Isempty in while loop not working

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!

Answer 1

[Stephen23]

"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.

[Diogo Nascimento]

amen to that! I was looking to the wrong part of the problem then :S

thanks!