What is the difference between require() and library()?

Question

What is the difference between require() and library()?

Answer 1

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.

Answer 2

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")     } }