当更新崩溃时我收到此消息：
W:Failed to fetch http://cran.at.r-project.org/dists/precise-backports/main/source/Sources 404 Not Found
E:Some index files failed to download. They have been ignored, or old ones used instead.
答案1
我认为你的链接是错误的，试试这个
deb http://cran.at.r-project.org/bin/linux/ubuntu precise/