How to expand dataframe - r

Question

I have this kind of data:
df <- data.frame(year=c(1999,1999,1999,2000,2000,2001,2011,2011,2011,2011), class=c("A","B","C","A","C","A","B","C","D","E"),
n=c(10,20,30,12,15,40,50,55,60,5), occurs=c(0,1,3,4,2,0,0,11,12,2))
> df
year class n occurs
1 1999 A 10 0
2 1999 B 20 1
3 1999 C 30 3
4 2000 A 12 4
5 2000 C 15 2
6 2001 A 40 0
7 2011 B 50 0
8 2011 C 55 11
9 2011 D 60 12
10 2011 E 5 2
I would like to expand this data like this:
year class n occurs
1 1999 A 1 0
1 1999 A 2 0
1 1999 A 3 0
...
1 1999 A 10 0
2 1999 B 0 0
2 1999 B 1 0
2 1999 B 2 0
...
2 1999 B 20 1
3 1999 C 1 1
3 1999 C 1 1
3 1999 C 1 0
3 1999 C 1 0
.. the rest of occurs is seq of zeros...because `n-occurs` = 27 zeros and seq of 3x `1`.
I want to expand the rows n times as indicated by column n and so that the occurs column expands to flag 0 or 1 n-times according to the number of occurs columns number so if column occurs has interger 5 and column n = 10 then there will be n rows (year and class the same) and flags occurs 5 times zero and 5 times number one.
EDIT: Please note the new sequence of occurs (seq only of 0 and 1) is based on n-occurs for number of 0s and number of 1 is determined by number occurs.

Answer 1

Consider do.call and lapply calls using the data.frame() constructor with build of occurs:
df_List <- lapply(seq(nrow(df)), FUN=function(d){
occ <- c(rep(1, df$occurs[[d]]), rep(0, df$n[[d]]-df$occurs[[d]]))
data.frame(year=df$year[[d]], class=df$class[[d]], n=seq(df$n[[d]]), occurs=occ)
})
finaldf <- do.call(rbind, df_List)
head(finaldf, 20)
# year class n occurs
# 1 1999 A 1 0
# 2 1999 A 2 0
# 3 1999 A 3 0
# 4 1999 A 4 0
# 5 1999 A 5 0
# 6 1999 A 6 0
# 7 1999 A 7 0
# 8 1999 A 8 0
# 9 1999 A 9 0
# 10 1999 A 10 0
# 11 1999 B 1 1
# 12 1999 B 2 0
# 13 1999 B 3 0
# 14 1999 B 4 0
# 15 1999 B 5 0
# 16 1999 B 6 0
# 17 1999 B 7 0
# 18 1999 B 8 0
# 19 1999 B 9 0
# 20 1999 B 10 0

Answer 2

Here is a base R method that is closely related to the linked post here and in my comment above. The answer is provides the method for generating the first two columns of the data.frame.
dat <- data.frame(df[1:2][rep(1:nrow(df), df$n),],
n=sequence(df$n),
occurs=unlist(mapply(function(x, y) rep(0:1, c(x-y, y)), df$n, df$occurs)))
Here, the first 2 columns are generated using that answer. n is generated using sequence, and occurs uses mapply and rep, returning a vector with unlist. This puts the 1s at the end. You could use 1:0 to put the 1s at the beginning or feed the resulting vector to sample within mapply to get a random ordering of 1s and 0s.
We can check that the data.frame has the proper number of rows:
nrow(dat) == sum(df$n)
[1] TRUE
The first 15 observations of
head(dat, 15)
year class n occurs
1 1999 A 1 0
1.1 1999 A 2 0
1.2 1999 A 3 0
1.3 1999 A 4 0
1.4 1999 A 5 0
1.5 1999 A 6 0
1.6 1999 A 7 0
1.7 1999 A 8 0
1.8 1999 A 9 0
1.9 1999 A 10 0
2 1999 B 1 0
2.1 1999 B 2 0
2.2 1999 B 3 0
2.3 1999 B 4 0
2.4 1999 B 5 0