my newton raphson nested while function is not iterating well. What do I do?

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Tamunopiriye Whyte 2018-11-9

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评论： Tamunopiriye Whyte 2018-11-9

Hey, I am having an issue with my code. It is supposed to iterate and produce the value 0.2094 as my rootA but it keeps jumping from 0.20 (My Estimate) to 0.2074 and the iteration stops there. I tried to give it an additional condition i<50 and it would still print 0.200 until the last step and put 0.2074. Thank you in advance.

 function [rootA,rootB,rootC,y,i] = Isonewton1(Tr,Pr)
 %Isonewton the Newton Raphson function to obtain roots of a polynomial
 %   Detailed explanation goes here
 i=1; %my first iteration
 Pr= 0.45
 Tr= 0.85
 MV= [0.24:0.35:1.3]
 root_estimates = [0.15 0.35 1.9] % these are my three estimates of 3 roots.
 rootA(i)= root_estimates (1); % assiging of each root value with a value from my root estimate row vector.
 y=((8*Tr)/(8.*rootA(i)-1))-(27/(64.*rootA(i)^2))-Pr;% this is my y-value
 g=(27/(32.*rootA(i)^3))-((64*Tr)/(((8.*rootA(i)-1))^2)); 
 h=y/g;
 while (h<=0.001)
     rootA(i)= root_estimates (1);
    y=8.*Tr/(8.*rootA(i)-1)-27/(64.*rootA(i).^2)-Pr;  % this is my y-value 
    g=27/(32.*rootA(i).^3)-64.*Tr/(8.*rootA(i)-1).^2; 
    h=y/g;  %adjustment
    rootA(i+1)=rootA(i)-h; 
     i=i+1 ;
     P=max(rootA);
  end

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Jim Riggs 2018-11-9

I do not even understand what function you are trying to find roots for. Why does your y value calculation contain the "rootA" variable? Can you write the mathematical function for which you are trying to find roots?

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Answer 1

Jim Riggs 2018-11-9

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I haven't spent the time to understand all of your code, but one thing jumps out at me from the start. Inside the while loop, you keep resetting the root to the estimate.

rootA(i)= root_estimates (1);

Perhaps this statement should be outside the while loop.