R Pipelining functions | 易学教程

问题

Is there a way to write pipelined functions in R where the result of one function passes immediately into the next? I'm coming from F# and really appreciated this ability but have not found how to do it in R. It should be simple but I can't find how. In F# it would look something like this:

let complexFunction x =
     x |> square 
     |> add 5 
     |> toString

In this case the input would be squared, then have 5 added to it and then converted to a string. I'm wanting to be able to do something similar in R but don't know how. I've searched for how to do something like this but have not come across anything. I'm wanting this for importing data because I typically have to import it and then filter. Right now I do this in multiple steps and would really like to be able to do something the way you would in F# with pipelines.

回答1:

We can use Compose from the functional package to create our own binary operator that does something similar to what you want

# Define our helper functions
square <- function(x){x^2}
add5 <- function(x){x + 5}

# functional contains Compose
library(functional)

# Define our binary operator
"%|>%" <- Compose

# Create our complexFunction by 'piping' our functions
complexFunction <- square %|>% add5 %|>% as.character
complexFunction(1:5)
#[1] "6"  "9"  "14" "21" "30"


# previously had this until flodel pointed out
# that the above was sufficient
#"%|>%" <- function(fun1, fun2){ Compose(fun1, fun2) }

I guess we could technically do this without requiring the functional package - but it feels so right using Compose for this task.

"%|>%" <- function(fun1, fun2){
    function(x){fun2(fun1(x))}
}
complexFunction <- square %|>% add5 %|>% as.character
complexFunction(1:5)
#[1] "6"  "9"  "14" "21" "30"

回答2:

Here is a functional programming approach using Reduce. It is in fact an example from ?Reduce

square <- function(x) x^2
add_5 <- function(x)  x+5
x <- 1:5
## Iterative function application:
Funcall <- function(f, ...) f(...)

Reduce(Funcall, list(as.character, add_5, square,x), right = TRUE)
## [1] "6"  "9"  "14" "21" "30"

Or even more simply using the functional package and Compose

This is nice as it will create the function for you

library(functional)
do_stuff <-   Compose(square,add_5,as.character )
do_stuff(1:5)
##  [1] "6"  "9"  "14" "21" "30"

I note that I would not consider either of these approaches idiomatically R ish (if that is even a phrase)

回答3:

I think that you might just want to write a function to do the steps you desire.

complexFunction <- function(x) {
    as.character(x^2 + 5)
}

Then just call complexFunction(x).

Edit to show what R is doing internally (@mnel) -- The way R parses the and evaluates as.character(x^2 + 5) does what you want

You can use codetools to investigate what R to see how the values are being passed to eachother

flattenAssignment(quote(as.character(x^2+5)))
[[1]]
[[1]][[1]]
x

[[1]][[2]]
`*tmp*`^2

[[1]][[3]]
`*tmp*` + 5


[[2]]
[[2]][[1]]
`as.character<-`(`*tmp*`, value = `*tmpv*`)

[[2]][[2]]
`+<-`(`*tmp*`, 5, value = `*tmpv*`)

[[2]][[3]]
`^<-`(x, 2, value = `*tmpv*`)

Or you can get the Lisp style representation to see how it is parsed (and the results passed)

showTree(quote(as.character(x^2+5)))
(as.character (+ (^ x 2) 5))

回答4:

Since this question was asked, the magrittr pipe has become enormously popular in R. So your example would be:

library (magrittr)

fx <- function (x) {
     x %>%
     `^` (2) %>%
     `+` (5)  %>%
     as.character ()
     }

Note that the backquote notation is because I'm literally using R's built-in functions and I need to specially quote them to use them in this manner. More normally-named functions (like exp or if I'd created a helper function add) wouldn't need backquotes and would appear more like your example.

Note also that %>% passes the incoming value as the first argument to the next function automatically, though you can change this. Note also that an R function returns the last value calculated so I don't need to return or assign the calculation in order to return it.

This is a lot like the nice special operators defined by other answers, but it uses a particular function that's widely used in R now.

回答5:

This works for R pretty similar in F#:

"%|>%" <- function(x, fun){
    if(is.function(x)) {
      function(...) fun(x(...))
    } else {
      fun(x)
    }
}

来源：https://stackoverflow.com/questions/13354048/r-pipelining-functions

标签

pipeline