Part 7 Data manipulation

Package dplyr introduces a grammar of data manipulation. See the nice cheat sheet

We will first introduce the 5 intuitively-named key functions from {dplyr}:

Table 7.1: the 5 core dplyr functions

name	what.it.does
mutate	adds new variables (columns) that are functions of existing variables
select	picks variables (columns) based on their names.
filter	picks observations (rows) based on their values.
summarise	collapses multiple values down to a single summary.
arrange	changes the ordering of the rows.

All 5 functions work in a similar and consistent way:

• The first argument is the input: a data frame or a tibble.
• The output is a new tibble.

Note that {dplyr} never modifies the input: you need to redirect the output and save in a new - or the same - object.

We will use the presidential data set from the ggplot2 package. It contains data of the terms of presidents of the USA, from Eisenhower to Obama:

• Name
• Term starting date
• Term ending date of mandate
• Political party

Table 7.2: presidential data set

name	start	end	party
Eisenhower	1953-01-20	1961-01-20	Republican
Kennedy	1961-01-20	1963-11-22	Democratic
Johnson	1963-11-22	1969-01-20	Democratic
Nixon	1969-01-20	1974-08-09	Republican
Ford	1974-08-09	1977-01-20	Republican
Carter	1977-01-20	1981-01-20	Democratic
Reagan	1981-01-20	1989-01-20	Republican
Bush	1989-01-20	1993-01-20	Republican
Clinton	1993-01-20	2001-01-20	Democratic
Bush	2001-01-20	2009-01-20	Republican
Obama	2009-01-20	2017-01-20	Democratic