我试着安装一个包，使用

install.packages("foobarbaz")

却收到了警告

Warning message:
package 'foobarbaz' is not available (for R version x.y.z)

为什么R认为这个包是不可用的?

参见这些问题的具体例子:

My package doesn't work for R 2.15.2 package 'Rbbg' is not available (for R version 2.15.2) package is not available (for R version 2.15.2) package doMC NOT available for R version 3.0.0 warning in install.packages Dependency ‘Rglpk’ is not available for package ‘fPortfolio’ What to do when a package is not available for our R version? Is the bigvis package for R not available for R version 3.0.1? package ‘syncwave’/‘mvcwt’ is not available (for R version 3.0.2) package ‘diamonds’ is not available (for R version 3.0.0) Is the plyr package for R not available for R version 3.0.2? Package bigmemory not installing on R 64 3.0.2 package "makeR" is not available (for version 3.0.2) package ‘RTN’ is not available (for R version 3.0.1) Trouble Installing geoR package package ‘twitterR’ is not available (for R version 3.1.0) How to install 'Rcpp, package? I got "package is not available" package ‘dataset’ is not available (for R version 3.1.1) "package ‘rhipe’ is not available (for R version 3.1.2)"