Why are exclamation marks used in Ruby methods

Question

In Ruby some methods have a question mark     that ask a question like include  that ask if the object in question is included  this then returns a true false   But why do some methods have exclamation marks     where others don t   What does it mean

User · Accepted Answer

In general  methods that end in   indicate that the method will modify the object it s called on  Ruby calls these as  dangerous methods  because they change state that someone else might have a reference to  Here s a simple example for strings   foo    A STRING     a string called foo foo downcase        modifies foo itself puts foo            prints modified foo   This will output   a string   In the standard libraries  there are a lot of places you ll see pairs of similarly named methods  one with the   and one without   The ones without are called  safe methods   and they return a copy of the original with changes applied to the copy  with the callee unchanged   Here s the same example without the     foo    A STRING       a string called foo bar   foo downcase    doesn t modify foo  returns a modified string puts foo              prints unchanged foo puts bar              prints newly created bar   This outputs   A STRING a string   Keep in mind this is just a convention  but a lot of Ruby classes follow it  It also helps you keep track of what s getting modified in your code

User · Answer

typically means that the method acts upon the object instead of returning a result   From the book Programming Ruby      Methods that are  dangerous   or modify the receiver  might be named with a trailing

User · Answer

Bottom line    methods just change the value of the object they are called upon  whereas a method without   returns a manipulated value without writing over the object the method was called upon    Only use   if you do not plan on needing the original value stored at the variable you called the method on    I prefer to do something like   foo    word  bar   foo capitalize puts bar   OR  foo    word  puts foo capitalize   Instead of   foo    word  foo capitalize  puts foo   Just in case I would like to access the original value again

User · Answer

Simple explanation   foo    BEST DAY EVER   assign a string to variable foo     gt  foo downcase  call method downcase  this is without any exclamation    best day ever    returns the result in downcase  but no change in value of foo     gt  foo  call the variable foo now    BEST DAY EVER   variable is unchanged     gt  foo downcase   call destructive version     gt  foo  call the variable foo now    best day ever   variable has been mutated in place    But if you ever called a method downcase  in the explanation above  foo would change to downcase permanently  downcase  would not return a new string object but replace the string in place  totally changing the foo to downcase  I suggest you don t use downcase  unless it is totally necessary

User · Answer

The exclamation point means many things  and sometimes you can t tell a lot from it other than  this is dangerous  be careful    As others have said  in standard methods it s often used to indicate a method that causes an object to mutate itself  but not always   Note that many standard methods change their receiver and don t have an exclamation point  pop  shift  clear   and some methods with exclamation points don t change their receiver  exit     See this article for example   Other libraries may use it differently   In Rails an exclamation point often means that the method will throw an exception on failure rather than failing silently   It s a naming convention but many people use it in subtly different ways   In your own code a good rule of thumbs is to use it whenever a method is doing something  dangerous   especially when two methods with the same name exist and one of them is more  dangerous  than the other    Dangerous  can mean nearly anything though

User · Answer

It is most accurate to say that methods with a Bang  are the more dangerous or surprising version  There are many methods that mutate without a Bang such as  destroy and in general methods only have bangs where a safer alternative exists in the core lib   For instance  on Array we have  compact and  compact   both methods mutate the array  but  compact  returns nil instead of self if there are no nil s in the array  which is more surprising than just returning self   The only non-mutating method I ve found with a bang is Kernel s  exit  which is more surprising than  exit because you cannot catch SystemExit while the process is closing   Rails and ActiveRecord continues this trend in that it uses bang for more  surprising  effects like  create  which raises errors on failure

User · Answer

I like to think of this as an explosive change that destroys all that has gone before it  Bang or exclamation mark means that you are making a permanent saved change in your code    If you use for example Ruby s method for global substitutiongsub the substitution you make is permanent    Another way you can imagine it  is opening a text file and doing find and replace  followed by saving    does the same in your code    Another useful reminder if you come from the bash world is sed -i has this similar effect of making permanent saved change

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From themomorohoax com   A bang can used in the below ways  in order of my personal preference      1  An active record method raises an error if the method does not do   what it says it will       2  An active record method saves the record or a method saves an   object  e g  strip          3  A method does something    extra     like posts to someplace  or does   some action      The point is  only use a bang when you   ve really thought about whether it   s necessary  to save other developers the annoyance of having to check why you are using a bang   The bang provides two cues to other developers      1  that it   s not necessary to save the object after calling the   method       2  when you call the method  the db is going to be changed    http   www themomorohoax com 2009 02 11 when-to-use-a-bang-exclamation-point-after-rails-methods

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This naming convention is lifted from Scheme      1 3 5  Naming conventions      By convention  the names of procedures   that always return a boolean value   usually end in        Such procedures   are called predicates       By convention  the names of procedures   that store values into previously   allocated locations  see section 3 4    usually end in        Such procedures   are called mutation procedures  By   convention  the value returned by a   mutation procedure is unspecified

User · Answer

Called  Destructive Methods  They tend to change the original copy of the object you are referring to   numbers  1 0 10 5 8  numbers collect  n  puts n 2    would multiply each number by two numbers  returns the same original copy numbers collect   n  puts n 2    would multiply each number by two and destructs the original copy from the array numbers     returns  nil nil nil nil nil

[ruby] Why are exclamation marks used in Ruby methods?

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