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

Answer 1

library(plyr)
ddply(data, .(MONTH-YEAR), nrow)

This will give you the answer, if "MONTH-YEAR" is a variable. First, try unique(data$MONTH-YEAR) and see if it returns unique values (no duplicates).

Then above simple split-apply-combine will return what you are looking for.