What is the difference between require and library

Question

What is the difference between require   and library

User · Answer

Always use library  Never use require  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  That is  use requireNamespace instead of require in these cases  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  quot Failed quot       success     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

User · Answer

Here seems to be the difference on an already loaded package  While it is true that both require and library do not load the package  Library does a lot of other things before it checks and exits   I would recommend removing  require  from the beginning of a function running 2mil times anyway  but if  for some reason I needed to keep it  require is technically a faster check   microbenchmark req   require microbenchmark   lib   library microbenchmark  times   100000  Unit  microseconds  expr    min     lq      mean median     uq        max neval   req  3 676  5 181  6 596968  5 655  6 177   9456 006 1e 05   lib 17 192 19 887 27 302907 20 852 22 490 255665 881 1e 05

User · Answer

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

User · Answer

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

User · Answer

Another benefit of require   is that it returns a logical value by default  TRUE if the packages is loaded  FALSE if it isn t    gt  test  lt - library  abc   Error in library  abc     there is no package called  abc   gt  test Error  object  test  not found  gt  test  lt - 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   gt  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

User · Answer

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

User · Answer

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   lt package1 gt      lt package2 gt           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

User · Answer

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

[r] What is the difference between require() and library()?

Always use `library`. Never use `require`.

Experienced R developers agree:

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