Generate entries in time series data - r

Question

I want to generate a row (with zero ammount) for each missing month (until the current) in the following dataframe. Can you please give me a hand in this? Thanks!
trans_date ammount
1 2004-12-01 2968.91
2 2005-04-01 500.62
3 2005-05-01 434.30
4 2005-06-01 549.15
5 2005-07-01 276.77
6 2005-09-01 548.64
7 2005-10-01 761.69
8 2005-11-01 636.77
9 2005-12-01 1517.58
10 2006-03-01 719.09
11 2006-04-01 1231.88
12 2006-05-01 580.46
13 2006-07-01 1468.43
14 2006-10-01 692.22
15 2006-11-01 505.81
16 2006-12-01 1589.70
17 2007-03-01 1559.82
18 2007-06-01 764.98
19 2007-07-01 964.77
20 2007-09-01 405.18
21 2007-11-01 112.42
22 2007-12-01 1134.08
23 2008-02-01 269.72
24 2008-03-01 208.96
25 2008-04-01 353.58
26 2008-05-01 756.00
27 2008-06-01 747.85
28 2008-07-01 781.62
29 2008-09-01 195.36
30 2008-10-01 424.24
31 2008-12-01 166.23
32 2009-02-01 237.11
33 2009-04-01 110.94
34 2009-07-01 191.29
35 2009-11-01 153.42
36 2009-12-01 222.87
37 2010-09-01 1259.97
38 2010-11-01 375.61
39 2010-12-01 496.48
40 2011-02-01 360.07
41 2011-03-01 324.95
42 2011-04-01 566.93
43 2011-06-01 281.19
44 2011-08-01 428.04
'data.frame': 44 obs. of 2 variables:
$ trans_date : Date, format: "2004-12-01" "2005-04-01" "2005-05-01" "2005-06-01" ...
$ ammount: num 2969 501 434 549 277 ...

Answer 1

you can use seq.Date and merge:
> str(df)
'data.frame': 44 obs. of 2 variables:
$ trans_date: Date, format: "2004-12-01" "2005-04-01" "2005-05-01" "2005-06-01" ...
$ ammount : num 2969 501 434 549 277 ...
> mns <- data.frame(trans_date = seq.Date(min(df$trans_date), max(df$trans_date), by = "month"))
> df2 <- merge(mns, df, all = TRUE)
> df2$ammount <- ifelse(is.na(df2$ammount), 0, df2$ammount)
> head(df2)
trans_date ammount
1 2004-12-01 2968.91
2 2005-01-01 0.00
3 2005-02-01 0.00
4 2005-03-01 0.00
5 2005-04-01 500.62
6 2005-05-01 434.30
and if you need months until current, use this:
mns <- data.frame(trans_date = seq.Date(min(df$trans_date), Sys.Date(), by = "month"))
note that it is sufficient to call simply seq instead of seq.Date if the parameters are Date class.

Answer 2

If you're using xts objects, you can use timeBasedSeq and merge.xts. Assuming your original data is in an object Data:
# create xts object:
# no comma on the first subset (Data['ammount']) keeps column name;
# as.Date needs a vector, so use comma (Data[,'trans_date'])
x <- xts(Data['ammount'],as.Date(Data[,'trans_date']))
# create a time-based vector from 2004-12-01 to 2011-08-01. The "m" denotes
# monthly time-steps. By default this returns a yearmon class. Use
# retclass="Date" to return a Date vector.
d <- timeBasedSeq(paste(start(x),end(x),"m",sep="/"), retclass="Date")
# merge x with an "empty" xts object, xts(,d), filling with zeros
y <- merge(x,xts(,d),fill=0)