While-loop only iterates a single time

I'm trying to find a value for h by first guessing a value (0.00001<h<0.003) and then using a while loop to find the correct value.

W = 0.3;        %Channel width (m).
S = 0.006;      %Channel slope (m/m).
ks = 0.0013;    %Nikuradse roughness length (m).
Q = 1e-3;       %Water flow (m^3/s)

h = 0.0004;        %Initial water depth to start while-loop.
htemp = .9*h;     %Controlling value to determine if we are getting a better value for h.

while (h-htemp)>(1e-6)
    R = (W * h) / (W + 2*h);        %Calculate hydraulic radius.
    C = 18*log10((12*R)/ks);        %Calculate grain-related Chezy number.
    u = C*(h*S)^(.5);               %Calculate flow velocity.
    htemp = Q / (W*u);              %Calculate depth of the water.
    h = (h+htemp)/2
end

However, I only seem to get the while loop to do a single iteration, even though the end result does not satisfy the initial condition. How could I get this to work?

Answer 1

[Image Analyst]

Simple debugging would have let you know. But to be more explicit, try this where I added fprintf's to your code:

W = 0.3;        %Channel width (m).
S = 0.006;      %Channel slope (m/m).
ks = 0.0013;    %Nikuradse roughness length (m).
Q = 1e-3;       %Water flow (m^3/s)

h = 0.0004;        %Initial water depth to start while-loop.
htemp = .9*h;     %Controlling value to determine if we are getting a better value for h.

while (h-htemp)>(1e-6)
    R = (W * h) / (W + 2*h);        %Calculate hydraulic radius.
    C = 18*log10((12*R)/ks);        %Calculate grain-related Chezy number.
    u = C*(h*S)^(.5);               %Calculate flow velocity.
    htemp = Q / (W*u);              %Calculate depth of the water.
    h = (h+htemp)/2;
  fprintf('h = %f\n(h-htemp) = %f\n', h, (h-htemp));
  if (h-htemp)>(1e-6)
    fprintf('It is greater than 1e-6 so the loop will iterate again.\n');
  else
    fprintf('It is NOT greater than 1e-6 so the loop will not iterate again.\n');
  end
end

You will see

h = 0.105771
(h-htemp) = -0.105371
It is NOT greater than 1e-6 so the loop will not iterate again.

[Thom21]

A friend also kindly pointed me to this obvious flaw in my code. I changed it to abs(h-htemp), which consistently gives me the same answer :)