Extending a Long Time Series Data Format with Missing Rows

Question

Let's say I have a dataframe:

df <- data.frame(group = c('A','A','A','B','B','B','C','C','C'), 
time = c(1,2,4,1,2,3,5,7,8), 
data = c(5,6,7,8,9,10,1,2,3))

What I want to do is insert data into a data frame where it was not in the sequence. So in the above example, I do not have enough data for time = 3 for group A, and time = 4 for group B and time = 6 for group C. I would essentially want to put NAs instead of the data column. How can I add these extra lines? I need a generalized solution. NOTE: I CHANGED THE QUESTION AS AN EARLIER ERROR. WE CAN'T ACCEPT THAT THERE WILL BE ONLY 4 REMARKS FOR EVERY GROUP.

Purpose:

  df <- data.frame(group = c('A','A','A','A','B','B','B','C','C','C','C'), 
    time = c(1,2,3,4,1,2,3,5,6,7,8), 
    data = c(5,6,NA,7,8,9,10,1,NA,2,3))

Answer 1

: data.table. 'data.frame' 'data.table' (setDT(df)), , '' min max 'time', on ' ".

library(data.table)
setDT(df)[df[, .(time = min(time):max(time)) , by = group], on = c("group", "time")]
#    group time data
# 1:     A    1    5
# 2:     A    2    6
# 3:     A    3   NA
# 4:     A    4    7
# 5:     B    1    8
# 6:     B    2    9
# 7:     B    3   10
# 8:     C    5    1
# 9:     C    6   NA
#10:     C    7    2
#11:     C    8    3