提取上次预测

Question

我有以下data.frame：

forecast_date      applicable_at        forecasted_value
 2008-01-01         2008-01-10            155
 2008-01-04         2008-01-10            152
 2008-01-06         2008-01-10            144
 2008-01-02         2008-01-10            145
 2008-01-03         2008-01-10            148
 2008-01-05         2008-01-10            151

.............................................

data.frame将持续到2015-12-20

我想从这个data.frame中提取最后一个预测：

forecast_date      applicable_at        forecasted_value
 2008-01-06         2008-01-10            144

我尝试使用data.table package和setDT功能，但我无法正确提取它。你能帮帮我吗？此外，我无法用子集函数解决这个问题。

我试过了：

newDF <- setDT(df)[, list(LastDate = max(forecast_date)), by = list(applicable_at)] 

和

newDF <- subset(df, df$applicable_at == df$applicable_at & max(df$forecast_date)) 

Answer 1

text1="forecast_date      applicable_at        forecasted_value
2008-01-01         2008-01-10            155
2008-01-04         2008-01-10            152
2008-01-06         2008-01-10            144
2008-01-02         2008-01-10            145
2008-01-03         2008-01-15            148
2008-01-05         2008-01-15            151"

dat <- read.table(text=text1, head=T, as.is=T)

使用dplyr：

library(dplyr)

dat %>% group_by(applicable_at) %>% filter(forecast_date==max(forecast_date))
# Source: local data frame [2 x 3]
# Groups: applicable_at

#   forecast_date applicable_at forecasted_value
# 1    2008-01-06    2008-01-10              144
# 2    2008-01-05    2008-01-15              151

使用data.table：

library(data.table)
dt <- data.table(dat)
setkey(dt, applicable_at)

dt[, V1:=max(forecast_date), by=applicable_at][forecast_date == V1]

<强>更新

setDT(dat)[, V1:=max(forecast_date), by=applicable_at][forecast_date == V1]