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

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

[Torsten]

fsolve works with vector inputs, not with matrix inputs.

Best wishes

Torsten.

[MaxPr]

@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?

Answer 1

[David Goodmanson]

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.

[MaxPr]

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.