[R] interpolating a lot of data at once

[David Hugh-Jones]

I have a big dataset containing a lot of values for 1970, 1980 and
1990. I want to interpolate values for the years in between, and also
if possible to extrapolate to 1968 and 1969. The method doesn't have
to be clever but I am looking for a function that will do all the data
at once. (Doing foreach, or apply, is just too slow!) Is there
something that will take

list(df$val.1970, df$val.1980, df$val.1990)

as inputs and output an interpolated matrix?

Cheers
David