R在去年同一月和同一天创建新的销售列

Question

Assuming I have a data table (not data frame) that has columns of Year, Month, Day, Sales, and a new column Sales Last Year (Similar to the picture below), how do I create that Sales Last Year column to grab the Sales from the same day and month but one year ago?

Data

dt <- data.table(Month = rep("January", times = 3),
                 Year = 2016:2018,
                 Day = rep(1, times = 3),
                 Sales = c(5000, 1000, 2000))

Expected result

Answer 1

Generate fake data to work with:

library(data.table)
dt <- data.table(
    day   = rep(1,3),
    month = rep(1, 3),
    year  = 2016:2018,
    sales = 1:3*500
)

A "merge" approach:

dtcopy <- copy(dt)
dtcopy[ , year := year + 1]
setnames(dtcopy, old="sales", new="sales_last_year")
merge(dt, dtcopy, by=c("month", "day", "year"), all.x=TRUE)

A "lag" approach:

dt <- dt[order(day, month, year)]
dt[ , sales_last_year := shift(sales), by=.(day, month)]