-zero#answer_271
https://www.mathworks.com/matlabcentral/answers/316889-why-two-equal-numbers-are-not-equal
https://www.mathworks.com/matlabcentral/answers/41536-why-is-2-24-10-22-4-not-equal-to-0
https://www.mathworks.com/matlabcentral/answers/20830-1-0010-does-not-equal-1-0010
etc, etc, etc
Always use a tolerance when comparing floating point values.
Related Question