Need Help, Error in for loop

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

Answer 1

[Walter Roberson]

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.

Answer 2

[John BG]

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

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