在 v1.8.3 之前，在 R {data.table} 中使用 `:=` 时如何抑制输出? [英] how to suppress output when using `:=` in R {data.table}, prior to v1.8.3?

问题描述

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

Is there a way to prevent data.table to print the new data.table after assigning a new column by reference? I gather standard behaviour is

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

即分配后表格打印到屏幕上.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

i.e. the table is printed to screen after assignment. is there a way to stop data.table from showing the new table after assigning the new column z? I know I can stop this behaviour by saying

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

但这违背了 := 的目的(旨在避免复制).

but that is defeating the purpose of := (which is designed to avoid copies).

推荐答案

由于 <-.data.table 不会复制，所以可以使用 <-:

Since <-.data.table doesn't make a copy, you can use <-:

创建一个data.table对象:

Create a data.table object:

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. 

<小时>

还值得记住的是，R 仅在交互模式下打印对象的值.

---

It is also worth remembering that R only prints the value of an object in interactive mode.

因此，在批处理模式下，您可以简单地使用:

So, in batch mode, you can simply use:

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

这在批处理模式下作为脚本运行时不会产生任何输出.

This will not produce any output when run as a script in batch mode.

来自 Matthew Dowle 的更多信息:

另请参阅常见问题解答 2.21 和 2.22:

Also see FAQ 2.21 and 2.22 :

2.21 为什么DT[i,col:=value] 返回整个DT?我希望没有可见值(与 <- 一致)，或者包含更新的行数的消息或返回值.数据确实被引用更新并不明显.

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.

这样复合语法就可以工作了；例如，DT[i,done:=TRUE][,sum(done)].当详细程度打开时，将返回更新的行数，基于每个查询或全局使用 options(datatable.verbose=TRUE).

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 好的，但是DT[i,col:=value]的返回值不能不可见的返回吗?

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

• 我们尝试过，但 R 在内部强制启用 [ 的可见性.的价值FunTab 的 [ 的 eval 列(参见 src/main/names.c)是 0 表示力R_Visible 开启(参见 R-Internals 第 1.6 节).因此，当我们尝试invisible() 或者自己直接设置R_Visible为0，eval中src/main/eval.c 会再次强制它打开.
• 在习惯了这种行为之后，您可能会越来越喜欢它(我们有).毕竟，我们使用 <- 进行了多少次子赋值，然后立即查看数据检查是否正常?
• 我们可以将 := 混入一个同样返回数据的 j 中；混合更新并在一个查询中选择.检测 j 是否单独更新(然后行为不同)可能会令人困惑.

• 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.

<小时>

Matthew Dowle 的第二次更新:

我们现在找到了解决方案，当使用 := 时，v1.8.3 不再打印结果.我们将更新 FAQ 2.21 和 2.22.

We have now found a solution and v1.8.3 no longer prints the result when := is used. We will update FAQ 2.21 and 2.22.

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