**R – Gerald Belton**, and kindly contributed to R-bloggers]. (You can report issue about the content on this page here)

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This week, we return to our “Getting Started With R” series. Today we are going to look at some tools from the “dplyr” package. Hadley Wickham, the creator of dplyr, calls it “A Grammar of Data Manipulation.”

## filter()

Use filter() for subsetting data by rows. It takes logical expressions as inputs, and returns all rows of your data for which those expressions are true.

To demonstrate, let’s start by loading the tidyverse library (which includes dplyr), and we’ll also load the gapminder data.

library(tidyverse) library(gapminder)

Here’s how filter() works:

filter(gapminder, lifeExp<30)

Produces this output:

# A tibble: 2 × 6 country continent year lifeExp pop gdpPercap1 Afghanistan Asia 1952 28.801 8425333 779.4453 2 Rwanda Africa 1992 23.599 7290203 737.0686 >

## The pipe operator

The pipe operator is one of the great features of the tidyverse. In base R, you often find yourself calling functions nested within functions nested within… you get the idea. The pipe operator `%>%`

takes the object on the left-hand side, and “pipes” it into the function on the right hand side.

For example:

> gapminder %>% head() # A tibble: 6 × 6 country continent year lifeExp pop gdpPercap1 Afghanistan Asia 1952 28.801 8425333 779.4453 2 Afghanistan Asia 1957 30.332 9240934 820.8530 3 Afghanistan Asia 1962 31.997 10267083 853.1007 4 Afghanistan Asia 1967 34.020 11537966 836.1971 5 Afghanistan Asia 1972 36.088 13079460 739.9811 6 Afghanistan Asia 1977 38.438 14880372 786.1134 >

This is the equivalent of saying “head(gapminder).” So far, that doesn’t seem a lot easier… but wait a bit and you’ll see the beauty of the pipe.

## select()

We talked about using filter() to subset data by rows. We can use select() to do the same thing for columns:

> select(gapminder, year, lifeExp) # A tibble: 1,704 × 2 year lifeExp1 1952 28.801 2 1957 30.332 3 1962 31.997 4 1967 34.020 5 1972 36.088 6 1977 38.438 7 1982 39.854 8 1987 40.822 9 1992 41.674 10 1997 41.763 # ... with 1,694 more rows

Here’s the same thing, but using pipes, and sending it through “head()” to make the display more compact:

> gapminder %>% + select(year, lifeExp) %>% + head(4) # A tibble: 4 × 2 year lifeExp1 1952 28.801 2 1957 30.332 3 1962 31.997 4 1967 34.020 >

We are going to be making some changes to the gapminder data, so let’s start by creating a copy of the data. That way, we don’t have to worry about changing the original data.

new_gap <- gapminder

## mutate()

mutate() is a function that defines a new variable and inserts it into your tibble. For example, gapminder has GDP per capita and population; if we multiply these we get the GDP.

new_gap %>% mutate(gdp = pop * gdpPercap)

Note that the above code creates the new field and displays the resulting tibble; we would have had to use the “<-” operator to save the new field in our tibble.

## arrange()

arrange() reorders the rows in a data frame. The gapminder data is currently arranged by country, and then by year. But what if we wanted to look at it by year, and then by country?

new_gap %>% arrange(year, country)

# A tibble: 1,704 × 6 country continent year lifeExp pop gdpPercap1 Afghanistan Asia 1952 28.801 8425333 779.4453 2 Albania Europe 1952 55.230 1282697 1601.0561 3 Algeria Africa 1952 43.077 9279525 2449.0082 4 Angola Africa 1952 30.015 4232095 3520.6103 5 Argentina Americas 1952 62.485 17876956 5911.3151 6 Australia Oceania 1952 69.120 8691212 10039.5956 7 Austria Europe 1952 66.800 6927772 6137.0765 8 Bahrain Asia 1952 50.939 120447 9867.0848 9 Bangladesh Asia 1952 37.484 46886859 684.2442 10 Belgium Europe 1952 68.000 8730405 8343.1051 # ... with 1,694 more rows

## group_by() and summarize()

The group_by() function adds grouping information to your data, which then allows you to do computations by groups. The summarize() function is a natural partner for group_by(). summarize() takes a dataset with *n* observations, calculates the requested summaries, and returns a dataset with 1 observation:

my_gap %>% group_by(continent) %>% summarize(n = n())

The functions you’ll apply within `summarize()`

include classical statistical summaries, like `mean()`

, `median()`

, `var()`

, `sd()`

, `mad()`

, `IQR()`

, `min()`

, and `max()`

. Remember they are functions that take

new_gap %>% group_by(continent) %>% summarize(avg_lifeExp = mean(lifeExp))

# A tibble: 5 × 2 continent avg_lifeExp1 Africa 48.86533 2 Americas 64.65874 3 Asia 60.06490 4 Europe 71.90369 5 Oceania 74.32621

## A wondrous example

To fully appreciate the wonders of the pipe command and the dplyr data manipulation commands, take a look at this example. It comes from Jenny Brian‘s excellent course, STAT545, at the University of British Columbia (to whom I owe a debt for much of the information included in this series of blog posts).

new_gap %>% select(country, year, continent, lifeExp) %>% group_by(continent, country) %>% ## within country, take (lifeExp in year i) - (lifeExp in year i - 1) ## positive means lifeExp went up, negative means it went down mutate(le_delta = lifeExp - lag(lifeExp)) %>% ## within country, retain the worst lifeExp change = smallest or most negative summarize(worst_le_delta = min(le_delta, na.rm = TRUE)) %>% ## within continent, retain the row with the lowest worst_le_delta top_n(-1, wt = worst_le_delta) %>% arrange(worst_le_delta)

Source: local data frame [5 x 3] Groups: continent [5] continent country worst_le_delta1 Africa Rwanda -20.421 2 Asia Cambodia -9.097 3 Americas El Salvador -1.511 4 Europe Montenegro -1.464 5 Oceania Australia 0.170

To quote Jenny: “Ponder that for a while. The subject matter and the code. Mostly you’re seeing what genocide looks like in dry statistics on average life expectancy.”

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**R – Gerald Belton**.

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