merge data frames to eliminate missing observations

Question

I have two data frames. One (df1) contains all columns and rows of interest, but includes missing observations. The other (df2) includes values t

Answer 1

aggregate can do this:

aggregate(. ~ county,
          data=merge(df1, df2, all=TRUE), # Merged data, including NAs
          na.action=na.pass,              # Aggregate rows with missing values...
          FUN=sum, na.rm=TRUE)            # ...but instruct "sum" to ignore them.
##   county year2 year3 year1
## 1     aa    20    30    10
## 2     bb     2     3     1
## 3     cc    10    15     5
## 4     dd   150   200   100