Multiple if loops with warning message variable appears to change size with every iteration

I am aware this might not be the only problem with the code but I honestly can't think of anything else.

Other than the fact that it keeps saying the matrix for the index is not compatible...

So if anyone can help in time I will be eternally greatful

function mymodel(x1,x2)
x1=3;
x2=5;
for i =[x1,x2]
    if (x1>0 && x2>0) 
        mymodel(i)=(x1\x2)
    elseif (x1>0||x2>0) 
        mymodel(i)= 2.*(x1\x2)
        if (x1<0 && x2<0) 
            mymodel(i)= (x2\x1)
        else mymodel(i) = 0
        end

Obviously it needs to compute a different variable for each iteration.

It does not seem to want to compute negative numbers at all. Which I would need.

Helps pls!