count number of rows in a data frame in R based on group

Question

I have a data frame in R like this:

  ID   MONTH-YEAR   VALUE
  110   JAN. 2012     1000
  111   JAN. 2012     2000
         .         .
         .         .
  121   FEB. 2012     3000
  131   FEB. 2012     4000
         .           .
         .           .

So, for each month of each year there are n rows and they can be in any order(mean they all are not in continuity and are at breaks). I want to calculate how many rows are there for each MONTH-YEAR i.e. how many rows are there for JAN. 2012, how many for FEB. 2012 and so on. Something like this:

 MONTH-YEAR   NUMBER OF ROWS
 JAN. 2012     10
 FEB. 2012     13
 MAR. 2012     6
 APR. 2012     9

I tried to do this:

n_row <- nrow(dat1_frame %.% group_by(MONTH-YEAR))

but it does not produce the desired output.How can I do that?

Answer 1

The count() function in plyr does what you want:

library(plyr)

count(mydf, "MONTH-YEAR")