[R] problem with NA and if

[Keld Jørn Simonsen]

Hi 

I would like to sum a number of time series, some of them having NA's

Standard action is here that if I sum a value with a NA, then the result
is NA. I would like it to just keep the value.

I then try to:

 a = NA; if (a == NA) { a = 0}

just to try it out, but it says

Error in if (a == NA) { : missing value where TRUE/FALSE needed

What is wrong, and can I do it smarter? I looked at na.action but I
don't see how it affects addition of vectors, nor time series.

Best regards
keld