# fsolve Finite difference - Problem: "indices must either be real positive integers or logicals"

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MaxPr on 1 Mar 2017
Edited: MaxPr on 1 Mar 2017
Hello!
So I'm fairly new to matlab and I seem to run into an error I do not unterstand.
"Subscript indices must either be real positive integers or logicals."
Maybe somebody can spot my error easily?
close all;
clear all;
clc;
eta0=0;
etaInf=9;
deltaEta=0.1; %stepsize eta
N=(etaInf-eta0)/deltaEta+1; %number of nodes +1; later + number of nodes for the belt & the film ;
x0=0;
xInf=1;
deltaX=0.01; %stepsize x
M=(xInf-x0)/deltaX+1; %number of nodes x
anfangswert=horzcat(zeros(M,N),zeros(M,N),zeros(M,N)); %starting values, just zeros for now
%%Aufruf des Solvers
options=optimset('FinDiffType','central','MaxIter',10000);
Sol = fsolve(@(x)Keller_2D_Diskr_c(x(1:M,1:N),x(1:M,N+1:2*N),...
x(1:M,2*N+1:3*N),N,deltaEta,M,deltaX),anfangswert,options);
And the function:
function fval = Keller_2D_Diskr_c(f,g,H,N,deltaEta,M,deltaX)
% initialising space
fval1=zeros(M,N);fval2=zeros(M,N);fval3=zeros(M,N);
% Define functions of the Form F(X)=0
for i=1:M-1
for j=1:N-1
fval1(i,j)=((f(i,j)-f(i,j-1))/deltaEta)-g(i,j);
fval2(i,j)=((g(i,j)-g(i,j-1))/deltaEta)-H(i,j);
fval3(i,j)=((H(i,j)-H(i,j-1))/deltaEta)+f(i,j)*H(i,j)+...
2*x(i,j)*(H(i,j)*((f(i,j)+f(i,j-1)-f(i-1,j)-f(i-1,j-1))...
/2*deltaX)-g(i,j)*((g(i,j)+g(i,j-1)-g(i-1,j)-g(i-1,j-1))...
/2*deltaX));
end
end
%BC's
fval(:,1)=[-f(:,1);1-g(:,1);fval2(:,1)]; %Boundary values at eta=0
for i=2:N-1
fval(:,j)=[fval1(:,j-1);fval2(:,j);fval3(:,i-j)];
end
fval(:,N)=[fval1(:,N-1);-g(:,N);fval3(:,N-1)]; %Boundary values at eta=9
##### 2 CommentsShowHide 1 older comment
MaxPr on 1 Mar 2017
@Thorsten Hmmm... Not sure what you mean. In the documentation it says:
Nonlinear system solver
Solves a problem specified by
F(x) = 0
for x, where F(x) is a function that returns a vector value.
x is a vector or a matrix; see Matrix Arguments.
Am I on the wrong path here?

David Goodmanson on 1 Mar 2017
Hi Max, You did not mention where in the code the error occurred, but certainly you will get such an error with
for j=1:N-1
fval1(i,j)=((f(i,j)-f(i,j-1))/deltaEta)-g(i,j); % etc.
when j=1, since the index j-1 is zero.
##### 1 CommentShowHide None
MaxPr on 1 Mar 2017
Oh!! Thanks!! I'm so stupid. It should be
for j=2:N-1
I knew it had to be a small thing, but I couldn't see it.

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