问题
The title is self explaining. I would like to know why R has chosen to recycle boolean values for selection/subsetting?
The documentation for "[" states Such vectors are recycled if necessary to match the corresponding extent. i, j
Are there any advantages of doing this? I could think of one as mentioned below, but I'd think the disadvantages might outweigh the benefits of ease of use.
df<- data.frame(C1=1:10,c2=101:110)
class(unclass(df)[1]) # df is a list of two lists, each a column of df
df
df[1] # selects 1st list (ie, first column)
df[2]
# However, indices are recycled if we use Logical indices
df[TRUE] # selects both columns
df[c(T,T),] # recycled row indices
df[c(T,T,F),] # recycled row indices
df[FALSE]
# This can actually lead to inadvertent errors
# For example, this has only 7 index elements instead of 10,
# but it's quite possible to miss out on the fact that these are being recycled
df[c(T,F,T,T,F,F,F),]
The only use of this recycling feature that I could think of was in skipping alternate rows
df[c(T,F),]
The context for asking this question is another one I saw on SO yesterday. It was later deleted as someone had pointed out the difference e between | and ||. I wonder if they realised they were also dealing with recycling here.
# An erronous use of && can land you in soup too
df [df$C1 >0 && df$c2 <102, ] #returns TRUE, will select all rows
Are there any other well known pitfalls of this nature that one should be wary of?
回答1:
Advantages:
Lets you select every nth row or column in a vector or data.frame or matrix:
> m <- matrix(1:20, 4)
> m[c(TRUE,FALSE), ]
[,1] [,2] [,3] [,4] [,5]
[1,] 1 5 9 13 17
[2,] 3 7 11 15 19
> m[, c(TRUE,FALSE) ]
[,1] [,2] [,3]
[1,] 1 9 17
[2,] 2 10 18
[3,] 3 11 19
[4,] 4 12 20
Every third column:
> m[, c(TRUE,FALSE,FALSE) ]
[,1] [,2]
[1,] 1 13
[2,] 2 14
[3,] 3 15
[4,] 4 16
The cited disadvantage is really an incorrect use of the && operator (which I think you do actually realize). That operator only ever returns a length-1 vector and is generally inappropriate when trying to do indexing. That was probably the confusion exhibited by the questioner who used the || operator.
Ultimately the answer is because the authors liked it that way. R is a clone in most semantics of S and it was built around the dawn of high level languages in the AT&T think-tank.
来源：https://stackoverflow.com/questions/40419203/r-rationale-for-recycling-boolean-indices-for-selection