code_01.R

library(plyr)

# Sample data
dat = data.frame(
    time = c(2,3,4,1,2,3,4,3,4,5),
    percent = c(2,23,100,7,40,60,100,40,60,100),
    variable = c(rep("a", 3), rep("b", 4), rep("c", 3))
)
dat

##    time percent variable
## 1     2       2        a
## 2     3      23        a
## 3     4     100        a
## 4     1       7        b
## 5     2      40        b
## 6     3      60        b
## 7     4     100        b
## 8     3      40        c
## 9     4      60        c
## 10    5     100        c

# Expand to fill missing cases
grid = expand.grid(
    time = seq(min(dat$time), max(dat$time)),
    variable = unique(dat$variable)
)
dat = merge(grid, dat, by = c("variable", "time"), all.x = TRUE)

# Fill with 0 or 100
dat = ddply(
    dat,
    "variable",
    transform,
    flag = as.logical(cumprod(is.na(percent)))
)
dat$percent[dat$flag & is.na(dat$percent)] = 0
dat$percent[!dat$flag & is.na(dat$percent)] = 100
dat$flag = NULL

# Result
dat

##    variable time percent
## 1         a    1       0
## 2         a    2       2
## 3         a    3      23
## 4         a    4     100
## 5         a    5     100
## 6         b    1       7
## 7         b    2      40
## 8         b    3      60
## 9         b    4     100
## 10        b    5     100
## 11        c    1       0
## 12        c    2       0
## 13        c    3      40
## 14        c    4      60
## 15        c    5     100