导致长度不等于 1 或 dplyr 中组长度的分组操作 [英] grouped operations that result in length not equal to 1 or length of group in dplyr

问题描述

我不确定使用哪个函数来执行以下操作:

I'm not sure which function to use to do the following:

library(data.table)
dt = data.table(a = 1:4, b = 1:2)

dt[, rep(a[1], 3), by = b]
#   b V1
#1: 1  1
#2: 1  1
#3: 1  1
#4: 2  2
#5: 2  2
#6: 2  2

summarise 和 mutate 都不满意这个长度:

Both summarise and mutate are unhappy with this length:

library(dplyr)
df = data.frame(a = 1:4, b = 1:2)

df %.% group_by(b) %.% summarise(rep(a[1], 3))
#Error: expecting a single value

df %.% group_by(b) %.% mutate(rep(a[1], 3))
#Error: incompatible size (3), expecting 2 (the group size) or 1

推荐答案

在 dplyr 0.2 版中，您可以使用 do 运算符执行此操作:

In dplyr version 0.2 you could do this using the do operator:

> df %>% group_by(b) %>% do(data.frame(a = rep(.$a[1], 3)))
#Source: local data frame [6 x 2]
#Groups: b
#
#  b a
#1 1 1
#2 1 1
#3 1 1
#4 2 2
#5 2 2
#6 2 2

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