r - 在 v1.8.3 之前，在 R {data.table} 中使用 `:=` 时如何抑制输出？

Question

有没有办法防止data.table通过引用分配新列后打印新的data.table？我收集标准行为是

library(data.table)
example(data.table)
DT
#    x y  v
# 1: a 1 42
# 2: a 3 42
# 3: a 6 42
# 4: b 1 11
# 5: b 3 11
# 6: b 6 11
# 7: c 1  7
# 8: c 3  8
# 9: c 6  9

DT[,z:=1:nrow(DT)]

#    x y  v z
# 1: a 1 42 1
# 2: a 3 42 2
# 3: a 6 42 3
# 4: b 1 11 4
# 5: b 3 11 5
# 6: b 6 11 6
# 7: c 1  7 7
# 8: c 3  8 8
# 9: c 6  9 9

即表格在分配后打印到屏幕上。有没有办法在分配新列 z 后阻止 data.table 显示新表？我知道我可以通过说来阻止这种行为

DT <- copy(DT[,z:=1:nrow(DT)])

但这违背了 := 的目的(旨在避免复制)。

Answer 1

自 <-.data.table不复制，你可以用<- :

创建一个 data.table 对象:

library(data.table)
di <- data.table(iris)

创建一个新列:

di <- di[, z:=1:nrow(di)]
di

#       Sepal.Length Sepal.Width Petal.Length Petal.Width Species  z
#  [1,]          5.1         3.5          1.4         0.2  setosa  1
#  [2,]          4.9         3.0          1.4         0.2  setosa  2
#  [3,]          4.7         3.2          1.3         0.2  setosa  3
#  [4,]          4.6         3.1          1.5         0.2  setosa  4
#  [5,]          5.0         3.6          1.4         0.2  setosa  5
#  [6,]          5.4         3.9          1.7         0.4  setosa  6
#  [7,]          4.6         3.4          1.4         0.3  setosa  7
#  [8,]          5.0         3.4          1.5         0.2  setosa  8
#  [9,]          4.4         2.9          1.4         0.2  setosa  9
# [10,]          4.9         3.1          1.5         0.1  setosa 10
# First 10 rows of 150 printed. 

还值得记住的是，R 仅在交互模式下打印对象的值。

因此，在批处理模式下，您可以简单地使用:

di[, z:=1:nrow(di)]

在批处理模式下作为脚本运行时，这不会产生任何输出。

来自马修·道尔的更多信息:

另请参阅常见问题解答 2.21 和 2.22 :

2.21 Why does DT[i,col:=value] return the whole of DT? I expected either no visible value (consistent with <-), or a message or return value containing how many rows were updated. It isn't obvious that the data has indeed been updated by reference.

So that compound syntax can work; e.g., DT[i,done:=TRUE][,sum(done)]. The number of rows updated is returned when verbosity is on, either on a per query basis or globally using options(datatable.verbose=TRUE).

2.22 Ok, but can't the return value of DT[i,col:=value] be returned invisibly, then?

• We tried to but R internally forces visibility on for [. The value of FunTab's eval column (see src/main/names.c) for [ is 0 meaning force R_Visible on (see R-Internals section 1.6). Therefore, when we tried invisible() or setting R_Visible to 0 directly ourselves, eval in src/main/eval.c would force it on again.
• After getting used to this behaviour, you might grow to prefer it (we have). After all, how many times do we subassign using <- and then immediately look at the data to check it's ok?
• We can mix := into a j which also returns data; a mixed update and select in one query. To detect whether j solely updates (and then behave dierently) could be confusing.

马修·道尔的第二次更新:

我们现在找到了解决方案，v1.8.3 不再打印 := 时的结果用来。我们将更新 FAQ 2.21 和 2.22。