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Intro to dplyr

When working with data frames in R, it is often useful to manipulate and summarize data. The dplyr package in R offers one of the most comprehensive group of functions to perform common manipulation tasks. In addition, the dplyr functions are often of a simpler syntax than most other data manipulation functions in R.

Elements of dplyr

There are several elements of dplyr that are unique to the library, and that do very cool things!

Functions for manipulating data

The text below was exerpted from the

Dplyr aims to provide a function for each basic verb of data manipulating, like:

• filter() (and slice())
  • filter rows based on values in specified columns
• arrange()
  • sort data by values in specified columns
• select() (and rename())
  • view and work with data from only specified columns
• distinct()
  • view and work with only unique values from specified columns
• mutate() (and transmute())
  • add new data to the data frame
• summarise()
  • calculate specified summary statistics on data
• sample_n() and sample_frac()
  • return a random sample of rows

Format of function calls

The single table verb functions share these features:

• The first argument is a data.frame (or a dplyr special class tbl_df, known as a 'tibble').
  • dplyr can work with data.frames as is, but if you're dealing with large data it's worthwhile to convert them to a tibble, to avoid printing a lot of data to the screen. You can do this with the function
    as_tibble().
  • Calling the class function on a tibble will return the vector c("tbl_df", "tbl", "data.frame").
• The subsequent arguments describe how to manipulate the data (e.g., based on which columns, using which summary statistics), and you can refer to columns directly (without using $).
• The result is always a new tibble.
• Function calls do not generate 'side-effects'; you always have to assign the results to an object

Grouped operations

Certain functions (e.g., group_by, summarise, and other 'aggregate functions') allow you to get information for groups of data, in one fell swoop. This is like performing database functions with knowing SQL or any other db specific code. Powerful stuff!

Piping

We often need to get a subset of data using one function, and then use another function to do something with that subset (and we may do this multiple times). This leads to nesting functions, which can get messy and hard to keep track of. Enter 'piping', dplyr's way of feeding the output of one function into another, and so on, without the hassleof parentheses and brackets.

Let's say we want to start with the data frame my_data, apply function1(), then function2(), and then function3(). This is what that looks like without piping:

function3(function2(function1(my_data)))

This is messy, difficult to read, and the reverse of the order our functions actually occur. If any of these functions needed additional arguments, the readability would be even worse!

The piping operator %>% takes everything in front of it and "pipes" it into the first argument of the function after. So now our example looks like this:

my_data %>%
  function1() %>%
  function2() %>%
  function3()

This runs identically to the original nested version!

For example, if we want to find the mean body weight of male mice, we'd do this:

	myMammalData %>%                     # start with a data frame
		filter(sex=='M') %>%               # first filter for rows where sex is male
		summarise (mean_weight = mean(weight))  # find the mean of the weight 
                                            # column, store as mean_weight

This is read as "from data frame myMammalData, select only males and return the mean weight as a new list mean_weight".

Download Small Mammal Data

Before we get started, we will need to download our data to investigate. To do so, we will use the loadByProduct() function from the neonUtilities package to download data straight from the NEON servers. To learn more about this function, please see the Download and Explore NEON data tutorial here.

Let's look at the NEON small mammal capture data from Harvard Forest (within Domain 01) for all of 2014. This site is located in the heart of the Lyme disease epidemic.

Read more about NEON terrestrial measurements here.

# load packages
library(dplyr)
library(neonUtilities)

# load the NEON small mammal capture data
# NOTE: the check.size = TRUE argument means the function 
# will require confirmation from you that you want to load 
# the quantity of data requested
loadData <- loadByProduct(dpID="DP1.10072.001", site = "HARV", 
                 startdate = "2014-01", enddate = "2014-12", 
                 check.size = TRUE) # Console requires your response!

# if you'd like, check out the data
str(loadData)

The loadByProduct() function calls the NEON server, downloads the monthly data files, and 'stacks' them together to form two tables of data called 'mam_pertrapnight' and 'mam_perplotnight'. It also saves the readme file, explanations of variables, and validation metadata, and combines these all into a single 'list' that we called 'loadData'. The only part of this list that we really need for this tutorial is the 'mam_pertrapnight' table, so let's extract just that one and call it 'myData'.

myData <- loadData$mam_pertrapnight

class(myData) # Confirm that 'myData' is a data.frame

## [1] "data.frame"

Use dplyr

For the rest of this tutorial, we are only going to be working with three variables within 'myData':

• scientificName a string of "Genus species"
• sex a string with "F", "M", or "U"
• identificationQualifier a string noting uncertainty in the species identification

filter()

This function:

• extracts only a subset of rows from a data frame according to specified conditions
• is similar to the base function subset(), but with simpler syntax
• inputs: data object, any number of conditional statements on the named columns of the data object
• output: a data object of the same class as the input object (e.g., data.frame in, data.frame out) with only those rows that meet the conditions

For example, let's create a new dataframe that contains only female Peromyscus mainculatus, one of the key small mammal players in the life cycle of Lyme disease-causing bacterium.

# filter on `scientificName` is Peromyscus maniculatus and `sex` is female. 
# two equals signs (==) signifies "is"
data_PeroManicFemales <- filter(myData, 
                   scientificName == 'Peromyscus maniculatus', 
                   sex == 'F')

# Note how we were able to put multiple conditions into the filter statement,
# pretty cool!

So we have a dataframe with our female P. mainculatus but how many are there?

# how many female P. maniculatus are in the dataset
# would could simply count the number of rows in the new dataset
nrow(data_PeroManicFemales)

## [1] 98

# or we could write is as a sentence
print(paste('In 2014, NEON technicians captured',
                   nrow(data_PeroManicFemales),
                   'female Peromyscus maniculatus at Harvard Forest.',
                   sep = ' '))

## [1] "In 2014, NEON technicians captured 98 female Peromyscus maniculatus at Harvard Forest."

That's awesome that we can quickly and easily count the number of female Peromyscus maniculatus that were captured at Harvard Forest in 2014. However, there is a slight problem. There are two Peromyscus species that are common in Harvard Forest: Peromyscus maniculatus (deer mouse) and Peromyscus leucopus (white-footed mouse). These species are difficult to distinguish in the field, leading to some uncertainty in the identification, which is noted in the 'identificationQualifier' data field by the term "cf. species" or "aff. species". (For more information on these terms and 'open nomenclature' please ). When the field technician is certain of their identification (or if they forget to note their uncertainty), identificationQualifier will be NA. Let's run the same code as above, but this time specifying that we want only those observations that are unambiguous identifications.

# filter on `scientificName` is Peromyscus maniculatus and `sex` is female. 
# two equals signs (==) signifies "is"
data_PeroManicFemalesCertain <- filter(myData, 
                   scientificName == 'Peromyscus maniculatus', 
                   sex == 'F',
                   identificationQualifier =="NA")

# Count the number of un-ambiguous identifications
nrow(data_PeroManicFemalesCertain)

## [1] 0

Woah! So every single observation of a Peromyscus maniculatus had some level of uncertainty that the individual may actually be Peromyscus leucopus. This is understandable given the difficulty of field identification for these species. Given this challenge, it will be best for us to consider these mice at the genus level. We can accomplish this by searching for only the genus name in the 'scientificName' field using the grepl() function.

grepl()

This is a function in the base package (e.g., it isn't part of dplyr) that is part of the suite of Regular Expressions functions. grepl uses regular expressions to match patterns in character strings. Regular expressions offer very powerful and useful tricks for data manipulation. They can be complicated and therefore are a challenge to learn, but well worth it!

Here, we present a very simple example.

• inputs: pattern to match, character vector to search for a match
• output: a logical vector indicating whether the pattern was found within each element of the input character vector

Let's use grepl to learn more about our possible disease vectors. In reality, all species of Peromyscus are viable players in Lyme disease transmission, and they are difficult to distinguise in a field setting, so we really should be looking at all species of Peromyscus. Since we don't have genera split out as a separate field, we have to search within the scientificName string for the genus -- this is a simple example of pattern matching.

We can use the dplyr function filter() in combination with the base function grepl() to accomplish this.

# combine filter & grepl to get all Peromyscus (a part of the 
# scientificName string)
data_PeroFemales <- filter(myData,
                   grepl('Peromyscus', scientificName),
                   sex == 'F')

# how many female Peromyscus are in the dataset
print(paste('In 2014, NEON technicians captured',
                   nrow(data_PeroFemales),
                   'female Peromyscus spp. at Harvard Forest.',
                   sep = ' '))

## [1] "In 2014, NEON technicians captured 558 female Peromyscus spp. at Harvard Forest."

group_by() + summarise()

An alternative to using the filter function to subset the data (and make a new data object in the process), is to calculate summary statistics based on some grouping factor. We'll use group_by(), which does basically the same thing as SQL or other tools for interacting with relational databases. For those unfamiliar with SQL, no worries - dplyr provides lots of additional functionality for working with databases (local and remote) that does not require knowledge of SQL. How handy!

The group_by() function in dplyr allows you to perform functions on a subset of a dataset without having to create multiple new objects or construct for() loops. The combination of group_by() and summarise() are great for generating simple summaries (counts, sums) of grouped data.

NOTE: Be continentious about using summarise with other summary functions! You need to think about weighting for means and variances, and summarize doesn't work precisely for medians if there is any missing data (e.g., if there was no value recorded, maybe it was for a good reason!).

Continuing with our small mammal data, since the diversity of the entire small mammal community has been shown to impact disease dynamics among the key reservoir species, we really want to know more about the demographics of the whole community. We can quickly generate counts by species and sex in 2 lines of code, using group_by and summarise.

# how many of each species & sex were there?
# step 1: group by species & sex
dataBySpSex <- group_by(myData, scientificName, sex)

# step 2: summarize the number of individuals of each using the new df
countsBySpSex <- summarise(dataBySpSex, n_individuals = n())

## `summarise()` regrouping output by 'scientificName' (override with `.groups` argument)

# view the data (just top 10 rows)
head(countsBySpSex, 10)

## # A tibble: 10 x 3
## # Groups:   scientificName [5]
##    scientificName          sex   n_individuals
##    <chr>                   <chr>         <int>
##  1 Blarina brevicauda      F                50
##  2 Blarina brevicauda      M                 8
##  3 Blarina brevicauda      U                22
##  4 Blarina brevicauda      <NA>             19
##  5 Glaucomys volans        M                 1
##  6 Mammalia sp.            U                 1
##  7 Mammalia sp.            <NA>              1
##  8 Microtus pennsylvanicus F                 2
##  9 Myodes gapperi          F               103
## 10 Myodes gapperi          M                99

Note: the output of step 1 (dataBySpSex) does not look any different than the original dataframe (myData), but the application of subsequent functions (e.g., summarise) to this new dataframe will produce distinctly different results than if you tried the same operations on the original. Try it if you want to see the difference! This is because the group_by() function converted dataBySpSex into a "grouped_df" rather than just a "data.frame". In order to confirm this, try using the class() function on both myData and dataBySpSex. You can also read the help documentation for this function by running the code: ?group_by()

# View class of 'myData' object
class(myData)

## [1] "data.frame"

# View class of 'dataBySpSex' object
class(dataBySpSex)

## [1] "grouped_df" "tbl_df"     "tbl"        "data.frame"

# View help file for group_by() function
?group_by()

Pipe functions together

We created multiple new data objects during our explorations of dplyr functions, above. While this works, we can produce the same results more efficiently by chaining functions together and creating only one new data object that encapsulates all of the previously sought information: filter on only females, grepl to get only Peromyscus spp., group_by individual species, and summarise the numbers of individuals.

# combine several functions to get a summary of the numbers of individuals of 
# female Peromyscus species in our dataset.

# remember %>% are "pipes" that allow us to pass information from one function 
# to the next. 

dataBySpFem <- myData %>% 
                  filter(grepl('Peromyscus', scientificName), sex == "F") %>%
                  group_by(scientificName) %>%
                  summarise(n_individuals = n())

## `summarise()` ungrouping output (override with `.groups` argument)

# view the data
dataBySpFem

## # A tibble: 3 x 2
##   scientificName         n_individuals
##   <chr>                          <int>
## 1 Peromyscus leucopus              455
## 2 Peromyscus maniculatus            98
## 3 Peromyscus sp.                     5

Cool!

Base R only

So that is nice, but we had to install a new package dplyr. You might ask, "Is it really worth it to learn new commands if I can do this is base R." While we think "yes", why don't you see for yourself. Here is the base R code needed to accomplish the same task.

# For reference, the same output but using R's base functions

# First, subset the data to only female Peromyscus
dataFemPero  <- myData[myData$sex == 'F' & 
                   grepl('Peromyscus', myData$scientificName), ]

# Option 1 --------------------------------
# Use aggregate and then rename columns
dataBySpFem_agg <-aggregate(dataFemPero$sex ~ dataFemPero$scientificName, 
                   data = dataFemPero, FUN = length)
names(dataBySpFem_agg) <- c('scientificName', 'n_individuals')

# view output
dataBySpFem_agg

##           scientificName n_individuals
## 1    Peromyscus leucopus           455
## 2 Peromyscus maniculatus            98
## 3         Peromyscus sp.             5

# Option 2 --------------------------------------------------------
# Do it by hand

# Get the unique scientificNames in the subset
sppInDF <- unique(dataFemPero$scientificName[!is.na(dataFemPero$scientificName)])

# Use a loop to calculate the numbers of individuals of each species
sciName <- vector(); numInd <- vector()
for (i in sppInDF) {
  sciName <- c(sciName, i)
  numInd <- c(numInd, length(which(dataFemPero$scientificName==i)))
}

#Create the desired output data frame
dataBySpFem_byHand <- data.frame('scientificName'=sciName, 
                   'n_individuals'=numInd)

# view output
dataBySpFem_byHand

##           scientificName n_individuals
## 1    Peromyscus leucopus           455
## 2 Peromyscus maniculatus            98
## 3         Peromyscus sp.             5