在data.table中按多个列分组创建一个新列 [英] create a new column in a data.table from group by multiple columns

问题描述

我正在处理一个包含X和Y列的data.table，我想创建一个新列Z，该列Z是具有相同值(X，Y)的所有记录的数目.

I'm working on a data.table that includes X and Y columns and I want to create a new column Z which is the number of all records with the same value of (X, Y).

我知道使用data.frame时的语法:

I know the syntax when working with a data.frame:

ddply(df,.(X,Y),nrow)

我测试了在该论坛上发现的不同语法，但是它们不起作用:

I tested different syntaxes I found on this forum but they didn't work:

dt[, Z := lapply(.SD,nrow), by="X,Y"] # or   
dt[, `:=`(Z = lapply(.SD,nrow)), by="X,Y"]   

我精确地表示X和Y是数字.

I precise X and Y are numeric.

推荐答案

从

library(data.table)
dt <- data.table(X = c(1, 1, 2), Y = c(1, 1, 2))

适当的语法是

dt[, Z := .N, by = c("X","Y")]

或

dt[, Z := .N, by = .(X,Y)]

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