What Is the Purpose of "!" and "" at the End of Method Names

What is the purpose of ! and ? at the end of method names?

It's "just sugarcoating" for readability, but they do have common meanings:

• Methods ending in ! perform some permanent or potentially dangerous change; for example:
  
  • Enumerable#sort returns a sorted version of the object while Enumerable#sort! sorts it in place.
  • In Rails, ActiveRecord::Base#save returns false if saving failed, while ActiveRecord::Base#save! raises an exception.
  • Kernel::exit causes a script to exit, while Kernel::exit! does so immediately, bypassing any exit handlers.
• Methods ending in ? return a boolean, which makes the code flow even more intuitively like a sentence — if number.zero? reads like "if the number is zero", but if number.zero just looks weird.

In your example, name.reverse evaluates to a reversed string, but only after the name.reverse! line does the name variable actually contain the reversed name. name.is_binary_data? looks like "is name binary data?".

What does the question mark at the end of a method name mean in Ruby?

It is a code style convention; it indicates that a method returns a boolean value (true or false) or an object to indicate a true value (or “truthy” value).

The question mark is a valid character at the end of a method name.

https://docs.ruby-lang.org/en/2.0.0/syntax/methods_rdoc.html#label-Method+Names

Why are exclamation marks used in Ruby methods?

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.

BASIC: What does an exclamation mark at the end of a function name mean?

It means the function returns a SINGLE. Exclamation point is a shortcut for As Single.

what does ? ? mean in ruby

Functions that end with ? in Ruby are functions that only return a boolean, that is, true, or false.

When you write a function that can only return true or false, you should end the function name with a question mark.

The example you gave shows a ternary statement, which is a one-line if-statement. .nil? is a boolean function that returns true if the value is nil and false if it is not. It first checks if the function is true, or false. Then performs an if/else to assign the value (if the .nil? function returns true, it gets nil as value, else it gets the File.join(root_dir, '/') as value.

It can be rewritten like so:

if root_dir.nil?
  prefix = nil
else
  prefix = File.join(root_dir, '/')
end

What does the exclamation mark mean before a function call?

"!" is a new dart operator for conversion from a nullable to a non-nullable type.
Read here and here about sound null safety.

Use it only if you are absolutely sure that the value will never be null and do not confuse it with the conditional property access operator.

What does an exclamation mark mean after the name of a function?

In Julia, it's a convention to append ! to names of functions that modify their arguments. The reason is Julia function arguments are passed-by-sharing, without this "bang" convention, it's not easy to know whether a function will change the content of input arguments or not.

What does the exclamation mark do before the function?

JavaScript syntax 101: here is a function declaration:

function foo() {}

Note that there’s no semicolon; this is just a function declaration. You would need an invocation, foo(), to actually run the function.

Now, when we add the seemingly innocuous exclamation mark: !function foo() {} it turns it into an expression. It is now a function expression.

The ! alone doesn’t invoke the function, of course, but we can now put () at the end: !function foo() {}(), which has higher precedence than ! and instantly calls the function.

function foo() {}() would be a syntax error because you can’t put arguments (()) right after a function declaration.

So what the author is doing is saving a byte per function expression; a more readable way of writing it would be this:

(function(){})();

Lastly, ! makes the expression return a boolean based on the return value of the function. Usually, an immediately invoked function expression (IIFE) doesn’t explicitly return anything, so its return value will be undefined, which leaves us with !undefined which is true. This boolean isn’t used.

What does an exclamation mark before a variable mean in JavaScript

! is the logical not operator in JavaScript.

Formally

!expression is read as:

• Take expression and evaluate it. In your case that's variable.onsubmit
• Case the result of that evaluation and convert it to a boolean. In your case since onsubmit is likely a function, it means - if the function is null or undefined - return false, otherwise return true.
• If that evaluation is true, return false. Otherwise return true.

In your case

In your case !variable.onsubmit means return true if there isn't a function defined (and thus is falsy), otherwise return false (since there is a function defined).

Simply put - !variable means take the truth value of variable and negate it.

Thus, if (!variable) { will enter the if clause if variable is false (or coerces to false)

In total

if (!variable.onsubmit || (variable.onsubmit() != false)) {

Means - check if variable.onsubmit is defined and truthy (thus true), then it checks if calling onsubmit returns a result that coerces to true. In a short line it checks if there is no onsubmit or it returns true.

Next time, how do I find this myself?

• MDN has a list of operators here.
• The language specification specifies such operators, though being the official specification it does contain some jargon which might be hard to understand.

Why does the println! function use an exclamation mark in Rust?

println! is not a function, it is a macro. Macros use ! to distinguish them from normal method calls. The documentation contains more information.

See also:

• What is the difference between macros and functions in Rust?

---

Rust uses the Option type to denote optional data. It has an unwrap method.

Rust 1.13 added the question mark operator ? as an analog of the try! macro (originally proposed via RFC 243).

An excellent explanation of the question mark operator is in The Rust Programming Language.

fn foo() -> Result<i32, Error> {
    Ok(4)
}

fn bar() -> Result<i32, Error> {
    let a = foo()?;
    Ok(a + 4)
}

The question mark operator also extends to Option, so you may see it used to unwrap a value or return None from the function. This is different from just unwrapping as the program will not panic:

fn foo() -> Option<i32> {
    None
}

fn bar() -> Option<i32> {
    let a = foo()?;
    Some(a + 4)
}

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