Here's one way to do it using plyr
and ddply
.
I use ddply sequentially, first to get the first and last rows of each month, and again to calculate the monthlyReturn.
(Perhaps using xts or zoo might be easier, I am not sure.)
#Using plyr and the data in df
df$Date <- as.POSIXlt(as.Date(df$Date, "%m/%d/%Y"))
df$Month <- (df$Date$mon + 1) #0 = January
sdf <- df[,-1] #drop the Date Column, ddply doesn't like it
library("plyr")
#this function is called with 2 row data frames
monthlyReturn<- function(df) {
(df$Value[2] - df$Value[1])/(df$Value[1])
}
adf <- ddply(sdf, .(Month), function(x) x[c(1, nrow(x)), ]) #get first and last values for each Month
mon.returns <- ddply(adf, .(Month), monthlyReturn)
Here's the data I used to test it out:
> df
Date Value
1 1/1/2000 10.0
2 1/31/2000 10.1
3 2/1/2000 10.2
4 2/28/2000 11.0
5 3/1/2000 10.0
6 3/31/2000 24.1
7 5/10/2000 510.0
8 5/22/2000 522.0
9 6/04/2000 604.0
10 7/03/2000 10.1
11 7/30/2000 7.2
12 12/28/2000 11.0
13 12/30/2000 3.0
> mon.returns
Month V1
1 1 0.01000000
2 2 0.07843137
3 3 1.41000000
4 5 0.02352941
5 6 0.00000000
6 7 -0.28712871
7 12 -0.72727273
Hope that helps.