Comparing the pipe with base methods

Some say, the pipe (#tidyverse) makes analyses in R easier. I agree. This post demonstrates some examples.

Let’s take the mtcars dataset as an example.

data(mtcars)
?mtcars

Say, we would like to compute the correlation between gasoline consumption (mpg) and horsepower (hp).

Base approach 1

cor(mtcars[, c("mpg", "hp")])

##            mpg         hp
## mpg  1.0000000 -0.7761684
## hp  -0.7761684  1.0000000

We use the [-operator (function) to select the columns; note that df[, c(col1, col2)] sees dataframes as matrices, and spits out a dataframe, not a vector:

class(mtcars[, c("mpg", "hp")])

## [1] "data.frame"

That’s ok, because cor expects a matrix or a dataframe as input. Alternatively, we can understand dataframes as lists as in the following example.

Base approach 2

cor.test(x = mtcars[["mpg"]], y = mtcars[["hp"]])

##
## 	Pearson's product-moment correlation
##
## data:  mtcars[["mpg"]] and mtcars[["hp"]]
## t = -6.7424, df = 30, p-value = 1.788e-07
## alternative hypothesis: true correlation is not equal to 0
## 95 percent confidence interval:
##  -0.8852686 -0.5860994
## sample estimates:
##        cor
## -0.7761684

the [[-operator extracts a column from a list (a dataframe is technically a list), and extracts it as a vector. This is useful as some functions, such as cor.test don’t digest dataframes, but want vectors as input (here x, y).

Pipe approach 1

We will use dplyr for demonstrating the pipe approach.

library(dplyr)

mtcars %>%
  select(mpg, hp) %>%
  cor

##            mpg         hp
## mpg  1.0000000 -0.7761684
## hp  -0.7761684  1.0000000

If you are not acquainted with dplyr, the %>% operator can be translated as then do. More specifically, the result of the the lefthand side (lhs) is transferred as input to the righthand side (rhs).

Easy, right?

Pipe approach 2

We will need broom here, a package that renders some R output into a nice (ie, tidy) dataframe. For example, cor.test does not spit a nice dataframe when left in the wild. Applying tidy() from broom on the output, we will get a nice dataframe:

library(broom)

cor.test(x = mtcars[["mpg"]], y = mtcars[["hp"]]) %>% tidy

##     estimate statistic      p.value parameter   conf.low  conf.high
## 1 -0.7761684 -6.742389 1.787835e-07        30 -0.8852686 -0.5860994
##                                 method alternative
## 1 Pearson's product-moment correlation   two.sided

# same:
tidy(cor.test(x = mtcars[["mpg"]], y = mtcars[["hp"]]))

##     estimate statistic      p.value parameter   conf.low  conf.high
## 1 -0.7761684 -6.742389 1.787835e-07        30 -0.8852686 -0.5860994
##                                 method alternative
## 1 Pearson's product-moment correlation   two.sided

This code can be made simpler using dplyr:

mtcars %>%
  do(tidy(cor.test(.$mpg, .$hp)))

##     estimate statistic      p.value parameter   conf.low  conf.high
## 1 -0.7761684 -6.742389 1.787835e-07        30 -0.8852686 -0.5860994
##                                 method alternative
## 1 Pearson's product-moment correlation   two.sided

The function do from dplyr runs any function, provided it spits a dataframe. That’s why we first apply tidy from broom, and run do afterwards.

The . dot refers to the dataframe as handed over from the last step. We need this piece because cor.test does not know any variable by the name mpg (unless you have attached mtcars beforehands).

This code produces the same result:

mtcars %>%
  do(cor.test(.$mpg, .$hp) %>% tidy) %>%
  knitr::kable()

Pipe appraoch 3

The package magrittr provides some pipe variants, most importantly perhaps the “exposition pipe”, %$%:

mtcars %$%
  cor.test(mpg, hp) %>%
  tidy

##     estimate statistic      p.value parameter   conf.low  conf.high
## 1 -0.7761684 -6.742389 1.787835e-07        30 -0.8852686 -0.5860994
##                                 method alternative
## 1 Pearson's product-moment correlation   two.sided

Why is it useful? Let’s spell out the code above in more detail.

• Line 1: “Hey R, pick up mtcars but do not simply pass over this dataframe, but pull out each column and pass those columns over”
• Line 2: “Run the function cor.test with hp and mpg” and then …
• Line 3: “Tidy the result up. Not necessary here but quite nice”.

Remember that cor.test does not accept a dataframe as input. It expects two vectors. That’s why we need to transform the dataframe mtcars to a bundle of vectors (ie., the columns).

Recap

In sum, I think the pipe makes life easier. Of course, one needs to get used to it. But after a while, it’s much simpler than working with deeply nested [ brackets.

Enjoy the pipe!