Error: Replacement has length zero R

I 'm trying to scrape http://then.gasbuddy.com/.

I'm running the next code in R

 library(RCurl)
 library(XML)
 doc <- htmlTreeParse('http://www.southcarolinagasprices.com/GasPriceSearch.aspx?typ=adv&fuel=A&srch=0&area=All%20Areas&station=All%20Stations&tme_limit=4')
rootNode <- xmlRoot(doc)

((rootNode[[2]][4])[1][[1]])[[15]][[1]][[11]][[1]][[1]][[2]][[8]][[1]][[2]][[1]][[1]][[1]][[1]][[1]][[1]]

#<div class="p1"/>

x <- matrix(, nrow = 20, ncol = 4)

x[1,1] <- xmlValue(((rootNode[[2]][4])[1][[1]])[[15]][[1]][[11]][[1]][[1]][[2]][[8]][[1]][[2]][[1]][[1]][[1]][[1]][[1]][[1]])

But I have this error

replacement has length zero

How can I subtract p1 and put it in a matrix?

Solution

You've come up with an interesting way to get around their price obfuscation. Since they didn't restrict scraping in their Terms of Service, here's one way you can scrape the prices:

library(xml2)

doc <- read_html('http://www.southcarolinagasprices.com/GasPriceSearch.aspx?typ=adv&fuel=A&srch=0&area=All%20Areas&station=All%20Stations&tme_limit=4')

prices <- xml_find_all(doc, xpath="//div[@class='sp_p']")

sapply(prices, function(x) {
  as.numeric(paste(gsub("d", "\\.", 
                        gsub("^p", "", 
                             unlist(xml_attrs(xml_find_all(x, "./div"))))),
                   collapse=""))
})

##   [1] 1.65 1.65 1.65 1.65 1.65 1.65 1.65 1.65 1.65 1.67 1.68 1.69 1.69 1.69 1.69 1.69 1.69 1.69 1.69
##  [20] 1.70 1.71 1.72 1.72 1.73 1.73 1.73 1.73 1.73 1.73 1.73 1.73 1.73 1.74 1.74 1.74 1.74 1.74 1.74
##  [39] 1.74 1.74 1.74 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.76 1.76
##  [58] 1.76 1.76 1.76 1.76 1.76 1.76 1.76 1.76 1.76 1.76 1.77 1.77 1.77 1.77 1.77 1.77 1.77 1.77 1.77
##  [77] 1.77 1.77 1.77 1.77 1.77 1.77 1.77 1.77 1.77 1.77 1.77 1.77 1.77 1.77 1.77 1.77 1.77 1.77 1.77
##  [96] 1.77 1.77 1.77 1.77 1.77 1.77 1.77 1.77 1.77 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78
## [115] 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78
## [134] 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79
## [153] 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79
## [172] 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79
## [191] 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79 1.79