Can anyone tell me why I'm getting an error here? "xold=xnew;'' is giving me an error in this statement and I'm not sure why.

Question

Brendan ，2023-2-22

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https://ww2.mathworks.cn/matlabcentral/answers/1916680-can-anyone-tell-me-why-i-m-getting-an-error-here-xold-xnew-is-giving-me-an-error-in-this-statem

回答： KSSV ，2023-2-22

clear all 
close all
clc
% Problem 6.7
while (1)
fx=@(x) x.^3-6*x.^2+11*x-6.1;
% Using the Modified Secant Method 
xold=3.5;
d=0.01;
% Iteration 1
xold=xnew;
xnew=xold-d*xold*fx(xold)/(fx(xold+d*xold)-fx(xold));
ea=abs((xnew-xold)/xnew)*100;
% Iteration 2
xold=xnew;
xnew=xold-d*xold*fx(xold)/(fx(xold+d*xold)-fx(xold));
ea=abs((xnew-xold)/xnew)*100;
% Iteration 3
xold=xnew;
xnew=xold-d*xold*fx(xold)/(fx(xold+d*xold)-fx(xold));
ea=abs((xnew-xold)/xnew)*100;
fprintf('The root using 3 iterations of Modified Secant Method is %8.4f with an approximate relative error of %6.3e\n',xnew,ea)
end

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Brendan 2023-2-22

I would also like to keep an argument attached to this code.

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Answer 1

KSSV 2023-2-22

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1916680-can-anyone-tell-me-why-i-m-getting-an-error-here-xold-xnew-is-giving-me-an-error-in-this-statem#answer_1177055

These lines:

xold=3.5;
d=0.01;
% Iteration 1
xold=xnew;
xnew=xold-d*xold*fx(xold)/(fx(xold+d*xold)-fx(xold));

will obviously thrwo an error becuase xnew is a new variable and it is not defined. You need to either comment this line or you may need to move to the next line. After calculating xnew then save xnew to xold.