install.packages() cannot open url

在老師的建議下開始學習資料爬蟲，但在安裝rvest包時出現了問題 警告如下：

> install.packages("rvest")
Warning in install.packages :
  unable to access index for repository https://cran.rstudio.com/src/contrib:
  cannot open URL 'https://cran.rstudio.com/src/contrib/PACKAGES'
Warning in install.packages :
  unable to access index for repository https://cran.rstudio.com/src/contrib:
  cannot open URL 'https://cran.rstudio.com/src/contrib/PACKAGES'
Warning in install.packages :
  package ‘rvest’ is not available (for R version 3.5.1)
Warning in install.packages :
  unable to access index for repository https://cran.rstudio.com/bin/windows/contrib/3.5:
  cannot open URL 'https://cran.rstudio.com/bin/windows/contrib/3.5/PACKAGES'
 

對該問題進行了查詢，發現是由於cran下載映象造成的。 我使用的映象是預設的外國映象。 在更改映象後rvest包正常安裝。