在for回圈中通過字串參考物件時為物件賦值

我經常難以理解如何R在回圈內分配值。期望的行為對我來說似乎很簡單，但我顯然沒有很好地掌握R.

例如，我有一堆資料物件，我想為每個物件添加注釋（它們是不相關的，因此在此分配之外的串列中一起使用它們是沒有意義的）。這是一個 MWE

my_comment <- paste0("these objects were created on ", date())

obj1 <- "content1"
obj2 <- "content2"

obj_l <- list(obj1, obj2)

for(obj in obj_l) {
comment(obj) <- my_comment
}

## get 'NULL', but want "these objects were created ..."
comment(obj1)

## get 'NULL'
comment(obj_l)

## assignment is only made to temp variable 'obj'
## This makes sense, but not the desired outcome. 
comment(obj)

我想解決方案將類似于以下偽代碼

obj_l <- c("obj1", "obj2")

for(name in obj_l)
 unknown_function(name, comment, my_comment, unknown_args)

}

或者

modify(obj_l, my_comment, unknown_syntax)

如果我的偽代碼在軌道上，有人可以幫我解決這些問題unknown_嗎？

uj5u.com熱心網友回復：

這些中的任何一個都可以對#3 的組件進行注釋，obj_l或者在#3 的情況下生成一個添加了注釋的新串列。我們展示了 #1 的物件已更改，但其他物件相同。

# 1
obj1 <- "content1"
obj2 <- "content2"
obj_l <- list(obj1, obj2)
for(i in seq_along(obj_l)) {
  comment(obj_l[[i]]) <- "some comment"
}
dput(obj_l)
## list(structure("content1", comment = "some comment"), 
##   structure("content2", comment = "some comment"))


# 2
obj1 <- "content1"
obj2 <- "content2"
obj_l <- mget(c("obj1", "obj2"))
for(nm in names(obj_l)) {
  comment(obj_l[[nm]]) <- "some comment"
}

# 3
obj1 <- "content1"
obj2 <- "content2"
obj_l <- list(obj1, obj2)
obj_2 <- lapply(obj_l, `comment<-`, "some comment")

如果你想要評論obj1然后obj2

# 4
obj1 <- "content1"
obj2 <- "content2"
nms <- c("obj1", "obj2")
e <- .GlobalEnv # environment with objects
for(nm in nms) comment(e[[nm]]) <- "some comment"

# 5
obj1 <- "content1"
obj2 <- "content2"
obj_l <- mget(c("obj1", "obj2")) # named list w obj1, obj2 elements
lapply(obj_l, `comment<-`, "some comment") |> list2env(.GlobalEnv)

uj5u.com熱心網友回復：

使用回圈中的串列，通常更容易回圈索引或名稱，而不是實際的物件：

for(i in seq_along(obj_l)) {
  comment(obj_l[[i]]) <- my_comment
}

comment(obj_l[[i]])
# [1] "these objects were created on Wed Nov  9 10:55:27 2022"

并且您已經為串列中的專案添加了評論，不要期望obj1有評論。（最好不要去想obj1，你所擁有的是obj_l有 2 個元素，obj_l[[1]]和obj_l[[2]].

uj5u.com熱心網友回復：

在閱讀了更多幫助頁面和反復試驗之后，這是我的解決方案

obj1 <- "content1"
obj2 <- "content2"

obj_l <- c("obj1", "obj2")

comment <- '"my comment!"'

for(x in obj_l) {
    my_exp <- paste0("comment(", x, ") <- ", comment)
    parse(text = my_exp)
}

comment(obj1)
# [1] "my comment"

uj5u.com熱心網友回復：

comment()我嘗試在不使用該功能的情況下回答實際問題。

見下文：

create_comment <- function(object, comment) {
  
  new_object <- list(Object = object, Comment = comment)
  return(new_object)
}

test <- map(c(obj1, obj2), create_comment, comment = my_comment)

我沒有使用該comment()函式，而是構建了一個為每個單獨物件創建一個命名串列物件的函式。這使得我可以創建一個包含所有存盤物件的大串列，并且它還將字串存盤my_comment在串列中。

測驗的輸出顯示...

[[1]]
[[1]]$Object
[1] "content1"

[[1]]$Comment
[1] "these objects were created on Wed Nov 09 16:04:04 2022"


[[2]]
[[2]]$Object
[1] "content2"

[[2]]$Comment
[1] "these objects were created on Wed Nov 09 16:04:04 2022"

然后我可以[[i]]用來訪問我的物件

例如：

test[[1]]

$Object
[1] "content1"

$Comment
[1] "these objects were created on Wed Nov 09 16:04:04 2022"

我可以通過使用進一步訪問我的個人評論或物件$

test[[1]]$Comment

[1] "these objects were created on Wed Nov 09 16:04:04 2022"

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