I try to make a WebCrawler which find links from a homepage and visit the found links again and again..
Now i have written a code w9ith a parser which shows me the found links and print there statistics of some tags of this homepage but i dont get it how to visit the new links in a loop and print there statistics too.
*
#visit = {}
#src = Net::HTTP.start(#url.host, #url.port) do |http|
http.get(#url.path)
#content = #src.body
*
def govisit
if #content =~ #commentTag
end
cnt = #content.scan(#aTag)
cnt.each do |link|
#visit[link] = []
end
puts "Links on this site: "
#visit.each do |links|
puts links
end
if #visit.size >= 500
exit 0
end
printStatistics
end
First of all you need a function that accepts a link and returns the body output. Then parse all the links out of the body and keep a list of links. Check that list if you didn't visit the link yet. Remove those visited links from the new links list and call the same function again and do it all over.
To stop the crawler at a certain point you need to build in a condition the while loop.
based on your code:
#visited_links = []
#new_links = []
def get_body(link)
#visited_links << link
#src = Net::HTTP.start(#url.host, #url.port) { |http| http.get(#url.path) }
#src.body
end
def get_links(body)
# parse the links from your body
# check if the content does not have the same link
end
start_link_body = get_body("http://www.test.com")
get_links(start_link_body)
while #visited_links < 500 do
body = get_body(#new_links.shift)
get_links(body)
end
Related
I'm new to Ruby and am using Nokogiri to parse html webpages. An error is thrown in a function when it gets to the line:
currentPage = Nokogiri::HTML(open(url))
I have verified the inputs of the function, url is a string with a webaddress. The line I previously mention works exactly as intended when used outside of the function, but not inside. When it gets to that line inside the function the following error is thrown:
WebCrawler.rb:25:in `explore': undefined method `+#' for #<Nokogiri::HTML::Document:0x007f97ea0cdf30> (NoMethodError)
from WebCrawler.rb:43:in `<main>'
The function the problematic line is in is pasted below.
def explore(url)
if CRAWLED_PAGES_COUNTER > CRAWLED_PAGES_LIMIT
return
end
CRAWLED_PAGES_COUNTER++
currentPage = Nokogiri::HTML(open(url))
links = currentPage.xpath('//#href').map(&:value)
eval_page(currentPage)
links.each do|link|
puts link
explore(link)
end
end
Here is the full program (It's not much longer):
require 'nokogiri'
require 'open-uri'
#Crawler Params
START_URL = "https://en.wikipedia.org"
CRAWLED_PAGES_COUNTER = 0
CRAWLED_PAGES_LIMIT = 5
#Crawler Functions
def explore(url)
if CRAWLED_PAGES_COUNTER > CRAWLED_PAGES_LIMIT
return
end
CRAWLED_PAGES_COUNTER++
currentPage = Nokogiri::HTML(open(url))
links = currentPage.xpath('//#href').map(&:value)
eval_page(currentPage)
links.each do|link|
puts link
explore(link)
end
end
def eval_page(page)
puts page.title
end
#Start Crawling
explore(START_URL)
require 'nokogiri'
require 'open-uri'
#Crawler Params
$START_URL = "https://en.wikipedia.org"
$CRAWLED_PAGES_COUNTER = 0
$CRAWLED_PAGES_LIMIT = 5
#Crawler Functions
def explore(url)
if $CRAWLED_PAGES_COUNTER > $CRAWLED_PAGES_LIMIT
return
end
$CRAWLED_PAGES_COUNTER+=1
currentPage = Nokogiri::HTML(open(url))
links = currentPage.xpath('//#href').map(&:value)
eval_page(currentPage)
links.each do|link|
puts link
explore(link)
end
end
def eval_page(page)
puts page.title
end
#Start Crawling
explore($START_URL)
Just to give you something to build from, this is a simple spider that only harvests and visits links. Modifying it to do other things would be easy.
require 'nokogiri'
require 'open-uri'
require 'set'
BASE_URL = 'http://example.com'
URL_FORMAT = '%s://%s:%s'
SLEEP_TIME = 30 # in seconds
urls = [BASE_URL]
last_host = BASE_URL
visited_urls = Set.new
visited_hosts = Set.new
until urls.empty?
this_uri = URI.join(last_host, urls.shift)
next if visited_urls.include?(this_uri)
puts "Scanning: #{this_uri}"
doc = Nokogiri::HTML(this_uri.open)
visited_urls << this_uri
if visited_hosts.include?(this_uri.host)
puts "Sleeping #{SLEEP_TIME} seconds to reduce server load..."
sleep SLEEP_TIME
end
visited_hosts << this_uri.host
urls += doc.search('[href]').map { |node|
node['href']
}.select { |url|
extension = File.extname(URI.parse(url).path)
extension[/\.html?$/] || extension.empty?
}
last_host = URL_FORMAT % [:scheme, :host, :port].map{ |s| this_uri.send(s) }
puts "#{urls.size} URLs remain."
end
It:
Works on http://example.com. That site is designed and designated for experimenting.
Checks to see if a page was visited previously and won't scan it again. It's a naive check and will be fooled by URLs containing queries or queries that are not in a consistent order.
Checks to see if a site was previously visited and automatically throttles the page retrieval if so. It could be fooled by aliases.
Checks to see if a page ends with ".htm", ".html" or has no extension. Anything else is ignored.
The actual code to write an industrial strength spider is much more involved. Robots.txt files need to be honored, figuring out how to deal with pages that redirect to other pages either via HTTP timeouts or JavaScript redirects is a fun task, dealing with malformed pages are a challenge....
I want to collect the names of users in a particular group, called Nature, in the photo-sharing website Fotolog. This is my code:
require 'rubygems'
require 'mechanize'
require 'csv'
def getInitUser()
agent1 = Mechanize.new
number = 0
while number<=500
address = 'http://http://www.fotolog.com/nature/participants/#{number}/'
logfile2 = File.new("Fotolog/Users.csv","a")
tryConut = 0
begin
page = agent1.get(address)
rescue
tryConut=tryConut+1
if tryConut<5
retry
end
return
end
arrayUsers= []
# search for the users
page.search("a[class=img_border_radius").map do |opt|
link = opt.attributes['href'].text
link = link.gsub("http://www.fotolog.com/","").gsub("/","")
arrayUsers << link
logfile2.print("#{link}\n")
end
number = number+100
end
return arrayUsers
end
arrayUsers = getInitUser()
arrayUsers.each do |user|
getFriend(user)
end
But the Users.csv file I am getting is empty. What's wrong here? I suspect it might have something to do with the "class" tag I am using. But from the inspect element, it seems to be the correct class, isn't it? I am just getting started with web crawling, so I apologise if this is a silly query.
I'm trying to create a simple web-crawler, so I wrote this:
(Method get_links take a parent link from which we will seek)
require 'nokogiri'
require 'open-uri'
def get_links(link)
link = "http://#{link}"
doc = Nokogiri::HTML(open(link))
links = doc.css('a')
hrefs = links.map {|link| link.attribute('href').to_s}.uniq.delete_if {|href| href.empty?}
array = hrefs.select {|i| i[0] == "/"}
host = URI.parse(link).host
links_list = array.map {|a| "#{host}#{a}"}
end
(Method search_links, takes an array from get_links method and search at this array)
def search_links(urls)
urls = get_links(link)
urls.uniq.each do |url|
begin
links = get_links(url)
compare = urls & links
urls << links - compare
urls.flatten!
rescue OpenURI::HTTPError
warn "Skipping invalid link #{url}"
end
end
return urls
end
This method finds most of links from the website, but not all.
What did I do wrong? Which algorithm I should use?
Some comments about your code:
def get_links(link)
link = "http://#{link}"
# You're assuming the protocol is always http.
# This isn't the only protocol on used on the web.
doc = Nokogiri::HTML(open(link))
links = doc.css('a')
hrefs = links.map {|link| link.attribute('href').to_s}.uniq.delete_if {|href| href.empty?}
# You can write these two lines more compact as
# hrefs = doc.xpath('//a/#href').map(&:to_s).uniq.delete_if(&:empty?)
array = hrefs.select {|i| i[0] == "/"}
# I guess you want to handle URLs that are relative to the host.
# However, URLs relative to the protocol (starting with '//')
# will also be selected by this condition.
host = URI.parse(link).host
links_list = array.map {|a| "#{host}#{a}"}
# The value assigned to links_list will implicitly be returned.
# (The assignment itself is futile, the right-hand-part alone would
# suffice.) Because this builds on `array` all absolute URLs will be
# missing from the return value.
end
Explanation for
hrefs = doc.xpath('//a/#href').map(&:to_s).uniq.delete_if(&:empty?)
.xpath('//a/#href') uses the attribute syntax of XPath to directly get to the href attributes of a elements
.map(&:to_s) is an abbreviated notation for .map { |item| item.to_s }
.delete_if(&:empty?) uses the same abbreviated notation
And comments about the second function:
def search_links(urls)
urls = get_links(link)
urls.uniq.each do |url|
begin
links = get_links(url)
compare = urls & links
urls << links - compare
urls.flatten!
# How about using a Set instead of an Array and
# thus have the collection provide uniqueness of
# its items, so that you don't have to?
rescue OpenURI::HTTPError
warn "Skipping invalid link #{url}"
end
end
return urls
# This function isn't recursive, it just calls `get_links` on two
# 'levels'. Thus you search only two levels deep and return findings
# from the first and second level combined. (Without the "zero'th"
# level - the URL passed into `search_links`. Unless off course if it
# also occured on the first or second level.)
#
# Is this what you intended?
end
You should probably be using mechanize:
require 'mechanize'
agent = Mechanize.new
page = agent.get url
links = page.search('a[href]').map{|a| page.uri.merge(a[:href]).to_s}
# if you want to remove links with a different host (hyperlinks?)
links.reject!{|l| URI.parse(l).host != page.uri.host}
Otherwise you'll have trouble converting relative urls to absolute properly.
I'm trying to extract the first href link from a website. Just the full link alone.
I am expecting to get http://www.iana.org/domains/example as the output but instead I am getting just http://www.iana.org/domains/ex
require 'net/http'
source = Net::HTTP.get('www.example.org', '/index.html')
def findhref(page) #returns rest of the html after href
return page[page.index('href')..-1]
end
def findlink(page)
text = findhref(page)
firstquote = text.index('"') #first position of quote
secondquote = text[firstquote+1..-1].index('"') #2nd quote
puts text #for debugging
puts firstquote+1 #for debugging
puts secondquote #for debugging
return text[firstquote+1..secondquote]
end
print findlink(source)
I would suggest using Nokogiri for HTML parsing. The solution to your problem would be as simple as:
doc = Nokogiri::HTML(open('www.example.org/index.html'))
first_anchor = doc.css('a').first
first_href = first_anchor['href']
So I am parsing a URL and want to get a list of all the links in a page using Nokogiri.
But I want to push the results returned into a two-dimensional array.
I am now doing this:
def my_list(url)
root = Nokogiri::HTML(open(url))
list = []
root.css("a").each do |link|
list << (link[:href])
end
end
This gives me just the http links. If I do list << link it gives me the full <a> tag.
What I want to do is to push just the text of the link (can use link.text) to say list[0][0], and then the href value (using link[:href]) to the other cell say list[0][1].
How do I do that?
Thanks.
def my_list(url)
root = Nokogiri::HTML(open(url))
root.css("a").map do |link|
[link.text, link[:href]]
end
end
def my_list(url)
root = Nokogiri::HTML(open(url))
list = []
root.css("a").each do |link|
list << [link.text,link[:href]]
end
end