动态构建查询多列的调用 [英] Dynamically build call for lookup multiple columns

问题描述

使用字符向量变量作为参数，如何动态地查找多个字段和通过引用添加。在下面的情况下，我想查找两列，并删除其中的 i。前缀。当然，他们可以覆盖已经存在的列，名称相同。

How can I dynamically lookup multiple fields and add by reference using character vector variable as argument. In below case I want to lookup two columns and get rid of i. prefix in them. Of course they can override already existing columns with the same name.

library(data.table)
set.seed(1)
ID <- data.table(id = 1:3, meta = rep(1,3), key = "id")
JN <- data.table(idd = sample(ID$id, 3, FALSE), value = sample(letters, 3, FALSE), meta = rep(1,3), key = "idd")
select <- c("value","meta") # my fields to lookup
j.lkp <- call(":=", select, lapply(paste0("i.",select), as.symbol))
j.lkp
# `:=`(c("value", "meta"), list(i.value, i.meta))
ID[JN, eval(j.lkp)]
# Error in eval(expr, envir, enclos) : could not find function "i.value"
ID[JN, `:=`(c("value", "meta"), list(i.value, i.meta))]
#    id meta value
# 1:  1    1     x
# 2:  2    1     v
# 3:  3    1     f

我知道类似问题，但是这个在加入期间要求向量化参数，直接建立调用 j 。

编辑：我知道我可以使用 .SDcols 但是我不能通过引用执行这个

I'm aware of similar question but this one asks for vectorized argument during join and directly building call for j.
edit: I'm aware I can do it using .SDcols but then I cannot perform this by reference

推荐答案

这似乎是我最直接的方式：

This seems to be the most straightforward way to me:

ID[JN, (select) := mget(paste0('i.', select))]

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