a simple way to sum the value of certain rows in R

Question

I have a dataframe like this:

count  date
1233 2012-10-12     
3232 2012-10-12     
2334 2012-10-12     
3330 2012-10-13    
1445 2012-10-13     
3455 2012-10-13     
7632 2012-10-13     

I'd like to know the sum of count of each day, is any simple way to do that? Also, how to select the data of certain date with certain logic?

Answer 1

Let dat be your data frame, to get aggregation per day, you can use

aggregate(count ~ date, data = dat, FUN = sum)

To select data of a certain date, say "2012-10-12", you may do

subset(dat, date == "2012-10-12")

Answer 2

Using the plyr package assuming your data is called df you can do:

ddply(df, "date", numcolwise(sum))

Answer 3

Another base R option is xtabs

xtabs(count~date, dat)