For the following code, It shows my 51 iterations, however i only want to display the last iteration

clc; clear all; close all;
prompt = 'What is the value of the relative pipe roughness, e/d? ';
m = input(prompt);
prompt = 'What is the value of the Reynolds number, Re? ';
Re = input(prompt);
g=@(f) 1/sqrt(f) + 2*log10((m*1/3.71)+2.51/(Re*sqrt(f)));

xLeft = 0.008; % lower limit x value initial guess
xRight = 0.08; % upper limit x value initial guess
fLeft = g(xLeft); % function of xLeft
fRight = g(xRight); % function of xRight
nIteration = 0;

if (fLeft*fRight>0)
    error('The fLeft and fRight value should have different signs');
end

for i = 0:50 
    err = abs((xLeft-xRight)/xLeft)*100; 
    xAvg = (xLeft+xRight)/2; 
    fAvg = g(xAvg); 
    
    if (fAvg*fLeft>0) 
        xLeft = xAvg;
        fLeft = fAvg; 
    else             
        xRight = xAvg;  
        fRight = fAvg;
    end
    nIteration = nIteration+1; 
    disp(['Error as percentage: ',num2str(err)]); 
    disp(['friction factor: ', num2str(xLeft)]);
    disp(['Solution reached in ',num2str(nIteration),' iterations']);
end

[Ben Spurr]

This is for the bisection method to find the roots of the Colebrook White equation.

Answer 1

[the cyclist]

Looks like you could just pull these lines out, and put them after the end statement of the loop:

disp(['Error as percentage: ',num2str(err)]); 
disp(['friction factor: ', num2str(xLeft)]);
disp(['Solution reached in ',num2str(nIteration),' iterations']);

[Ben Spurr]

Thanks for the quick repsonse.

I just tried that and still has same issue.

Is there a way to stop the loop when the 'Error as percentage' and 'friction factor' repeat, as after 26 itterations these become the same for 26 onwards.

[the cyclist]

Sorry, are you saying that if you run the code

clc; clear all; close all;
prompt = 'What is the value of the relative pipe roughness, e/d? ';
m = input(prompt);
prompt = 'What is the value of the Reynolds number, Re? ';
Re = input(prompt);
g=@(f) 1/sqrt(f) + 2*log10((m*1/3.71)+2.51/(Re*sqrt(f)));
xLeft = 0.008; % lower limit x value initial guess
xRight = 0.08; % upper limit x value initial guess
fLeft = g(xLeft); % function of xLeft
fRight = g(xRight); % function of xRight
nIteration = 0;
if (fLeft*fRight>0)
    error('The fLeft and fRight value should have different signs');
end
for i = 0:50 
    err = abs((xLeft-xRight)/xLeft)*100; 
    xAvg = (xLeft+xRight)/2; 
    fAvg = g(xAvg); 
    
    if (fAvg*fLeft>0) 
        xLeft = xAvg;
        fLeft = fAvg; 
    else             
        xRight = xAvg;  
        fRight = fAvg;
    end
    nIteration = nIteration+1; 
end

% These lines are now outside the for loop
disp(['Error as percentage: ',num2str(err)]); 
disp(['friction factor: ', num2str(xLeft)]);
disp(['Solution reached in ',num2str(nIteration),' iterations']);

you display all 51 iterations? That's not what happens for me.

To break out of the loop early, based on some condition, you can do

if %<code the condition here>
    break
end

I recommend not basing the break-out condition on a strict equality between floating point numbers, because of possible round-off error. Instead, set some tolerance, and test if the condition is met within that tolerance.

[Ben Spurr]

Thank you so much!

Yes if i run the code it displays all of the iterations, using the break function is a good idea.

Will check it out and look at setting a tolerance, that makes sense.

Thanks again, was very helpful.

[the cyclist]

I'd be curious to see the output you are seeing.

What are you using as the inputs? I tried lots of different values, but I keep hitting your error message.

[Ben Spurr]

The example i have been using is the following:

e/d = 0.002

Re = 2e6

The error message usually comes up if the values are greater

[the cyclist]

Running your code right here, and I see only the last iteration's results displayed. Not sure why you have something else.

clc; clear all; close all;
% prompt = 'What is the value of the relative pipe roughness, e/d? ';
% m = input(prompt);
m = 0.002;
% prompt = 'What is the value of the Reynolds number, Re? ';
% Re = input(prompt);
Re = 2e6;
g=@(f) 1/sqrt(f) + 2*log10((m*1/3.71)+2.51/(Re*sqrt(f)));
xLeft = 0.008; % lower limit x value initial guess
xRight = 0.08; % upper limit x value initial guess
fLeft = g(xLeft); % function of xLeft
fRight = g(xRight); % function of xRight
nIteration = 0;
if (fLeft*fRight>0)
    error('The fLeft and fRight value should have different signs');
end
for i = 0:50 
    err = abs((xLeft-xRight)/xLeft)*100; 
    xAvg = (xLeft+xRight)/2; 
    fAvg = g(xAvg); 
    
    if (fAvg*fLeft>0) 
        xLeft = xAvg;
        fLeft = fAvg; 
    else             
        xRight = xAvg;  
        fRight = fAvg;
    end
    nIteration = nIteration+1; 
end

% These lines are now outside the for loop
disp(['Error as percentage: ',num2str(err)]); 

Error as percentage: 2.6577e-13

disp(['friction factor: ', num2str(xLeft)]);

friction factor: 0.023498

disp(['Solution reached in ',num2str(nIteration),' iterations']);

Solution reached in 51 iterations

[Ben Spurr]

I have just adjusted my code and the output gives just the values i want. It seems that the problem was that the disp functions should be after the initial for loop closes. Thanks for the help and looks good now!