How do I do a conditional sum which only looks between certain date criteria

Question

Say I have data that looks like

date, user, items_bought, event_number
2013-01-01, x, 2, 1
2013-01-02, x, 1, 2
2013-01-03, x, 0, 3
2013-01-04, x, 0, 4
2013-0         


        

Answer 1

Here is a fairly simple method:

# replicate your data, shifting the days ahead by your required window,
# and rbind into a single data frame
d <- do.call(rbind,lapply(0:2, function(x) transform(data,date=date+x)))

# use aggregate to add it together, subsetting out "future" days
aggregate(items_bought~date+user,subset(d,date<=max(data$date)),sum)
         date user items_bought
1  2013-01-01    x            2
2  2013-01-02    x            3
3  2013-01-03    x            3
4  2013-01-04    x            1
5  2013-01-05    x            3
6  2013-01-06    x            4
7  2013-01-01    y            1
8  2013-01-02    y            2
9  2013-01-03    y            2
10 2013-01-04    y            6
11 2013-01-05    y           11
12 2013-01-06    y           12