What is the difference between require() and libra

Question

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

Answer 1

?library

and you will see:

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.

Answer 2

require() loads all additional packages

Answer 3

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 4

In addition to the good advice already given, I would add this:

It is probably best to avoid using require() unless you actually will be using the value it returns e.g in some error checking loop such as given by thierry.

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

Answer 5

My initial theory about the difference was that library loads the packages whether it is already loaded or not, i.e. it might reload an already loaded package, while require just checks that it is loaded, or loads it if it isn't (thus the use in functions that rely on a certain package). The documentation refutes this, however, and explicitly states that neither function will reload an already loaded package.

Answer 6

Always use library. Never¹ use require.

_{(¹ Almost never. Maybe.)}

In a nutshell, this is because, when using require, your code might yield different, erroneous results, without signalling an error. This is rare but not hypothetical! Consider this code, which yields different results depending on whether {dplyr} can be loaded:

require(dplyr)

x = data.frame(y = seq(100))
y = 1
filter(x, y == 1)

This can lead to subtly wrong results. Using library instead of require throws an error here, signalling clearly that something is wrong. This is good.

It also makes debugging all other failures more difficult: If you require a package at the start of your script and use its exports in line 500, you’ll get an error message “object ‘foo’ not found” in line 500, rather than an error “there is no package called ‘bla’”.

The only acceptable use case of require is when its return value is immediately checked, as some of the other answers show. This is a fairly common pattern but even in these cases it is better (and recommended, see below) to instead separate the existence check and the loading of the package.

More technically, require actually calls library internally (if the package wasn’t already attached — require thus performs a redundant check, because library also checks whether the package was already loaded). Here’s a simplified implementation of require to illustrate what it does:

require = function (package) {
    already_attached = paste('package:', package) %in% search()
    if (already_attached) return(TRUE)
    maybe_error = try(library(package, character.only = TRUE)) 
    success = ! inherits(maybe_error, 'try-error')
    if (! success) cat("Failed")
    success
}

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Experienced R developers agree:

Yihui Xie, author of {knitr}, {bookdown} and many other packages says:

Ladies and gentlemen, I've said this before: require() is the wrong way to load an R package; use library() instead

Hadley Wickham, author of more popular R packages than anybody else, says

Use library(x) in data analysis scripts. […] You never need to use require() (requireNamespace() is almost always better)