Index must be a positive integer or logical. Very simple program.

Hey guys, I am pretty new to Matlab and to this Forum, so excuse me my question if stupid and obvious. I need to write a recursive code for a Newton sequence for f(x) (found in the matlab-code). Now the principe is simple, and I should get a line-vector with the entries a(1), a(2), ... Again: I am really new to Matlab, so maybe the error is really simple :P I googled for the same error, and simply don't get other peoples codes with the same error, so the answers are pretty useless for me.

function [a]=a5folge(n)

      function [f]=f(x)

          f(x)=((x^5)/5)-((2*x^3)/3)+x;

      end

      function [f1]=f1(x)

          f1(x)=(x^4)-(2*x^2)+1;

      end

      function [psi]=psi(x)

          psi(x)=x-(f(x)/f1(x));

      end

a=zeros(n+1);
a(1)=sqrt((25+2*sqrt(55))/27);

for k=1:n

      a(k+1)=psi(a(k));

end

end

thx in advance.

Answer 1

[Brian B]

You cannot use the same name for a function and for its output argument. Try

      function [val]=f(x)

          val=((x^5)/5)-((2*x^3)/3)+x;

      end

and similarly for the others.

[Vladislav]

Thanks a lot :) Now it's working how it should more or less :P.

Answer 2

[Image Analyst]

You don't say f(x) or f1(x), just say f and f1.

function fout = f(x)
  fout = ((x^5)/5)-((2*x^3)/3)+x;
end

When you had the (x) in there, it was thinking that you were trying to access the xth element of an existing array called f, which you don't have yet. And don't have the output be the same as the function name - this isn't Visual Basic (where you could do that).

[Vladislav]

Ok, thanks :) Going to remember that array-element-explanation. Helpful :)

Answer 3

[Azzi Abdelmalek]

function a=a5folge(n)
a=zeros(n+1);
a(1)=sqrt((25+2*sqrt(55))/27);
for k=1:n
  a(k+1)=psi(a(k));
end
end
function psi1=psi(x)
psi1=x-f(x)/f1(x);
end
function ff=f(x)
ff=((x^5)/5)-((2*x^3)/3)+x;
end
function ff1=f1(x)
ff1=(x^4)-(2*x^2)+1;
end