考虑这个数据集：
> DATA <- data.frame(Agreement_number = c(1,1,1,1,2,2,2,2),
+ country = c("Canada","Canada", "USA", "USA", "Canada","Canada", "USA", "USA"),
+ action = c("signature", "ratification","signature", "ratification", "signature", "ratification","signature", "ratification"),
+ signature_date = c(2000,NA,2000,NA, 2001, NA, 2002, NA),
+ ratification_date = c(NA, 2001, NA, 2002, NA, 2001, NA, 2002))
> DATA
Agreement_number country action signature_date ratification_date
1 Canada signature 2000 NA
1 Canada ratification NA 2001
1 USA signature 2000 NA
1 USA ratification NA 2002
2 Canada signature 2001 NA
2 Canada ratification NA 2001
2 USA signature 2002 NA
2 USA ratification NA 2002
如您所见，一半的行有重复的信息。对于像这样的小数据集，删除重复数据非常容易。我可以使用coalesce函数（dplyr package），删除“action”列，然后删除所有不相关的行。虽然，还有很多其他方法。最终结果应如下所示：
> DATA <- data.frame( Agreement_number = c(1,1,2,2),
+ country = c("Canada", "USA", "Canada","USA"),
+ signature_date = c(2000,2000,2001,2002),
+ ratification_date = c(2001, 2002, 2001, 2002))
> DATA
Agreement_number country signature_date ratification_date
1 Canada 2000 2001
1 USA 2000 2002
2 Canada 2001 2001
2 USA 2002 2002
问题是，我的真实数据集更大（102000 x 270）并且还有更多变量。实际数据也更不规则，缺少更多值。 coalesce函数似乎很慢。到目前为止我能做的最好的循环仍然需要5-10分钟才能运行。
有一种简单的方法可以做得更快吗？我觉得R中必须有一些功能用于那种操作，但我找不到任何功能。
答案 0 :(得分：4)
我认为你需要dcast。 data.table库中的版本称自己为“快速”，根据我的经验，它在大型数据集上的速度很快。
首先，让我们创建一个signature_date或ratification_date列，具体取决于操作
library(data.table)
setDT(DATA)[, date := ifelse(action == "ratification", ratification_date, signature_date)]
现在，让我们将其转换为操作是列，值是日期
wide <- dcast(DATA, Agreement_number + country ~ action, value.var = 'date')
如此宽阔的样子
Agreement_number country ratification signature
1 1 Canada 2001 2000
2 1 USA 2002 2000
3 2 Canada 2001 2001
4 2 USA 2002 2002
答案 1 :(得分：3)
data.table。
我知道Harland也建议使用data.table和dcast()，但下面的解决方案是另一种方法。它以正确的顺序显示行，并将ratification_date复制到签名行。经过一些清理后，我们得到了理想的结果。
library(data.table)
# coerce to data.table,
# make sure that the actions are ordered properly, not alphabetically
setDT(DATA)[, action := ordered(action, levels = c("signature", "ratification"))]
# order the rows to make sure that signature row and ratification row are
# subsequent for each agreement and country
setorder(DATA, Agreement_number, country, action)
# copy the ratification date from the row below but only within each group
result <- DATA[, ratification_date := shift(ratification_date, type = "lead"),
by = c("Agreement_number", "country")][
# keep only signature rows, remove action column
action == "signature"][, action := NULL]
result
Agreement_number country signature_date ratification_date dummy1 dummy2 1: 1 Canada 2000 2001 2 D 2: 1 USA 2000 2002 3 A 3: 2 Canada 2001 2001 1 B 4: 2 USA 2002 2002 4 C
OP已经提到他的生产数据有270列。为了模拟这个，我添加了两个虚拟列：
set.seed(123L)
DATA <- data.frame(Agreement_number = c(1,1,1,1,2,2,2,2),
country = c("Canada","Canada", "USA", "USA", "Canada","Canada", "USA", "USA"),
action = c("signature", "ratification","signature", "ratification", "signature", "ratification","signature", "ratification"),
signature_date = c(2000,NA,2000,NA, 2001, NA, 2002, NA),
ratification_date = c(NA, 2001, NA, 2002, NA, 2001, NA, 2002),
dummy1 = rep(sample(4), each = 2L),
dummy2 = rep(sample(LETTERS[1:4]), each = 2L))
请注意，set.seed()用于抽样时的可重复结果。
Agreement_number country action signature_date ratification_date dummy1 dummy2 1 1 Canada signature 2000 NA 2 D 2 1 Canada ratification NA 2001 2 D 3 1 USA signature 2000 NA 3 A 4 1 USA ratification NA 2002 3 A 5 2 Canada signature 2001 NA 1 B 6 2 Canada ratification NA 2001 1 B 7 2 USA signature 2002 NA 4 C 8 2 USA ratification NA 2002 4 C
dcast()附加列 Harland建议使用data.table和dcast()。除了答案中的其他几个缺陷外，它还没有处理OP提到的额外列。
下面的dcast()方法还会返回其他列：
library(data.table)
# coerce to data table
setDT(DATA)[, action := ordered(action, levels = c("signature", "ratification"))]
# use already existing column to "coalesce" dates
DATA[action == "ratification", signature_date := ratification_date]
DATA[, ratification_date := NULL]
# dcast from long to wide form, note that ... refers to all other columns
result <- dcast(DATA, Agreement_number + country + ... ~ action,
value.var = "signature_date")
result
Agreement_number country dummy1 dummy2 signature ratification 1: 1 Canada 2 D 2000 2001 2: 1 USA 3 A 2000 2002 3: 2 Canada 1 B 2001 2001 4: 2 USA 4 C 2002 2002
请注意，此方法将更改列的顺序。
答案 2 :(得分：2)
这是使用uwe-block的data.frame的另一个data.table解决方案。它类似于uwe-block的方法，但使用max来折叠数据。
# covert data.frame to data.table and factor variables to character variables
library(data.table)
setDT(DATA)[, names(DATA) := lapply(.SD,
function(x) if(is.factor(x)) as.character(x) else x)]
# collapse data set, by agreement and country. Take max of remaining variables.
DATA[, lapply(.SD, max, na.rm=TRUE), by=.(Agreement_number, country)][,action := NULL][]
lapply运行未包含在by语句中的变量，并在删除NA值后计算最大值。链中的下一个链接删除不需要的操作变量，最后（不必要的）链接打印输出。
返回
Agreement_number country signature_date ratification_date dummy1 dummy2
1: 1 Canada 2000 2001 2 D
2: 1 USA 2000 2002 3 A
3: 2 Canada 2001 2001 1 B
4: 2 USA 2002 2002 4 C