在 data.table 中查找按 2 个因子分组的两个变量的变化

Question

I have plots which were visited at irregular intervals to record biomass of several species. I would like to record the change in each species's biomass, and the duration of the interval, at the beginning of the interval

Sample data are

plot <- c(rep(1,4), rep(2,3))
species <- c(rep(c('a','b'), 2), rep('a',3))
year <- c(1,1,3,3,2,5,13)
biom <- c(5,2,8,4,3,9,18)

DT <- data.table(plot=plot, sp=sp,year=year,biom=biom)

The desired output would look like

elapsed = c(2,2,NA,NA,3,8,NA)
dbiom = c(3,2,NA,NA,6,9,NA)

(eg, change in biomass of species a in plot 1, first survey in year 1 to second survey in year 2, was +3, and the elapsed time was 2 years)

I have been using the 'shift' operator in data.table but I cannot get it to work

setkey(DT, plot, sp , year)
cols = c("year","biom")
anscols = paste("lead", cols, sep="_")

b4 <- b3[ , (anscols) := shift(.SD, 1, NA, type = "lead"), 
     .SDcols=cols, by = c(plot, sp)]

I keep getting 'Error in eval(bysub, x, parent.frame()): object '1' not found'

Answer 1

Would something like this do the trick?

library(data.table)

plot <- c(rep(1,4), rep(2,3))
sp <- c(rep(c('a','b'), 2), rep('a',3))
year <- c(1,1,3,3,2,5,13)
biom <- c(5,2,8,4,3,9,18)

DT <- data.table(plot=plot, sp=sp,year=year,biom=biom)

elapsed = c(2,2,NA,NA,3,8,NA)
dbiom = c(3,2,NA,NA,6,9,NA)

DT[
  order(plot, year),
  .(year, biom,
    elapsed = year - shift(year),
    dbiom = biom - shift(biom)
  ),
  by = c("sp", "plot")
  ]