How to implement a web crawler using Golang

Web crawler, also known as web crawler and web spider, is an automated program used to crawl information on the Internet. Web crawlers can be used to obtain large amounts of data, analyze and process the data. This article will introduce how to use Golang to implement a web crawler.

1. Introduction to Golang
Golang, also known as Go language, was developed by Google and released in 2009. Golang is a statically typed, compiled language with features such as efficiency, reliability, security, simplicity, and concurrency. Due to Golang's efficiency and simplicity, more and more people are starting to use Golang to implement web crawlers.

2. Implementation steps

1. Installing Golang
   First you need to install Golang on your local computer. Golang can be downloaded and installed through the Golang official website (https://golang.org/).
2. Import dependency packages
   When using Golang to implement a web crawler, you need to use some third-party packages, such as "net/http", "io/ioutil", "regexp" and other packages. These packages can be installed using the go get command:
   go get -u github.com/PuerkitoBio/goquery
   go get -u golang.org/x/net/html
   go get -u golang.org /x/text/encoding/unicode
   go get -u golang.org/x/text/transform

Among them, the "goquery" package is used to parse HTML documents, and the "html" package is used For a given HTML document parser, the "unicode" package is used to parse the encoding, and the "transform" package is used to convert the encoding.

3. Determine the target website and the information that needs to be crawled
   Before implementing a web crawler, you need to determine the target website and the information that needs to be crawled. Taking Douban Movies as an example, the information we need to crawl includes movie names, ratings and comments.
4. Parse HTML document
   Use the GoQuery package to parse the HTML document, use the http GET method to obtain the HTML document from the target website, and use the GoQuery package to parse the information in the HTML document. The following is the code to parse the HTML document:

resp, err := http.Get(url)
if err != nil {
log.Fatal(err)
}
defer resp.Body.Close()
doc, err := goquery.NewDocumentFromReader(resp.Body)

5. Extract information from Extract the required information from the HTML document. The following is the code to extract information:
   

doc.Find(".hd").Each(func(i int, s *goquery.Selection) {

title := s.Find( "span.title").Text()
rating := s.Find("span.rating_num").Text()
comment := s.Find("span.inq").Text()
})

Storing information
6. Store the extracted information in a data file or database. Here is the code to store the information into a CSV file:
   

f, err := os.Create("movies.csv")

if err != nil {
log. Fatal(err)
}
defer f.Close()
w := csv.NewWriter(f)
w.Write([]string{"title", "rating", "comment "})
for i := 0; i < len(titles); i {
record := []string{titles[i], ratings[i], comments[i]}
w.Write(record)
}
w.Flush()

Full code

import (

"encoding/csv"
"github.com/PuerkitoBio/goquery"
"log"
"net/http"
"os"
"regexp"
)
func Crawl(url string) {
resp, err := http.Get(url)
if err != nil {

  log.Fatal(err)

}

defer resp.Body.Close()
doc, err := goquery.NewDocumentFromReader(resp.Body)
if err != nil {

  log.Fatal(err)

}

titles := []string{}

ratings := []string{}
comments := []string{}
re := regexp.MustCompile(
s ) doc.Find(".hd").Each(func(i int, s *goquery.Selection) {

  title := s.Find("span.title").Text()
  title = re.ReplaceAllString(title, "")
  rating := s.Find("span.rating_num").Text()
  comment := s.Find("span.inq").Text()
  titles = append(titles, title)
  ratings = append(ratings, rating)
  comments = append(comments, comment)

})

f, err := os.Create("movies.csv")
if err != nil {

  log.Fatal(err)

}

defer f.Close()
w := csv.NewWriter(f)
w.Write([]string{"title", "rating", "comment"})
for i := 0; i < len(titles); i {

  record := []string{titles[i], ratings[i], comments[i]}
  w.Write(record)

}

w.Flush()
}

Conclusion
8. Use Golang to implement Web crawlers need to master certain programming knowledge, including HTML document parsing, regular expression use, and file operations. By implementing a web crawler through the steps introduced in this article, you can obtain information on the target website and store the information on your local computer.
   

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