In R, Merge two data frames, fill down the blanks

Question

Say I have these two data frames:

big.table <- data.frame(\"idx\" = 1:100)

small.table <- data.frame(\"idx\" = sample(1:100, 10), \"color\" = sample(c         


        

Answer 1

A data.table solution:

require(data.table)
b <- data.table(big.table, key="idx")
s <- data.table(small.table, key="idx")
s[b, roll=T]

#      idx          color
#   1:   1             NA
#   2:   2             NA
#   3:   3             NA
#   4:   4          blue3
#   5:   5          blue3
#   6:   6          blue3
#   7:   7          blue3
#   8:   8          blue3
#   9:   9          blue3
#  10:  10          blue3
#  11:  11   navajowhite1
#  12:  12   navajowhite1
#  . . . .

Answer 2

It sounds like you need na.locf from the package zoo (stands for last observation carried forward):

library(zoo)
tbl <- merge(small.table, big.table, by = "idx", all.y=TRUE)
tbl$color2 <- na.locf(tbl$color,na.rm = FALSE)