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

Another option unsing reshape2 and working in the long format :

library(reshape2)
## reshape to long format
df1.m <- melt(df1)
df2.m <- melt(df2)
## get common values
idx <- df1.m$county %in% df2.m$county & 
       df1.m$variable%in% df2.m$variable
## replace NA values 
df1.m[idx,]$value <- ifelse(is.na(df1.m[idx,]$value),
                            df2.m$value , 
                            df1.m[idx,]$value)
## get the wide format
dcast(data=df1.m,county~variable)

  county year1 year2 year3
1     aa    10    20    30
2     bb     1     2     3
3     cc     5    10    15
4     dd   100   150   200