dplyr & Functions stat 480 Heike Hofmann

Transcription

1 dplyr & Functions stat 480 Heike Hofmann

2 Outline dplyr functions and package Functions

3 library(dplyr) data(baseball, package= plyr ))

4 Your Turn Use data(baseball, package="plyr") to make the baseball dataset active in R. Subset the data on your favorite player (you will need the Lahmann ID, e.g. Sammy Sosa sosasa01, Barry Bonds bondsba01, Babe Ruth ruthba01) Compute your player s batting averages for each season (batting average = #Hits/#at bats). Define a new variable experience in the data set as year - min(year) Plot averages by #years of experience. Compute an alltime batting average for your player.

5 summarise What does the summarise function do? Read up on it on its help pages: help(summarise)

6 summarise # overall batting average summarise(baseball,! mba = sum(h)/sum(ab) ) summarise(baseball,! first = min(year),!!!!! # first year of baseball records! duration = max(year) - min(year),! # duration of record taking in years! nteams = length(unique(team)),!! # different number of teams! nplayers = length(unique(id))!! # number of baseball players # in the dataset )

7 dplyr package introduces workflow that makes working with large datasets (relatively) easy main functionality: group_by, summarise, mutate, filter vignettes/introduction.html

8 group_by group_by(data, var1,...) is a function that takes a dataset and introduces a group for each (combination of) level(s) of the grouping variable(s) Power combination: group_by and summarise for a grouped dataframe, the summary statistics will be calculated for every group

9 library(dplyr) data(baseball, package="plyr") summarise(baseball, seasons = max(year)-min(year)+1, atbats = sum(ab), avg = sum(h, na.rm=t)/sum(ab, na.rm=t) ) seasons atbats avg summarise(group_by(baseball, id), seasons = max(year)-min(year)+1, atbats = sum(ab), avg = sum(h, na.rm=t)/sum(ab, na.rm=t) ) id seasons atbats avg 1 perezne walketo sweenma schmija loaizes

10 Chaining operator %.% x %.% f(y) is equivalent to f(x, y) baseball %.% group_by(id) is equivalent to group_by(baseball, id) Read %.% as then i.e. take data, then group it by player s id, then summarise it to

11 Chained version of example baseball %.% group_by(id) %.% summarise( seasons = max(year)-min(year)+1, atbats = sum(ab), avg = sum(h, na.rm=t)/sum(ab, na.rm=t) )

12 Your Turn Use dplyr statements to get (a) the life time batting average for each player (mba) (b) the life time number of times each player was at bats. (nab) Plot nab versus mba.

13 filter filter(data, expr1,...) is a function that takes a dataset and subsets it according to a set of expressions filter() works similarly to subset() except that you can give it any number of filtering conditions which are joined together with the logical AND &. You can use other boolean operators explicitly

14 Your Turn Use dplyr statements to get the number of team members on a team for each season (think of unique) Has the number of homeruns per season changed over time? Summarize the data with dplyr routines first, then visualize.

15 Functions in R Have been using functions a lot, now we want to write them ourselves! Idea: avoid repetitive coding (errors will creep in) Instead: extract common core, wrap it in a function, make it reusable

16 Basic Structure Name Input arguments names, default values Body Output values

17 A first function mean <- function(x) { return(sum(x)/length(x)) } mean(1:15) mean(c(1:15, NA)) mean <- function(x, na.rm=f) { if (na.rm) x <- na.omit(x) } return(sum(x)/length(x)) mean(1:15) mean(c(1:15, NA), na.rm=t)

18 Function mean Name: Input arguments mean x, na.rm=t names, default values Body Output values if(na.rm) x <- na.omit(x) return(sum(x)/length(x))

19 Function Writing Start simple, then extend Test out each step of the way Don t try too much at once help(browser)

20 Practice Write a function called mba input: playerid output: life-time batting average for playerid what does mba( bondsba01 )do? write a function called pstats input: playerid output: life-time batting average for playerid number of overall at bats

21 Checkpoint Submit all of your code for the last Your Turn at

22 Testing Always test the functions you ve written Even better: let somebody else test them for you Switch seats with your neighbor, test their functions!