data.table：如何在一个键执行二进制搜索两个（数值）值：exemple包括 [英] data.table: How to do binary search for two (numeric) values at one key: exemple included

问题描述

范例资料：

library(data.table)
DT <- data.table(a = c(1, 3, 5, 9, 15), 
                 b = c("a", "c", "d", "e", "f"))

我想得到两行， a == 3 | a == 9 ，即

# a b
# 3 c
# 9 e

我知道如果我这样做： DT [，a： = as.character（a）] 然后 setkey（DT，a）和 DT [c（3 9）] ，我可以得到想要的结果。但我想知道，如果有其他方法来做这种二进制搜索，而不提前改变 a 到字符？

I know if I do: DT[, a:=as.character(a)] then setkey(DT, a) and DT[c("3", "9")], I can get the wanted result. But I would like to know, if there are other methods to do this kind of binary search without changing a to character in advance?

谢谢。

推荐答案

您不需要将它转换为字符向量更有意义）

You don't need to convert it into a character vector (although integer would make more sense)

 DT <- data.table(a = c(1, 3, 5, 9, 15), b = c("a", "c", "d", "e", "f"))
 setkey(DT, a)
 DT[J(c(3, 9))]

此外，如果您拥有CRAN的最新版本，自动使用二进制搜索

Moreover, if you have the latest version in CRAN, the second time you use a in i will automatically uses binary search

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