How to get a single value of an array through a function inside a script?

Hello,

I have two questions, how can we use a variable defined in a script to be used later inside a function in the same script?

2nd, I am having trouble in trying to get a single value of an array which is processed inside a function inside a script. The function should return the u'th value of the array gE by storing it in the Eg(function return) but I get an error of

Index exceeds matrix dimensions.

Error in Slab_002>Esrc (line 187)

Eg = gE(u);

How can we fix this? All answers are welcome.

script something something

a = something;
gE = zeros(1, something);
dt = something;
STEPS = something;

function Eg = Esrc(u)
    
    a = dt*u;             %% I want to use the a that is defined above in the code.
    
    for i = 1:STEPS
        gE(i) = exp(-((t-t0-a)/tau).^2);        
    end
    
    Eg = gE(u);
    
 end

Answer 1

[Kevin Phung]

function Eg = Esrc(u,a)
    
    a = dt*u;             %% I want to use the a that is defined above in the code.
    
    for i = 1:STEPS
        gE(i) = exp(-((t-t0-a)/tau).^2);        
    end
    
    Eg = gE(u);
    
 end

[Stephen23]

This does not make much sense:

function Eg = Esrc(u,a)
    a = dt*u;
    ...
end

You added the inut argument a, but totally ignore that input argument inside the function by reallocating that variable name. What is the point of that?

[Zahid Saleem]

Well, I am trying to simulate maxwells equations using FDTD(Finite Difference Time domain), ome side of equation is temporal whereas the other side is spatial. I am fairly new to MATLAB so this was one of the easiest(inefficient) way to manage those equations differentials.