R包install失败：cannot open URL 'https://cran.rstudio.com/src/contrib/PACKAGES'
问题：
> install.packages("scatterplot3d")
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 ‘scatterplot3d’ 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'
Warning: unable to access index for repository https://cran.rstudio.com/src/contrib: cannot open URL 'https://cran.rstudio.com/src/contrib/PACKAGES'
> update.packages()
Warning: 无法在貯藏處https://cran.rstudio.com/src/contrib中读写索引: cannot open URL 'https://cran.rstudio.com/src/contrib/PACKAGES' Warning: unable to access index for repository https://cran.rstudio.com/src/contrib: cannot open URL 'https://cran.rstudio.com/src/contrib/PACKAGES'
Warning: unable to access index for repository https://cran.rstudio.com/src/contrib: cannot open URL 'https://cran.rstudio.com/src/contrib/PACKAGES'
> options(repos='http://cran.rstudio.com/')
> install.packages("scatterplot3d")
trying URL 'http://cran.rstudio.com/bin/windows/contrib/3.5/scatterplot3d_0.3-41.zip' Content type 'application/zip' length 338005 bytes (330 KB) downloaded 330 KB
package ‘scatterplot3d’ successfully unpacked and MD5 sums checked The downloaded binary packages are in C:\Users\Administrator\AppData\Local\Temp\Rtmpumb3sX\downloaded_packages
解决办法：
将https换成http，虽然我们浏览器不管是http、https都能打开.
options(repos='http://cran.rstudio.com/')
注：重启R后又会变回原来的https的...
然后要改个R studio设置： Tools -> Global Options -> Packages and unchecking the "Use Internet Explorer library/proxy for HTTP" option.
