Quantiles of a subset of data

That does it, thanks. Do you think you help me a little bit further?

I actually have 4 columns, YEAR, DAY, TEMP , and IBI. They are all
numeric. I need to calculate the average TEMP and IBI values between the
10% and 90% quantiles  for each YEAR.

The code 

by(data$day,data$year,day,c(0.1,0.9))

 was correct in that it calculated the quantile values as intended, but I
don't know how to then calculate the mean TEMP and IBI values encompasses
within those quantiles.

Thanks again,
David