Question

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我错过了什么？为什么dd = data.table(a = c(1,1), b = c(1,2), v = c(1, NA)) dd # a b v # 1: 1 1 1 # 2: 1 2 NA setkey(dd, a,b) dd[.(1,2), roll = TRUE, rollends = c(TRUE, TRUE)] # a b v # 1: 1 2 NA没有结转？

Answer 1

滚动连接不需要在此处滚动，因为您匹配确切的行(1, 2)。当实际值不匹配时进行滚动匹配，在您的情况下，它具有完全匹配。请参阅以下示例，我修改了dd，因此.(1,2)上没有匹配。

library(data.table)
dd = data.table(a = c(1,1), b = c(1,3), v = c(1, NA))
dd[.(1,2), roll = TRUE, rollends = c(TRUE, TRUE)]
#   a b v
#1: 1 2 1

请参阅?data.table // roll（强调我的）：

当i为data.table且其行与匹配除最后一个x加入列以外的所有内容时，以及其在i中的值}连接列落在一个间隙中（包括在该组的x中的最后一次观察之后），然后：


+Inf（或TRUE）在x向前滚动盛行值。它也被称为最后一次观察（LOCF）


...

R data.table与roll连接

1 个答案: