Attempted to access y(2); index out of bounds because numel(y)=1

I am trying to make sir epidemic model uding ode45 but constantly I am getting this error "Attempted to access y(2); index out of bounds because numel(y)=1".please suggest some method to remove this.
thank you

    if true
    % code
function sir_model = sir_model(t,y)
    a=1;
    b=1;
sir_model(1) = -a*y(1)*y(2);
sir_model(2)= a*y(1)*y(2) -b*y(2);
sir_model(3)= b*y(2);
sir_model=[sir_model(1) sir_model(2) sir_model(3)]';
  end

Answer 1

[Youssef Khmou]

hi y's length must >2 : try :

 H=sir_model(t,rand(2,1));

Answer 2

[Image Analyst]

Are you trying to do a recursive function? If so, I see no exit condition. Or are you just use to Visual Basic where you set the return argument of a function by setting the function name equal to something?

Since sir_model is a function, I don't think this will work:

sir_model(1) = -a*y(1)*y(2);

Then to make it worse, you're recursing back into sir_model three separate times with this statement:

sir_model=[sir_model(1) sir_model(2) sir_model(3)]';

As if that weren't bad enough, you're passing in a y that's just a single value (1, 2, or 3), not an array of two numbers.

By chance do you mean this:

function sir_model_output = sir_model(t,y)
    a = 1;
    b = 1;
    term1 = -a * y(1) * y(2);
    term2 =  a * y(1) * y(2) - b * y(2);
    term3 =  b * y(2);
    sir_model_output = [term1, term2, term3]';