用by参数连接data.table [英] Joining data.table with by argument

问题描述

我有两个数据.表 dx 和 dy

dx <- data.table(a = c(1,1,1,1,2,2), b = 3:8)
dy <- data.table(a = c(1,1,2), c = 7:9)

我想将 dy 加入 dx 的每一行，下面是所需的输出

I want to join dy to each row of dx, and below is the desired output

data.table(plyr::ddply(dx, c("a", "b"), function(d) merge(d, dy, by = "a")))
    a b c
 1: 1 3 7
 2: 1 3 8
 3: 1 4 7
 4: 1 4 8
 5: 1 5 7
 6: 1 5 8
 7: 1 6 7
 8: 1 6 8
 9: 2 7 9
10: 2 8 9

但是，我仅使用 data.table 或 merge 的 [] 内部的操作无法输出?我累了

However, I failed to make the output only using operation inside [] of data.table or merge? I have tired

merge(dx, dy, by = "a", all = TRUE)

vecseq(f__，len__，if(allow.cartesian || notjoin ||！anyDuplicated(f__，:连接结果为10行；大于9 = nrow(x)+ nrow(i).检查i中是否有重复的键值，每个键值都一遍又一遍地连接到x中的同一组.如果可以，请尝试按= .EACHI为每个组运行j，以避免分配过多.如果确定要继续，请使用allow.cartesian = TRUE重新运行.否则，请在FAQ，Wiki，堆栈溢出和数据表帮助中搜索此错误消息，以获取建议.

dy[dx,on="a"]

vecseq(f__，len__，if(allow.cartesian || notjoin ||！anyDuplicated(f__，:连接结果为10行；大于9 = nrow(x)+ nrow(i).检查i中是否有重复的键值，每个键值都一遍又一遍地连接到x中的同一组.如果可以，请尝试按= .EACHI为每个组运行j，以避免分配过多.如果确定要继续，请使用allow.cartesian = TRUE重新运行.否则，请在FAQ，Wiki，堆栈溢出和数据表帮助中搜索此错误消息，以获取建议.

dx[, merge(dy, by = "a"), by = c("a", "b")]

is.data.table(y)中的错误:缺少参数"y"，没有默认值

dx[, merge(.SD, dy, by = "a"), by = c("a", "b")]

merge.data.table(.SD，dy，by ="a")中的错误: by 中列出的元素必须是x和y中的有效列名

Error in merge.data.table(.SD, dy, by = "a") : Elements listed in by must be valid column names in x and y

我该怎么做呢?

谢谢！

推荐答案

如果我正确理解了您的要求，则可以使用直接合并选项，

If, I understood your requirement correctly, There is a direct merge option that you can use,

dx <- data.table(a = c(1,1,2,2), b = 3:6)
dy <- data.table(a = c(1,1,2), c = 7:9)
merge(x = dx, y = dy, by = "a", all = TRUE)

它给出您所提到的所需输出.如何连接(合并)数据框(内部，外部，左，右)?

It gives your desired output that you mentioned. How to join (merge) data frames (inner, outer, left, right)?

我希望它能消除您的疑问，我很抱歉.

I hope it clears your doubt if not, I am sory.

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