Summing rows by month in R

Question

So I have a data frame that has a date column, an hour column and a series of other numerical columns. Each row in the data frame is 1 hour of 1 day for an entire year.

Answer 1

Another base R solution

# to sum by date
rowsum(dat[-1], dat$Date)
#           Hour Melbourne Southern Flagstaff
#2009-05-01   21         0      496       715

# or by month and year
rowsum(dat[-1], format(dat$Date, "%b-%y") )
#       Hour Melbourne Southern Flagstaff
#May-09   21         0      496       715