A detailed guide to learning Go and writing crawlers

Start from scratch: Detailed steps for writing crawlers using Go language

Introduction:
With the rapid development of the Internet, crawlers are becoming more and more important. A crawler is a technical means that automatically accesses and obtains specific information on the Internet through a program. In this article, we will introduce how to write a simple crawler using Go language and provide specific code examples.

Step 1: Set up the Go language development environment
First, make sure you have correctly installed the Go language development environment. You can download it from the Go official website and follow the prompts to install it.

Step 2: Import the required libraries
Go language provides some built-in libraries to help us write crawler programs. In this example, we will use the following library:

import (
    "fmt"
    "net/http"
    "io/ioutil"
    "regexp"
)

• "fmt" for formatted output.
• "net/http" is used to send HTTP requests.
• "io/ioutil" is used to read the content of HTTP response.
• "regexp" is used to parse page content using regular expressions.

Step 3: Send an HTTP request
Sending an HTTP request is very simple using the "net/http" library of the Go language. Here is a sample code:

func fetch(url string) (string, error) {
    resp, err := http.Get(url)
    if err != nil {
        return "", err
    }
    defer resp.Body.Close()
    body, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        return "", err
    }
    return string(body), nil
}

In the above sample code, we define a function called fetch, which takes a URL as a parameter and returns the content of the HTTP response. First, we send a GET request using the http.Get function. We then use the ioutil.ReadAll function to read the contents of the response. Finally, we convert the contents of the response into a string and return it.

Step 4: Parse the page content
Once we get the content of the page, we can use regular expressions to parse it. The following is a sample code:

func parse(body string) []string {
    re := regexp.MustCompile(`<a[^>]+href="?([^"s]+)"?`)
    matches := re.FindAllStringSubmatch(body, -1)
    var result []string
    for _, match := range matches {
        result = append(result, match[1])
    }
    return result
}

In the above sample code, we used the regular expression <a[^>] href="?([^"s] )"? to match all links in the page. Then, we extract each link by looping through it and add it to a results array.

Step 5: Use the crawler
Now , we can use the function defined above to write a simple crawler program. The following is a sample code:

func spider(url string, depth int) {
    visited := make(map[string]bool)
    var crawl func(url string, depth int)
    crawl = func(url string, depth int) {
        if depth <= 0 {
            return
        }
        visited[url] = true
        body, err := fetch(url)
        if err != nil {
            return
        }
        links := parse(body)
        for _, link := range links {
            if !visited[link] {
                crawl(link, depth-1)
            }
        }
    }
    crawl(url, depth)
    for link := range visited {
        fmt.Println(link)
    }
}

In the above sample code, we first define a map named visited to record visited past links. Then we define an anonymous function called crawl, which is used to crawl the links recursively. On each link, we get the content of the page and parse out the links in it. Then, we continue to crawl recursively Take unvisited links until the specified depth is reached.

Conclusion:
Through the above steps, we have learned how to write a simple crawler program using Go language. Of course, this is just a simple example , you can expand and optimize according to actual needs. I hope this article will help you understand and apply Go language for crawler development.

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