你可以使用require()如果你想安装包当且仅当必要时，例如:

if (!require(package, character.only=T, quietly=T)) {
    install.packages(package)
    library(package, character.only=T)
}

对于多个包，您可以使用

for (package in c('<package1>', '<package2>')) {
    if (!require(package, character.only=T, quietly=T)) {
        install.packages(package)
        library(package, character.only=T)
    }
}

专业提示：

在脚本中使用时，可以通过指定install.packages()的repos参数来避免出现对话框屏幕，例如 安装。包(包,回购= " http://cran.us.r-project.org ") 你可以在suppressPackageStartupMessages()中包装require()和library()来抑制包启动消息，也可以使用参数require(…，安静=T，警告。冲突=F)如果需要保持安装安静。

在日常工作中，“一”并不多。

然而，根据这两个函数的文档(通过输入?在函数名和按enter键之前)，require在函数内部使用，因为它会输出一个警告，如果没有找到包则继续，而library将抛出一个错误。

require()的另一个好处是它默认返回一个逻辑值。如果包已加载，则为TRUE，否则为FALSE。

> test <- library("abc")
Error in library("abc") : there is no package called 'abc'
> test
Error: object 'test' not found
> test <- require("abc")
Loading required package: abc
Warning message:
In library(package, lib.loc = lib.loc, character.only = TRUE, logical.return = TRUE,  :
  there is no package called 'abc'
> test
[1] FALSE

所以你可以在下面这样的结构中使用require()。如果你想把你的代码分发到我们的R安装包可能不会被安装，这非常方便。

if(require("lme4")){
    print("lme4 is loaded correctly")
} else {
    print("trying to install lme4")
    install.packages("lme4")
    if(require(lme4)){
        print("lme4 installed and loaded")
    } else {
        stop("could not install lme4")
    }
}

除了已经提出的好建议之外，我还要补充一点:

最好避免使用require()，除非你真的要使用它返回的值，例如在一些错误检查循环中，比如thierry给出的。

In most other cases it is better to use library(), because this will give an error message at package loading time if the package is not available. require() will just fail without an error if the package is not there. This is the best time to find out if the package needs to be installed (or perhaps doesn't even exist because it it spelled wrong). Getting error feedback early and at the relevant time will avoid possible headaches with tracking down why later code fails when it attempts to use library routines

我最初的理论是，库加载包，不管它是否已经加载，也就是说，它可能会重新加载一个已经加载的包，而require只是检查它是否加载，或者如果它没有加载就加载它(因此在依赖于某个包的函数中使用)。然而，文档反驳了这一点，并明确指出这两个函数都不会重新加载已经加载的包。

?library

你会看到:

library(package) and require(package) both load the package with name package and put it on the search list. require is designed for use inside other functions; it returns FALSE and gives a warning (rather than an error as library() does by default) if the package does not exist. Both functions check and update the list of currently loaded packages and do not reload a package which is already loaded. (If you want to reload such a package, call detach(unload = TRUE) or unloadNamespace first.) If you want to load a package without putting it on the search list, use requireNamespace.