What is the difference between require() and library()?
require()和library()之间有什么区别？
There's not much of one in everyday work.
However, according to the documentation for both functions (accessed by putting a ? before the function name and hitting enter), require is used inside functions, as it outputs a warning and continues if the package is not found, whereas library will throw an error.
Another benefit of require() is that it returns a logical value by default. TRUE if the packages is loaded, FALSE if it isn't.
> 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
So you can use require() in constructions like the one below. Which mainly handy if you want to distribute your code to our R installation were packages might not be installed.
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")
}
}
You can use require() if you want to install packages if and only if necessary, such as:
if (!require(package, character.only=T, quietly=T)) {
install.packages(package)
library(package, character.only=T)
}
For multiple packages you can use
for (package in c('<package1>', '<package2>')) {
if (!require(package, character.only=T, quietly=T)) {
install.packages(package)
library(package, character.only=T)
}
}
Pro tips:
When used inside the script, you can avoid a dialog screen by specifying the repos parameter of install.packages() , such as
install.packages(package, repos="http://cran.us.r-project.org")
You can wrap require() and library() in suppressPackageStartupMessages() to, well, suppress package startup messages, and also use the parameters require(..., quietly=T, warn.conflicts=F) if needed to keep the installs quiet.
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