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POSearch for links in webpage, follow them, and return pages with no links in R
 

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    <p>I am writing an update to my <a href="http://cran.r-project.org/web/packages/rNOMADS/index.html" rel="nofollow">rNOMADS package</a> to include all the models on the NOMADS web site. To do this, I must search the html directory tree for each model. I do not know how deep this tree is, or how many branches it contains, beforehand. Therefore I am writing a simple web crawler to recursively search the top page for links, follow each link, and return the URLs of pages that have no more links. Such a page is the download page for model data. <a href="http://nomads.ncep.noaa.gov/cgi-bin/filter_cmcens.pl" rel="nofollow">Here is an example</a> of a URL that must be searched.</p> <p>I want to get the addresses of all web pages below this one. I have attempted this code:</p> <pre><code>library(XML) url &lt;- "http://nomads.ncep.noaa.gov/cgi-bin/filter_cmcens.pl" WebCrawler &lt;- function(url) { doc &lt;- htmlParse(url) links &lt;- xpathSApply(doc, "//a/@href") free(doc) if(is.null(links)) { #If there are no links, this is the page we want, return it! return(url) } else { for(link in links) { #Call recursively on each link found print(link) return(WebCrawler(link)) } } } </code></pre> <p>However, I have not figured out a good way to return a list of all the "dead end" pages. Instead, this code will only return one model page, not the whole list of them. I could declare a global variable and have the URLS saved to that variable, but I am wondering if there is a better way to go about this. How should I go about constructing this function to give me a list of <em>every single page</em>?</p>
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