A Quick Appreciation of the R transform Function

转载自：http://www.win-vector.com/blog/2018/09/a-quick-appreciation-of-the-r-transform-function/

John Mount
发表于

R users who also use the dplyr package will be able to quickly understand the following code that adds an estimated area column to a data.frame.

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suppressPackageStartupMessages(library("dplyr"))

iris %>%
 mutate(
 ., 
 Petal.Area = (pi/4)*Petal.Width*Petal.Length) %>%
 head(.)

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## Sepal.Length Sepal.Width Petal.Length Petal.Width Species Petal.Area
## 1 5.1 3.5 1.4 0.2 setosa 0.2199115
## 2 4.9 3.0 1.4 0.2 setosa 0.2199115
## 3 4.7 3.2 1.3 0.2 setosa 0.2042035
## 4 4.6 3.1 1.5 0.2 setosa 0.2356194
## 5 5.0 3.6 1.4 0.2 setosa 0.2199115
## 6 5.4 3.9 1.7 0.4 setosa 0.5340708

The notation we used above is the “explicit argument” variation we recommend for readability. What a lot of dplyr users do not seem to know is: base-R already has this functionality. The function is called transform().

To demonstrate this, let’s first detach dplyr to show that we are not using functions from dplyr.

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detach("package:dplyr", unload = TRUE)

Now let’s write the equivalent pipeline using exclusively base-R.

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iris ->.
 transform(
 ., 
 Petal.Area = (pi/4)*Petal.Width*Petal.Length) ->.
 head(.)

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## Sepal.Length Sepal.Width Petal.Length Petal.Width Species Petal.Area
## 1 5.1 3.5 1.4 0.2 setosa 0.2199115
## 2 4.9 3.0 1.4 0.2 setosa 0.2199115
## 3 4.7 3.2 1.3 0.2 setosa 0.2042035
## 4 4.6 3.1 1.5 0.2 setosa 0.2356194
## 5 5.0 3.6 1.4 0.2 setosa 0.2199115
## 6 5.4 3.9 1.7 0.4 setosa 0.5340708

The “->.” notation is a the end-of-line variation of the Bizarro Pipe. The transform() function has been part of R since 1998. dplyr::mutate() was introduced in 2014.

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git log --all -p --reverse --source -S 'transform <-'

commit 41c2f7338c45dbf9eac99c210206bc3657bca98a refs/remotes/origin/tags/R-0-62-4
Author: pd <pd@00db46b3-68df-0310-9c12-caf00c1e9a41>
Date: Wed Feb 11 18:31:12 1998 +0000

 Added the frametools functions subset() and transform()
 
 git-svn-id: https://svn.r-project.org/R/trunk@709 00db46b3-68df-0310-9c12-caf00c1e9a41

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