Need Help, Error in for loop

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CM
CM 2017-3-9
回答： John BG 2017-3-9
I have to find the root of an equation, but I get an error saying 'Error using * , and also an error in line 12 but.
T=1.2;
P=(0:2:10);
A=0.45724.*(P/T.^2).*(1+0.392.*(1-T.^0.5)).^2;
B=0.0778.*(P/T);
f=@(Z) Z.^3-(1-B).*Z.^2+(A-B.*2-3.*B.^2).*Z-(A.*B-B.^2-B.^3);
xl=0;
xu=6;
xm=0;
maxiter=15;
for i=1:maxiter;
xold=xm;
if f(xm)*f(xu)<0; %<-- Line 12
xl=xm;
else
xu=xm;
end
xm=(xu+xl)/2;
ea=abs((xm-xold)/xm);
xold=xm;
end

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采纳的回答

Walter Roberson
Walter Roberson 2017-3-9
You have
P=(0:2:10);
so P is a vector.
B=0.0778.*(P/T);
B involves P, so B is a vector.
f=@(Z) Z.^3-(1-B).*Z.^2+(A-B.*2-3.*B.^2).*Z-(A.*B-B.^2-B.^3);
f(Z) involves B, so f(Z) is a vector.
Your code is expecting f(Z) to be a scalar.

更多回答（1 个）

John BG
John BG 2017-3-9
Hi CM
got your code to work, but not sure the result is what you expect, please confirm
T=1.2;
maxiter=15;
P=[0:2:10];
% P=linspace(0,10,maxiter)
A=0.45724.*(P/T).^2.*(1+0.392.*(1-T.^0.5)).^2;
B=0.0778.*(P/T);
f=@(Z,A,B) Z^3-(1-B)*Z^2+(A-B*2-3*B^2)*Z-(A*B-B^2-B^3);
% Y=Z^3-(1-B)*Z^2+(A-B*2-3*B^2)*Z-(A*B-B^2-B^3);
xl=0;
xu=6;
xm=0;
for i=1:maxiter;
xold=xm;
if f(xm,A,B)*f(xu,A,B)<0; %<-- Line 12
xl=xm;
else
xu=xm;
end
xm=(xu+xl)/2;
ea=abs((xm-xold)/xm);
xold=xm;
end
I have added as comment the lines
P=linspace(0,10,maxiter)
Y=Z^3-(1-B)*Z^2+(A-B*2-3*B^2)*Z-(A*B-B^2-B^3);
because there may be a way to simplify, remove at least the for loop.
if you find this answer useful would you please be so kind to mark my answer as Accepted Answer?
To any other reader, please if you find this answer of any help,
please click on the thumbs-up vote link,
thanks in advance
John BG
<mailto:jgb2012@sky.com jgb2012@sky.com>

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