Day 13: Functional programming with purrr

Do this before class

Read R4DS chapter 21.

Complete the course Foundations of Functional Programming with purrr at DataCamp.

During class

Solve exercises in R4DS chapter 21.5.3.

A likelihood function

If \(x\) is a vector of \(n\) independent \(Exponential(\lambda)\)-distributed random variables, the likelihood function for \(\lambda\) is

\[ L(\lambda)= \prod_{i=1}^n\lambda\exp(-x_i\lambda) \] This can be computed given a value of \(\lambda\) using

L_exp <- function(x, lambda) prod(dexp(x, lambda))

We try with

set.seed(1)
x <- rexp(100, 1)
L_exp(x, 1)

## [1] 1.730976e-45

which seems to work. If we want to plot the function, we need to compute it over a sequence of \(\lambda\)-values. Trying

lambda <- seq(0.5, 2, length.out = 10)
L_exp(x, lambda)

## [1] 7.026489e-50

gives a scalar instead of the intended vector of \(L\)-values, what is computed? Use map_dbl to compute the vector \((L(\lambda_1), \ldots, L(\lambda_{10}))\).

More dramatic

Modify the function repertoir from day 12 such that it generates a suitable result if the API call fails (e.g. due to an empty repertoir). Use it in combination with e.g. map_df in order to create a table of all sets from 1908-2017. Who is the most active director?

More SHL-players

The function get_player below fetches data on an SHL-player given an url of the type "http://www.shl.se/lag/087a-087aTQv9u__frolunda-hc/qQ9-a5b4QRqdS__ryan-lasch"

get_player <- function(url){
    response <- GET(paste0(url, "/statistics"))
    player_xml <- content(response, "text") %>% 
        read_html(player_page)
    css <- list(".rmss_c-squad__player-header-name-h", # Namn
                ".rmss_c-squad__player-header-name-info-position", #Position
                ".rmss_c-squad__player-header-info-items-item-value") # Info
    map(css, html_nodes, x = player_xml) %>% 
        map(html_text) %>% 
        unlist() %>% 
        set_names(c("Namn", "Position", "Födelsedatum", "Ålder", "Nationalitet", "Längd", "Vikt", "Skjuter"))
}

make sure it returns a suitable error-message if the GET-call fails (in a successful call, response$status_code equals 200). Construct a new function get_team, that given an url of the form "http://www.shl.se/lag/2459-2459QTs1f__djurgarden-hockey/roster" generates a data.frame of players and informations from get_player.