data.tableassigne-by-reference`:=`的标准形式和功能形式的不同结果 [英] different results for standard form and functional form of data.table assigne-by-reference `:=`

问题描述

data.tabel 的引用分配 := 与功能形式的标准之间似乎存在细微差别.

There seems to be a minor difference between data.tabel's assignment by reference := in the standard to the functinal form.

标准形式将 RHS 强制转换为向量，而函数形式则不会.一个细节，但没有像我相信的那样记录.

Standard form coerces RHS to vector, the functional form does not. A detail, but not documented as I believe.

library(data.table)
dt <- data.table(a = c('a','b','c'))
v <- c('A','B','C')
l <- list(v)

all.equal(copy(dt)[, new := v], copy(dt)[, `:=` (new = v)])
# [1] TRUE
all.equal(copy(dt)[, new := l], copy(dt)[, `:=` (new = l)])
# [1] "Datasets have different column modes. First 3: new(character!=list)"

copy(dt)[, new := l][]
#    a new
# 1: a   A
# 2: b   B
# 3: c   C

copy(dt)[, `:=` (new = l)][]
#    a   new
# 1: a A,B,C
# 2: b A,B,C
# 3: c A,B,C

这是我最初提出这个问题的方式的主要编辑.

This is a major Edit of how I asked this question originally.

推荐答案

这是一个很好的问题，涉及到关于 := 运算符的设计决策.

This is very good question that touches design decision about := operator.

对于使用 := 作为运算符的简单调用，例如 col := val，我们决定自动将 val 包装到列表中.做出此决定是为了让用户更方便地分配单列.

For simple calls using := as an operator, like col := val, we decided to wrap val into a list automatically. This decision was made to make it more convenient for users to assign single column.

当您使用函数调用形式时，":="(col = val) 我们不再将 val 包装到列表中.它已经是扩展形式.:= 充当 list 的别名，但更新 就地.您始终可以通过将 := 更改为 list(或 .)来检查更新的列，例如 .(col = val).

When you are using function call form, ":="(col = val) we are not wrapping val into list anymore. It is extended form already. := behaves as an alias to list but updating in-place. You can always check what will be the updated column by changing := into list (or .) like .(col = val).

即使使用 := 作为运算符，您仍然必须提供 RHS 作为您正在创建 2+ 列的列表，c("col1","col2"):= list(val1, val2).

Not that even when using := as an operator, you still have to provide RHS as list of you are creating 2+ columns, c("col1","col2") := list(val1, val2).

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