Why won't my while loop terminate?

I'm basically trying to get the value of "F" be within 10% of the "W" value. When I run this code and watch the output, I can see the "forceErr" values drop below 10% but my while loop won't terminate for some reason. Can anyone figure out why?

format long

r = 0.1;
h = 0.5;
m = 6;
g = 9.81;
W = m*g;
rhoFluid = 1000;
forceErr = 1;

while forceErr > 0.1
    for z = 0:0.01:0.5
        pressure = rhoFluid*g*z;
        for theta = 0:pi/50:2*pi
         A = 1/2*r^2*theta;
         F = A*pressure;
         forceErr = abs((F - W)/W)
        end       
    end
end

Answer 1

[Cris LaPierre]

Because forceErr is never <= 0.1.

Your while loop is actually unnecessary here. It doesn't add to the solution. It will just recompute the exact same results as before. Everything is actually happening inside your for loops.

What you might want to do is replace the outer for loop with a while loop. Before the while loop, define an initial value for z. Then, inside the while loop, increment that value of z by some amount each time. This will allow your value for pressure to continue changing until a result is found that meets your stopping criteria.

Answer 2

[James Tursa]

The forceErr you are calculating happens for every iteration of the for-loop. The forceErr that the while-loop sees is only for the last iteration of the for-loop. I'm guessing you need to change your logic to look at each iteration. E.g., something like this logic:

for z = 0:0.01:0.5
    pressure = rhoFluid*g*z;
    for theta = 0:pi/50:2*pi
        A = 1/2*r^2*theta;
        F = A*pressure;
        forceErr = abs((F - W)/W)
        if forceErr <= 0.1
            break;
        end       
    end
    % this is where the break takes you
end

You will have to modify the above to code what needs to happen once you reach the desired forceErr condition. Maybe break out of the outer for-loop also? Are you trying to find the first place where z and theta give you the desired forceErr condition?