[R] behavior of ewma function

[Leeds, Mark (IED)]

I have the ewma function as shown below. I think I copied it from an
oldSplus help page on filter and
then modified it with a lot of help from Achim.

ewma<-function(x,lambda = 1, init = x[1]) {

rval<-filter(lambda*coredata(x),filter=(1-lambda),method="recursive",ini
t=init)
rval<-zoo(coredata(rval),index(x))
rval
}

It sort of works but , if there are NA values in the input series such
as

x<-c(NA,2,3,4,5,NA,6,7,8,9,10,11,12,13,14,15)

y<-ewma(x,.01)

y

  1         2         3         4         5         6         7
8         9        10        11        12        13        14        15
16 

0.0000000 0.0200000 0.0498000 0.0893020 0.1384090  NA 0.0000000
0.0700000 0.1493000 0.2378070 0.3354289 0.4420746 0.5576539 0.6820774
0.8152566 0.9571040

So, ewma starts off with a zero if there is an NA and this is misleading
in terms of the exponentially weighted
moving average numbers calculated going forward. Maybe this has
something to do
Woth the fact that I set init equal x[1] ? The NA in the 6th spot is
fine but is there any way to 

1) return an NA value when the input into the series is an NA ?

2) start the exponentially weighted moving average at the next non NA
value ?

So, essentually, for the example above, I would prefer the output to be

NA, 2, 2*.01 + .99*3 = 2.99, 2.99*.01 + .99*4 = 3.9899, 3.9899*.01+
0.99*5 = 4.989, 4.989 ( because of the NA ),
4.989*.01 + 0.99* 6 = 5.989 etc.

This might be too complex to get but atleast is it possible to get NA
where the inputs were NA  and not use zeros
for smoothing going forward ? Thanks.








 





P.S : The code in ewma may look a little different than one is used to
but that is because it is sometimes run with
The inputs being zoo objects so coredate and index are needed.
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