Functions to get question and answer sets from Stack Overflow, and to extract the code blocks from those questions.

SOPageLoaderHelper.R

#'Download a Stack Overflow question page as a character vector
#'
#'Helper function for other Stack Overflow related functions. Downloads the 
#'page as a character vector and extracts the portion that is needed for other
#'functions.
#'
#'@param qid The numeric question ID.
#'@return A character vector
#'
#'@author Ananda Mahto
#'
SOQuestionPage <- function(qid) {
  T1 <- suppressWarnings(
    readLines(paste("http://stackoverflow.com/q", 
                    as.character(qid), sep = "/")))
  cStart <- which(
    grepl(
      '<div itemscope itemtype="http://schema.org/Article">', T1, 
      fixed = TRUE)) + 3
  cEnd <- which(grepl("<a name='new-answer'></a>", T1, fixed = TRUE)) - 1
  T1[cStart:cEnd]
}
NULL


#'Clean text within \code{<pre><code>} tag blocks
#'
#'Text within \code{<pre><code>} tag blocks need to be "cleaned". For R code
#'that usually means changing \code{&lt;} and \code{&gt;} to \code{<} and 
#'\code{>}. 
#'
#'@param x The input character vector
#'@return A character vector
#'@author Ananda Mahto
#'
PreTagClean <- function(x) {
  x <- gsub(".*<pre><code>", "", x)
  x <- gsub("</code></pre>.*", "", x)
  x <- gsub("&lt;", "<", x)
  x <- gsub("&gt;", ">", x)
  pattern <- "</?\\w+((\\s+\\w+(\\s*=\\s*(?:\".*?\"|'.*?'|[^'\">\\s]+))?)+\\s*|\\s*)/?>"
  x <- gsub(pattern, "\\1", x)
  x
}
NULL


#'Create a list of code blocks from an input character vector
#'
#'A helper function for \code{\link{SOCodeBlocks}} to extract the lines in 
#'the character vector corresponding to a code block.
#'
#'@param x The input character vector
#'@return A character vector of just the code blocks from a Stack Overflow question.
#'@author Ananda Mahto
#'
CodeList <- function(x) {
  T1 <- data.frame(start = which(grepl("<pre><code>", x)),
                   end = which(grepl("</code></pre>", x)))
  T1 <- data.frame(t(T1))
  codeList <- lapply(T1, function(z) x[z[1]:z[2]])
  codeList <- lapply(codeList, PreTagClean)
  codeList
}

SOPrintMethods.R

print.SOCodeBlocks <- function(x, ...) {
  lapply(seq_along(x), 
         function(y) {
           cat("\n>>> Block", y, "\n\n")
           cat("", paste(x[[y]], "\n"))
           cat("", paste(rep("-", 60), sep = "", collapse = ""), "\n")
         })
  invisible(x)
}

print.QAList <- function(x, ...) {
  y <- lapply(x, PreTagClean)
  z <- c(paste("Question      : ", attr(x, "QTitle"), 
               "\n>>>  Question ID   : ", attr(x, "QID"), 
               "\n>>>  Question Tags : ", attr(x, "QTags"), sep =""),
         paste("Answer ID:", attr(x, "AnswerIDs")))
  lapply(seq_along(y), 
         function(Y) {
           cat("\n>>> ", z[Y], "\n\n")
           cat("", paste(y[[Y]], "\n"))
           cat("", paste(rep("-", 60), sep = "", collapse = ""), "\n")
         })
  invisible(x)
} 

StackOverflow.R

#'Print a list of question and answers from a Stack Overflow page
#'
#'Print a list of question and answers from a Stack Overflow page (entered by
#'question ID number).
#'
#'Currently hard-coded ot Stack Overflow, but may be generalized for other 
#'Stack Exchange sites.
#'
#'@param qid The numeric question ID.
#'@return A list of the class \code{c("QAList", "list")}
#'@note The formatting is handled by the \code{print.QAList} print method. The
#'HTML is still viewable by removing \code{"QAList"} from the list class, or 
#'by accessing the individual list items.
#'@author Ananda Mahto
#'@examples
#'
#'temp <- QAList(15332195)
#'temp
#'temp[[1]]
#'
#'\dontshow{rm(temp)}
#'
QAList <- function(qid) {
  T1 <- SOQuestionPage(qid)
  QTitle <- strsplit(T1[grepl("<h1 itemprop=\"name\">", T1)], ">|<")[[1]][5]
  AnswerIDs <- gsub(".*answer-([0-9]+).*", "\\1", 
                    T1[grepl("<div id=\"answer-", T1)])
  QTags <- T1[which(grepl("<div class=\"post-taglist\">", T1)) + 1]
  QTags <- paste(
    strsplit(QTags, "rel=\"tag\">|</a>")[[1]][c(FALSE, TRUE)], collapse = ", ")
  QAEnds <- which(grepl("<table class=\"fw\">", T1))
  Question <- data.frame(
    Start = which(
      grepl("<div class=\"post-text\" itemprop=\"description\">", T1)) + 1,
    End = QAEnds[1] - 6)
  Answers <- data.frame(
    Start = which(grepl("<td class=\"answercell\">", T1)) + 1,
    End = QAEnds[-1] - 2)
  ParsedPage <- apply(rbind(Question, Answers), 1, function(z) T1[z[1]:z[2]])
  ParsedPage <- lapply(ParsedPage, 
                       function(x) gsub("<div class=\"post-text\">", "", x))
  attr(ParsedPage, "QTitle") <- QTitle
  attr(ParsedPage, "QID") <- as.character(qid)
  attr(ParsedPage, "QTags") <- QTags
  attr(ParsedPage, "AnswerIDs") <- as.character(AnswerIDs)
  class(ParsedPage) <- c("QAList", class(ParsedPage))
  ParsedPage
}



#'Return a list of the code blocks at a Stack Overflow question page
#'
#'Return a list of the code blocks at a Stack Overflow question page (specified
#'by numeric question ID).
#'
#'Currently hard-coded ot Stack Overflow, but may be generalized for other 
#'Stack Exchange sites.
#'
#'@param qid The numeric question ID
#'@return A list of the class c("SOCodeBlocks", "list")
#'@note The formatting is handled by the \code{print.SOCodeBlocks} print method. 
#'The HTML is still viewable by removing "SOCodeBlocks" from the list class, or 
#'by accessing the individual list items.
#'@author Ananda Mahto
#'@examples
#'
#'temp <- SOCodeBlocks(15332195)
#'temp
#'temp[[6]]
#'
#'\dontshow{rm(temp)}
#'
SOCodeBlocks <- function(qid) {
  T1 <- QAList(qid)
  T2 <- unlist(lapply(T1, CodeList), recursive=FALSE, use.names=FALSE)
  class(T2) <- c("SOCodeBlocks", class(T2))
  T2
} 

The last function seems like it could be really useful. Perhaps sourcing all code from a question? Consider the case where there are multiple code chunks in a reproducible example and several intermediate objects. I don't have the expertise, but that would also make a pretty neat Firefox/Chrome extension.

One possible improvement might be to read it in with with an HTML parser (XML package?). I don't know much about the problems, but this answer seems to have some adamant opinions.