问题
While looking at an answer posted recently on SO, I noticed an unfamiliar assignment statement. Instead of the usual form of myVar<- myValue
, it used the for myVar[]<- myValue
. Personally, I had never seen such an assignment, but it had a highly useful effect-- it reshaped the assigned data myValue to the shape of myVar.
I would like to use this in my code. However the documentation for "<-"
seems to be silent on it.
Is this a well established feature and one can rely on it to work in all cases?
Also, my guess is that it might be a side effect of a function call stack, i.e. calling <-
and [
in sequence, but I could not figure out how. Can someone throw some light on that?
Here's an example--
# A dataframe
df1<- data.frame(a=1:4, b=11:14)
# simple assignment assigns to class of RHS
df1<- c(21:24,31:34)
df1
#[1] 21 22 23 24 31 32 33 34
class(df1)
#[1] "integer"
#assignment with [] casts to class of LHS
df1<- data.frame(a=1:4, b=11:14)
df1[]<- c(21:24,31:34)
df1
# a b
# 1 21 31
# 2 22 32
# 3 23 33
# 4 24 34
# recycling to preserve shape
df1[]<- c(101:102)
df1
# a b
# 1 101 101
# 2 102 102
# 3 101 101
# 4 102 102
class(df1)
#data.frame
# reshaping
df1<- data.frame(a=1:4, b=11:14)
df1[] <- matrix(1:8, 2,4)
df1 #matrix reshaped
class(df1)
#[1] "data.frame"
# flattening
x<- 1:8
x[]<- matrix(1:8,4,2)
x
#[1] 1 2 3 4 5 6 7 8
回答1:
This is an intentional and documented feature. As joran mentioned, the documentation page "Extract" includes this in the "Atomic Vectors" section:
An empty index selects all values: this is most often used to replace all the entries but keep the attributes.
However, in the case of recursive objects (data.frames
or lists
, for example), the attributes are only kept for the subsetted object. Its parts don't get such protection.
Here's an example:
animals <- factor(c('cat', 'dog', 'fish'))
df_factor <- data.frame(x = animals)
rownames(df_factor) <- c('meow', 'bark', 'blub')
str(df_factor)
# 'data.frame': 3 obs. of 1 variable:
# $ x: Factor w/ 3 levels "cat","dog","fish": 1 2 3
df_factor[] <- 'cat'
str(df_factor)
# 'data.frame': 3 obs. of 1 variable:
# $ x: chr "cat" "cat" "cat"
rownames(df_factor)
# [1] "meow" "bark" "blub"
df_factor
kept its rownames
attribute, but the x
column is just the character vector used in the assignment instead of a factor. We can keep the class and levels of x
by specifically replacing its values:
df_factor <- data.frame(x = animals)
df_factor$x[] <- 'cat'
str(df_factor)
# 'data.frame': 3 obs. of 1 variable:
# $ x: Factor w/ 3 levels "cat","dog","fish": 1 1 1
So replacement with empty subsetting is very safe for vectors, matrices, and arrays, because their elements can't have their own attributes. But it requires some care when dealing with list-like objects.
来源:https://stackoverflow.com/questions/41191623/easy-assignments-with-empty-square-brackets-x