"Array indices must be positive integers or logical values."

%define parameter
u= 1; %Liq velocity
D= 1; %Diffusion Coef

%domain and step

Lx= 81450; %length pipeline (m)

nx= 50; %num step in space(x)
nt= 100; %num time step
dx= Lx/(nx-1); %width space step

%satisfy CFL condition (ensure stability)
c=1; %speed
C=0.1; %Courant number (CFL condition <1)
dt= C*dx/c; %width each time step

%field variable
A = zeros(1,nx); %H2S concentration
x= linspace (0,Lx, nx); %distance

%initial condition
A(i)= 1; %conc. at initial
t=0; %at time=0

%loop
for n=1:nt 
    Ac= A; %save concentration into Ac for later used
    
    t=t+dt; %new time
    
    %new concentration
    
    for i=2:nx-1
        A(i)= Ac(i) + ((dt* D)/(dx*dx))* ((Ac(i+1)- 2*Ac(i)+ Ac(i-1)));
        
        
    end
    
    %boundary condition
    A(1)=0; A(end)=0; %Dirichlet
    
    %visualize
    plot(x,A); set(gca,'ylis', [0 100]);
    xlabel('Distance in pipeline'); ylabel('H2S concentration');
    title(sprintf ('H2S diffusion'));
    pause(0.01);
      
end

return;

[nurhamizah husna]

why there is array error and how to solve it?

[David Hill]

Your problem is you have not defined the variable "i" when you call "A(i) = 1;". So matlab doesn't know what index in A you want set to 1.

[Adam Danz]

It's not that Matlab doesn't know what i is, assuming it's not defined by the user. It's that i takes on the value of the imaginary unit when not otherwise defined.

>> clear
>> i
ans =
            0 +          1i

[nurhamizah husna]

Thank you for the explanation !

Answer 1

[Adam Danz]

If this is the complete script, the error is caused in this line where i is not defined and therefore takes the default value of the imaginary unit which is not a positive integer nor a logical value.

A(i)= 1; %conc. at initial

If this is not where the error is generated, provide the entire error message and indicate which line causes the error.