Question

I have a very long data frame with different plots, dates of sampling and values.
An example of the data frame is below.

How could I calculate sums of column “value” for plot A and for plot B if the end dates are different?

For examples for plot A end date is 5.1.2010 and the sum would be 25 and for plot B is 3.1.2010 so the sum would be 12.
I hope the question is understandable?

Data frame look like this:

date <- c("1.1.10", "1.1.10", "2.1.10", "2.1.10", "3.1.10", "3.1.10", "4.1.10", "4.1.10", "5.1.10", "5.1.10")
date <- as.Date(date, "%d.%m.%y")
plot <- c("A","B","A","B","A","B","A","B","A","B")
value <- seq(1:10)
data.frame(date, plot, value)

Answer 1

first, names like ‘plot’ are already taken in R, so it’s good form to not assign values to them. First stick it into a data.frame, or similar, then use logicals:

    dates <- as.Date(c("1.1.10", "1.1.10", "2.1.10", "2.1.10", "3.1.10", "3.1.10",     "4.1.10", "4.1.10", "5.1.10", "5.1.10"), "%d.%m.%y")
    plots <- c("A","B","A","B","A","B","A","B","A","B")
    value <- seq(1:10)
    mydf <- data.frame(dates, plots, value)
    endA <- as.Date("5.1.10", "%d.%m.%y")
    endB <- as.Date("3.1.10", "%d.%m.%y")
    sum(mydf$value[mydf$plots=="A" & mydf$dates <= endA]) # 25
    sum(mydf$value[mydf$plots=="B" & mydf$dates <= endB]) # 12