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

Question

Ben Spurr 2021-3-4

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https://ww2.mathworks.cn/matlabcentral/answers/762951-for-the-following-code-it-shows-my-51-iterations-however-i-only-want-to-display-the-last-iteration

评论： Ben Spurr 2021-3-4

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

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Ben Spurr 2021-3-4

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

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Answer 1

the cyclist 2021-3-4

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https://ww2.mathworks.cn/matlabcentral/answers/762951-for-the-following-code-it-shows-my-51-iterations-however-i-only-want-to-display-the-last-iteration#answer_639176

在 MATLAB Online 中打开

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']);

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the cyclist 2021-3-4

编辑：the cyclist 2021-3-4

在 MATLAB Online 中打开

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.

the cyclist 2021-3-4

在 MATLAB Online 中打开

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 2021-3-4

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!

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