magrittr

You can think about the following sequence of actions - find key, unlock car, start car, drive to school, park.

Expressed as a set of nested functions in R pseudocode this would look like:

park(drive(start_car(find("keys")), to="campus"))

Writing it out using pipes give it a more natural (and easier to read) structure:

find("keys") %>%
    start_car() %>%
    drive(to="campus") %>%
    park()

Approaches

All of the following are find, it mostly amounts to preference.

Nested:

h( g( f(x), y=1), z=1 )

Piped:

f(x) %>% g(y=1) %>% h(z=1)

Intermediate:

res = f(x)
res = g(res, y=1)
res = h(res, z=1)

What about other arguments?

Sometimes we want to send our results to an function argument other than first one or we want to use the previous result for multiple arguments. In these cases we can refer to the previous result using ..

data.frame(a=1:3,b=3:1) %>% lm(a~b,data=.)

## 
## Call:
## lm(formula = a ~ b, data = .)
## 
## Coefficients:
## (Intercept)            b  
##           4           -1

data.frame(a=1:3,b=3:1) %>% .[[1]]

## [1] 1 2 3

data.frame(a=1:3,b=3:1) %>% .[[length(.)]]

## [1] 3 2 1

rvest

rvest is a package from Hadley Wickham that makes basic processing and manipulation of HTML data straight forward.

Core functions:

• read_html - read HTML data from a url or character string.

• html_nodes - select specified nodes from the HTML document usign CSS selectors.

• html_table - parse an HTML table into a data frame.

• html_text - extract a tag pairs’ content.

• html_name - extract tags’ names.

• html_attrs - extract all of each tag’s attributes.

• html_attr - extract tags’ attribute value by name.