List and description of all packages in CRAN from within R

Question

I can get a list of all the available packages with the function:

ap <- available.packages()

But how can I also get a description of these packages from within R, so I can have a data.frame with two columns: package and description?

Answer 1

Dirk has provided an answer that is terrific and after finishing my solution and then seeing his I debated for some time posting my solution for fear of looking silly. But I decided to post it anyway for two reasons:

1. it is informative to beginning scrapers like myself
2. it took me a while to do and so why not :)

I approached this thinking I'd need to do some web scraping and choose crantastic as the site to scrape from. First I'll provide the code and then two scraping resources that have been very helpful to me as I learn:

library(RCurl)
library(XML)

URL <- "http://cran.r-project.org/web/checks/check_summary.html#summary_by_package"
packs <- na.omit(XML::readHTMLTable(doc = URL, which = 2, header = T, 
    strip.white = T, as.is = FALSE, sep = ",", na.strings = c("999", 
        "NA", " "))[, 1])
Trim <- function(x) {
    gsub("^\\s+|\\s+$", "", x)
}
packs <- unique(Trim(packs))
u1 <- "http://crantastic.org/packages/"
len.samps <- 10 #for demo purpose; use:
#len.samps <- length(packs) # for all of them
URL2 <- paste0(u1, packs[seq_len(len.samps)]) 
scraper <- function(urls){ #function to grab description
    doc   <- htmlTreeParse(urls, useInternalNodes=TRUE)
    nodes <- getNodeSet(doc, "//p")[[3]]
    return(nodes)
}
info <- sapply(seq_along(URL2), function(i) try(scraper(URL2[i]), TRUE))
info2 <- sapply(info, function(x) { #replace errors with NA
        if(class(x)[1] != "XMLInternalElementNode"){
            NA
        } else {
            Trim(gsub("\\s+", " ", xmlValue(x)))
        }
    }
)
pack_n_desc <- data.frame(package=packs[seq_len(len.samps)], 
    description=info2) #make a dataframe of it all

Resources:

1. talkstats.com thread on web scraping (great beginner examples)
2. w3schools.com site on html stuff (very helpful)

Answer 2

I wanted to try to do this using a HTML scraper (rvest) as an exercise, since the available.packages() in OP doesn't contain the package Descriptions.

library('rvest')
url <- 'https://cloud.r-project.org/web/packages/available_packages_by_name.html'
webpage <- read_html(url)
data_html <- html_nodes(webpage,'tr td')
length(data_html)

P1 <- html_nodes(webpage,'td:nth-child(1)') %>% html_text(trim=TRUE)  # XML: The Package Name
P2 <- html_nodes(webpage,'td:nth-child(2)') %>% html_text(trim=TRUE)  # XML: The Description
P1 <- P1[lengths(P1) > 0 & P1 != ""]  # Remove NULL and empty ("") items
length(P1); length(P2);

mdf <- data.frame(P1, P2, row.names=NULL)
colnames(mdf) <- c("PackageName", "Description")

# This is the problem! It lists large sets column-by-column,
# instead of row-by-row. Try with the full list to see what happens.
print(mdf, right=FALSE, row.names=FALSE)

# PackageName Description                                                             
# A3          Accurate, Adaptable, and Accessible Error Metrics for Predictive\nModels
# abbyyR      Access to Abbyy Optical Character Recognition (OCR) API                 
# abc         Tools for Approximate Bayesian Computation (ABC)                        
# abc.data    Data Only: Tools for Approximate Bayesian Computation (ABC)             
# ABC.RAP     Array Based CpG Region Analysis Pipeline                                
# ABCanalysis Computed ABC Analysis

# For small sets we can use either:
# mdf[1:6,] #or# head(mdf, 6)

However, although working quite well for small array/dataframe list (subset), I ran into a display problem with the full list, where the data would be shown either column-by-column or unaligned. I would have been great to have this paged and properly formatted in a new window somehow. I tried using page, but I couldn't get it to work very well.

---

EDIT: The recommended method is not the above, but rather using Dirk's suggestion (from the comments below):

db <- tools::CRAN_package_db()
colnames(db)
mdf <- data.frame(db[,1], db[,52])
colnames(mdf) <- c("Package", "Description")
print(mdf, right=FALSE, row.names=FALSE)

However, this still suffers from the display problem mentioned...