在利用R进行爬虫，时安装“rvest”包，时出现了错误，出现的错误提示如下：
Warning in install.packages :
package ‘rvest’ is not available (for R version 3.5.3)
Error in install.packages : error reading from connection
因此，具体操作如下几步
1.首先，需要将Rstudio默认的下载镜像切换成国内的下载镜像；
2.有多个国内下载镜像，还不行就多试几次，我选择的是北京大学的下载镜像；
3.输入：install.packages("rvest")，等待安装成功；
> install.packages("rvest")
also installing the dependencies ‘sys’, ‘askpass’, ‘jsonlite’, ‘mime’, ‘openssl’, ‘xml2’, ‘httr’, ‘selectr’
There is a binary version available but the source version is later:
binary source needs_compilation
jsonlite 1.6 1.6.1 TRUE
Binaries will be installed
trying URL 'https://mirrors.tuna.tsinghua.edu.cn/CRAN/bin/windows/contrib/3.5/sys_3.3.zip'
Content type 'application/zip' length 59526 bytes (58 KB)
downloaded 58 KB
trying URL 'https://mirrors.tuna.tsinghua.edu.cn/CRAN/bin/windows/contrib/3.5/askpass_1.1.zip'
Content type 'application/zip' length 64381 bytes (62 KB)
downloaded 62 KB
trying URL 'https://mirrors.tuna.tsinghua.edu.cn/CRAN/bin/windows/contrib/3.5/jsonlite_1.6.zip'
Content type 'application/zip' length 1160669 bytes (1.1 MB)
downloaded 1.1 MB
trying URL 'https://mirrors.tuna.tsinghua.edu.cn/CRAN/bin/windows/contrib/3.5/mime_0.8.zip'
Content type 'application/zip' length 48015 bytes (46 KB)