如何有条件地合并/复制不同的行[R]答案

【问题标题】：How to merge/copy different rows into one conditionally [R]如何有条件地合并/复制不同的行[R]
【发布时间】：2021-06-22 02:56:57
【问题描述】：

我有一个带有名称的大型数据框和一个名为 sequence 的“分类”变量。 sequence 说明关于其他行的位置。它有两个值：first 和 additional。问题是这些值的分布不均匀，即每个 first 没有一个 additional，每个 letters 值都是唯一的. 数据框长这样（简化版）：

letters <- sample(LETTERS, 20)
sequence <- c("first","additional","first","first","first","first","first","additional","additional","additional","first","first","additional","first","additional","additional","first","additional","first","first")
df <- data.drame(sequence, letters)

现在，我要做的是将 letters 中的每个 additional 值粘贴到 中对应的 first 值中字母。因此，例如，letters 列中的第二个（行）值将被粘贴到第一个中，因为它是对应的附加。此外，letters中的第八、第九和第十个值应粘贴在letters的第七个值的内部（旁边）（例如，first；附加；附加；附加）。

我已经尝试了以下明显限制，即它只查看紧邻的下一个值，

library(dplyr)    
df <- df %>% mutate(letters_ok = if_else(sequence == "additional",
                                            paste(letters, lag(letters), sep = "; "), letters))

突出我的问题：我如何设法有条件地滞后于 sequence 中的值，以便我可以根据 first 粘贴 letters 中的值还是附加分类？

由于每个 letters 值都是唯一的，并且与特定的 sequence 值相关联，因此我没有使用 group_by。其他所有解决方案都无法解决我目前对字符串/字符争论的了解，所以我非常感谢任何帮助。

【问题讨论】：

标签： r dataframe if-statement conditional-statements lag

【解决方案1】：

这是data.table 方法。我稍微修改了您的示例数据，因为letters 不是一个非常方便的列名。另外，添加set.seed(123) 用于复制目的。

样本数据

set.seed(123)
letter <- sample(LETTERS, 20)
sequence <- c("first","additional","first","first","first","first","first","additional","additional","additional","first","first","additional","first","additional","additional","first","additional","first","first")
df <- data.frame(sequence, letter)

#      sequence letter
# 1       first      O
# 2  additional      S
# 3       first      N
# 4       first      C
# 5       first      J
# 6       first      R
# 7       first      K
# 8  additional      E
# 9  additional      X
# 10 additional      Y
# 11      first      W
# 12      first      T
# 13 additional      I
# 14      first      L
# 15 additional      U
# 16 additional      M
# 17      first      P
# 18 additional      H
# 19      first      B
# 20      first      G

代码

library( data.table )
#convert to data.table format
setDT(df)
#add id-column
df[, id := .I ]
#perform rolling join
temp <- df[ sequence == "first", ][ df[ sequence == "additional", ], 
                                    .( x.letter, i.letter, i.id, x.id), 
                                    on = .(id), 
                                    roll = Inf ]
#summarise
temp <- temp[, paste0( `i.letter`, collapse = ";" ), by = .(x.id) ]
#join, drop id column
df[sequence == "first", ][ temp, letter := paste( letter, i.V1, sep = ";"), on = .(id = `x.id`) ][, id := NULL]

输出

#    sequence  letter
# 1:    first     O;S
# 2:    first       N
# 3:    first       C
# 4:    first       J
# 5:    first       R
# 6:    first K;E;X;Y
# 7:    first       W
# 8:    first     T;I
# 9:    first   L;U;M
#10:    first     P;H
#11:    first       B
#12:    first       G

【讨论】：