recursive calculations of tseries

Topics: Times Series
Jun 6, 2014 at 7:19 PM
Hi,

I'm wondering if it's possible to recursively calculate 'var1' as shown in the following example in IRIS. All variables are tseries objects.

e.g.
var1(date range) = var1{-1} + var2(and other calculations)

The "+ var2" part here is just for illustration, it could be any kind of calculations.

Many thanks.
Coordinator
Jun 6, 2014 at 8:27 PM
Edited Jun 6, 2014 at 8:34 PM
Yes. Use the function arf (type help tseries/arf to see help on this function).

In general, if you want to evaluate recursively an expression like

x = a1 x{-1} + a2 x{-2} + ... + ak x{-k} + y

you call the function in the following way:
x = arf(x,[1,-a1,-a2,...,-ak],y,range)
where range is the range on which the expression will be evaluated; of course, the initial values must exist in the tseries object x before you run this function. Note also how the autoregressive polynomial is entered: [1,-a1,-a2,...,-ak].

In your particular example, you'd run
var1 = arf(var1,[1,-1],var2,date_range);
Hope this helps.

Best,
Jaromir
Jun 6, 2014 at 9:04 PM
Many thanks, Jaromir. It definitely helps!

Can you apply other functions for var1, for example:

var1(date_range) =exp(var1{-1}) + var2
Jun 6, 2014 at 9:11 PM
or this

var1(date_range) = log((var2+1) * exp(var1{-1}))

Thanks a lot!
Coordinator
Jun 6, 2014 at 9:17 PM
Unfortunately, the function arf only works for linear models at the moment.

But I can create a general function that would evaluate any recursive expression. Feel free to submit a request for such a function.

Best,
Jaromir
Jun 9, 2014 at 1:15 PM
Thank you very much Jaromir. Will submit a request.