尝试在 R 版本 3.2.2 的 Windows 操作系统（Windows 7 专业版）上安装 swirl 时获得的错误消息：
Installing package into ‘C:/Users/Amrita/Documents/R/win-library/3.2’
(as ‘lib’ is unspecified)
--- Please select a CRAN mirror for use in this session ---
Error in download.file(url, destfile = f, quiet = TRUE) :
cannot open URL 'https://cran.r-project.org/CRAN_mirrors.csv'
In addition: Warning message:
In download.file(url, destfile = f, quiet = TRUE) :
InternetOpenUrl failed: ''
Warning: unable to access index for repository cran.rstudio.com/src/contrib
Warning: unable to access index for repository www.stats.ox.ac.uk/pub/RWin/src/contrib
Warning: unable to access index for repository cran.rstudio.com/bin/windows/contrib/3.2
Warning: unable to access index for repository www.stats.ox.ac.uk/pub/RWin/bin/windows/contrib/3.2
Warning message:
package ‘swirl’ is not available (for R version 3.2.2)