R语言 解决Package is not available (for R version XXX)

安装doMC包的时候出错：

Warning in install.packages :
package ‘doMC’ is not available for this version of R

A version of this package for your version of R might be available elsewhere,
see the ideas at
https://cran.r-project/doc/manuals/r-patched/R-admin.html#Installing-packages

出现这个问题很有可能是现在用的镜像中未纳入这个包。

安装doMC包，指定镜像源：
install.packages(“doMC”, repos=”http://R-Forge.R-project”)