[R] Correctly applying aggregate.ts()

[Rich Shepard]

   I've read ?aggregate and several blog posts on using aggregate() yet I
still haven't applied it correctly to my dataframe. The sample data are:

structure(list(sampdate = c("2005-01-01", "2005-01-02", "2005-01-03", 
"2005-01-04", "2005-01-05", "2005-01-06", "2005-01-07", "2005-01-08", 
"2005-01-09", "2005-01-10", "2005-01-11", "2005-01-12", "2005-01-13", 
"2005-01-14", "2005-01-15", "2005-01-16", "2005-01-17", "2005-01-18", 
"2005-01-19", "2005-01-20", "2005-01-21", "2005-01-22", "2005-01-23", 
"2005-01-24", "2005-01-25", "2005-01-26", "2005-01-27", "2005-01-28", 
"2005-01-29", "2005-01-30", "2005-01-31", "2005-02-01", "2005-02-02", 
"2005-02-03", "2005-02-04", "2005-02-05", "2005-02-06", "2005-02-07", 
"2005-02-08", "2005-02-09", "2005-02-10", "2005-02-11", "2005-02-12", 
"2005-02-13", "2005-02-14", "2005-02-15", "2005-02-16", "2005-02-17", 
"2005-02-18", "2005-02-19", "2005-02-20", "2005-02-21", "2005-02-22", 
"2005-02-23", "2005-02-24", "2005-02-25", "2005-02-26", "2005-02-27", 
"2005-02-28", "2005-03-01", "2005-03-02", "2005-03-03"), prcp = c(0.59, 
0.08, 0.1, 0, 0, 0.02, 0.05, 0.1, 0, 0.02, 0, 0.05, 0.2, 0, 0, 
0.5, 0.41, 0.84, 0.01, 0.1, 0.01, 0, 0, 0, 0, 0.21, 0.24, 0.13, 
1.12, 0.01, 0.09, 0, 0, 0, 0.35, 0.18, 0.65, 0.16, 0, 0, 0, 0, 
0.55, 0.21, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0.17, 0.05, 
0.01, 0)), row.names = c(NA, 62L), class = "data.frame")

   What I need to learn how to do is to calculate monthly sum, median, and
maximum rainfall amounts from the full data set which has daily rainfall
amounts. My most current effort to calculate monthly sums uses this syntax:

monthly.rain <- aggregate.ts(x = dp['sampdate','prcp'], by = list(month = \
substr(dp$sampdate, 1, 7)), FUN = sum, na.rm = TRUE)

(entered on a single line) which produces this result:

head(monthly.rain)
[1] NA

   The sample data has 62 of the 113K rows in the dataframe. A larger set can
be provided if needed.

   An explanation of what I've missed is needed.

Regards,

Rich