I am having a problem to run this code. It may not seem stopped when run. What's the solution?

g = @(f) 1/sqrt(f) + 2 * log10((RR)/3.7 + 2.51/(Re * sqrt(f)));
x_lower=0.008;
x_upper=0.0001;
x_mid=(x_lower* x_upper)/2;
while abs (g(x_mid))>0.0001
    if (g(x_mid))<0
        x_lower=x_mid
    elseif (g(x_mid))>0
        x_upper=x_mid
    else
        x_upper=x_lower
        fprintf('%f', x_mid)
        return
    end
    x_mid=(x_lower+x_upper)/2;
end
disp(x_mid)

[Voss]

What are the values of RR and Re?

And are you sure there is a zero in the region you are looking for it (knowing RR and Re will tell us this)?

Also, is this a typo or do you mean to have x_mid be half the product of x_lower and x_upper?

x_mid=(x_lower* x_upper)/2;

Later x_mid is the arithmetic mean (half the sum):

x_mid=(x_lower+x_upper)/2;

Also, it may not really matter because it's just names, but you have x_lower > x_upper. Is this intentional?

[Akash Talapatra]

The vaules of RR and Re are 0.005 and 30000. And yes, you identified my problems here in this code and really solved my problem. Your help is highly appreciable.

Answer 1

[Image Analyst]

Unrecognized function or variable 'RR'.

Error in test7>@(f)1/sqrt(f)+2*log10((RR)/3.7+2.51/(Re*sqrt(f))) (line 13)

g = @(f) 1/sqrt(f) + 2 * log10((RR)/3.7 + 2.51/(Re * sqrt(f)));

Error in test7 (line 17)

while abs (g(x_mid))>0.0001

So the fix is that you have to define RR. Use a failsafe to limit the number of iterations. Use break instead of return. You also need to use ismembertol() to see if your x_mid is close enough to your prior/last value (since you can't check equality with ==).

See the FAQ: https://matlab.fandom.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F

Possible fix:

% Demo by Image Analyst
% Initialization Steps.
clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;

% Define variables
RR = 10
Re = 9
maxIterations = 1000; % Failsafe.  Way more iterations than you ever expect.
loopCounter = 0; % Failsafe.  Way more iterations than you ever expect.
g = @(f) 1/sqrt(f) + 2 * log10((RR)/3.7 + 2.51/(Re * sqrt(f)));
x_lower=0.008;
x_upper=0.0001;
x_mid=(x_lower* x_upper)/2;
% Bisection search:
while (abs(g(x_mid)) > 0.0001) && (loopCounter < maxIterations)
    if (g(x_mid)) < 0
        x_lower=x_mid;
        fprintf('%d, case 1: Original x_mid = %.9f, x_lower = %.9f, x_upper = %.9f, ', loopCounter, x_mid, x_lower, x_upper)
    elseif (g(x_mid))>0
        x_upper=x_mid;
        fprintf('%d, case 2: Original x_mid = %.9f, x_lower = %.9f, x_upper = %.9f, ', loopCounter, x_mid, x_lower, x_upper)
    else
        x_upper=x_lower;
        fprintf('%d, case 3: Original x_mid = %.9f, x_lower = %.9f, x_upper = %.9f, ', loopCounter, x_mid, x_lower, x_upper)
        break
    end
    % Capture old value for comparison.
    x_mid_Prior = x_mid;
    % Create new value.
    x_mid=(x_lower+x_upper)/2;
    fprintf(' final x_mid = %.9f\n', x_mid);
    loopCounter = loopCounter + 1;
    % Break out if the value is not changing much.
    if ismembertol(x_mid, x_mid_Prior, 1e-7)
        fprintf('Success!  Found a solution at iteration %d with x_mid = %.9f.\n', loopCounter, x_mid)
        break;
    end
end
if loopCounter == maxIterations
    % If you bailed out because of the failsafe, you get here.
    warningMessage = sprintf('WARNING: loop exited after %d iterations without finding a solution.', loopCounter)
    uiwait(warndlg(warningMessage))
end