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?
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.
