Filling in values in a data frame in R?

Suppose I have this data frame:

  times vals
1     1    2
2     3    4
3     7    6

set up with

foo <- data.frame(times=c(1,3,7), vals=c(2,4,6))

and I want this one:

  times vals
1     1    2
2     2    2
3     3    4
4     4    4
5     5    4
6     6    4
7     7    6

That is, I want to fill in all the times from 1 to 7, and fill in the vals from the latest time that is not greater than the given time.

I have some code to do it using dplyr, but it is ugly. Suggestions for better?

library(dplyr)

foo <- merge(foo, data.frame(times=1:max(foo$times)), all.y=TRUE)
foo2 <- merge(foo, foo, by=c(), suffixes=c('', '.1'))

foo2 <- foo2 %>% filter(is.na(vals) & !is.na(vals.1) & times.1 <= times) %>%
  group_by(times) %>% arrange(-times.1) %>% mutate(rn = row_number()) %>%
  filter(rn == 1) %>%
  mutate(vals = vals.1,
         rn = NULL,
         vals.1 = NULL,
         times.1 = NULL)

foo <- merge(foo, foo2, by=c('times'), all.x=TRUE, suffixes=c('', '.2'))
foo <- mutate(foo,
              vals = ifelse(is.na(vals), vals.2, vals),
              vals.2 = NULL)

Answer

library(data.table)

setDT(foo)[.(1:7), on = 'times', roll = T]
#   times vals
#1:     1    2
#2:     2    2
#3:     3    4
#4:     4    4
#5:     5    4
#6:     6    4
#7:     7    6

setDT(foo)[.(times = 1:7), on = 'times', roll = T]

Attribution
Source : Link , Question Author : dfrankow , Answer Author : eddi