# Methods¶ ↑

Methods implement the functionality of your program. Here is a simple method definition:

    def one_plus_one
      1 + 1
    end

A method definition consists of the `def` keyword, a method name, the body of the method, `return` value and the `end` keyword. When called the method will execute the body of the method. This method returns `2`.

This section only covers defining methods. See also the syntax documentation on calling methods.

## `Method` Names¶ ↑

`Method` names may be one of the operators or must start a letter or a character with the eight bit set. It may contain letters, numbers, an `_` (underscore or low line) or a character with the eight bit set. The convention is to use underscores to separate words in a multiword method name:

    def method_name
      puts "use underscores to separate words"
    end

Ruby programs must be written in a US-ASCII-compatible character set such as UTF-8, ISO-8859-1 etc. In such character sets if the eight bit is set it indicates an extended character. Ruby allows method names and other identifiers to contain such characters. Ruby programs cannot contain some characters like ASCII NUL (`\x00`).

The following are examples of valid Ruby methods:

    def hello
      "hello"
    end

    def こんにちは
      puts "means hello in Japanese"
    end

Typically method names are US-ASCII compatible since the keys to type them exist on all keyboards.

`Method` names may end with a `!` (bang or exclamation mark), a `?` (question mark), or `=` (equals sign).

The bang methods (`!` at the end of the method name) are called and executed just like any other method. However, by convention, a method with an exclamation point or bang is considered dangerous. In Ruby's core library the dangerous method implies that when a method ends with a bang (`!`), it indicates that unlike its non-bang equivalent, permanently modifies its receiver. Almost always, the Ruby core library will have a non-bang counterpart (method name which does NOT end with `!`) of every bang method (method name which does end with `!`) that does not modify the receiver. This convention is typically true for the Ruby core library but may or may not hold true for other Ruby libraries.

Methods that end with a question mark by convention return boolean, but they may not always return just `true` or `false`. Often, they will return an object to indicate a true value (or “truthy” value).

Methods that end with an equals sign indicate an assignment method. For assignment methods, the return value is ignored and the arguments are returned instead.

These are method names for the various Ruby operators. Each of these operators accepts only one argument. Following the operator is the typical use or name of the operator. Creating an alternate meaning for the operator may lead to confusion as the user expects plus to add things, minus to subtract things, etc. Additionally, you cannot alter the precedence of the operators.

`+`

  add

`-`

  subtract

`*`

  multiply

`**`

  power

`/`

  divide

`%`

  modulus division, `String#%`

`&`

  AND

`^`

  XOR (exclusive OR)

`>>`

  right-shift

`<<`

  left-shift, append

`==`

  equal

`!=`

  not equal

`===`

  case equality. See `Object#===`

`=~`

  pattern match. (Not just for regular expressions)

`!~`

  does not match

`<=>`

  comparison aka spaceship operator. See `Comparable`

`<`

  less-than

`<=`

  less-than or equal

`>`

  greater-than

`>=`

  greater-than or equal

To define unary methods minus, plus, tilde and not (`!`) follow the operator with an `@` as in `+@` or `!@`:

    class C
      def -@
        puts "you inverted this object"
      end
    end

    obj = C.new

    -obj # prints "you inverted this object"

Unary methods accept zero arguments.

Additionally, methods for element reference and assignment may be defined: `[]` and `[]=` respectively. Both can take one or more arguments, and element reference can take none.

    class C
      def [](a, b)
        puts a + b
      end

      def []=(a, b, c)
        puts a * b + c
      end
    end

    obj = C.new

    obj[2, 3]     # prints "5"
    obj[2, 3] = 4 # prints "10"

## Return Values¶ ↑

By default, a method returns the last expression that was evaluated in the body of the method. In the example above, the last (and only) expression evaluated was the simple sum `1 + 1`. The `return` keyword can be used to make it explicit that a method returns a value.

    def one_plus_one
      return 1 + 1
    end

It can also be used to make a method return before the last expression is evaluated.

    def two_plus_two
      return 2 + 2
      1 + 1  # this expression is never evaluated
    end

Note that for assignment methods the return value will be ignored when using the assignment syntax. Instead, the argument will be returned:

    def a=(value)
      return 1 + value
    end

    p(self.a = 5) # prints 5

The actual return value will be returned when invoking the method directly:

    p send(:a=, 5) # prints 6

## Scope¶ ↑

The standard syntax to define a method:

    def my_method
      # ...
    end

adds the method to a class. You can define an instance method on a specific class with the `class` keyword:

    class C
      def my_method
        # ...
      end
    end

A method may be defined on another object. You may define a “class method” (a method that is defined on the class, not an instance of the class) like this:

    class C
      def self.my_method
        # ...
      end
    end

However, this is simply a special case of a greater syntactical power in Ruby, the ability to add methods to any object. Classes are objects, so adding class methods is simply adding methods to the `Class` object.

The syntax for adding a method to an object is as follows:

    greeting = "Hello"

    def greeting.broaden
      self + ", world!"
    end

    greeting.broaden # returns "Hello, world!"

`self` is a keyword referring to the current object under consideration by the compiler, which might make the use of `self` in defining a class method above a little clearer. Indeed, the example of adding a `hello` method to the class `String` can be rewritten thus:

    def String.hello
      "Hello, world!"
    end

A method defined like this is called a “singleton method”. `broaden` will only exist on the string instance `greeting`. Other strings will not have `broaden`.

## Overriding¶ ↑

When Ruby encounters the `def` keyword, it doesn't consider it an error if the method already exists: it simply redefines it. This is called overriding. Rather like extending core classes, this is a potentially dangerous ability, and should be used sparingly because it can cause unexpected results. For example, consider this irb session:

    >> "43".to_i
    => 43
    >> class String
    >>   def to_i
    >>     42
    >>   end
    >> end
    => nil
    >> "43".to_i
    => 42

This will effectively sabotage any code which makes use of the method `String#to_i` to parse numbers from strings.

## Arguments¶ ↑

A method may accept arguments. The argument list follows the method name:

    def add_one(value)
      value + 1
    end

When called, the user of the `add_one` method must provide an argument. The argument is a local variable in the method body. The method will then add one to this argument and return the value. If given `1` this method will return `2`.

The parentheses around the arguments are optional:

    def add_one value
      value + 1
    end

Multiple arguments are separated by a comma:

    def add_values(a, b)
      a + b
    end

When called, the arguments must be provided in the exact order. In other words, the arguments are positional.

### Default Values¶ ↑

Arguments may have default values:

    def add_values(a, b = 1)
      a + b
    end

The default value does not need to appear first, but arguments with defaults must be grouped together. This is ok:

    def add_values(a = 1, b = 2, c)
      a + b + c
    end

This will raise a SyntaxError:

    def add_values(a = 1, b, c = 1)
      a + b + c
    end

### `Array` Decomposition¶ ↑

You can decompose (unpack or extract values from) an `Array` using extra parentheses in the arguments:

    def my_method((a, b))
      p a: a, b: b
    end

    my_method([1, 2])

This prints:

    {:a=>1, :b=>2}

If the argument has extra elements in the `Array` they will be ignored:

    def my_method((a, b))
      p a: a, b: b
    end

    my_method([1, 2, 3])

This has the same output as above.

You can use a `*` to collect the remaining arguments. This splits an `Array` into a first element and the rest:

    def my_method((a, *b))
      p a: a, b: b
    end

    my_method([1, 2, 3])

This prints:

    {:a=>1, :b=>[2, 3]}

The argument will be decomposed if it responds to to_ary. You should only define to_ary if you can use your object in place of an `Array`.

Use of the inner parentheses only uses one of the sent arguments. If the argument is not an `Array` it will be assigned to the first argument in the decomposition and the remaining arguments in the decomposition will be `nil`:

    def my_method(a, (b, c), d)
      p a: a, b: b, c: c, d: d
    end

    my_method(1, 2, 3)

This prints:

    {:a=>1, :b=>2, :c=>nil, :d=>3}

You can nest decomposition arbitrarily:

    def my_method(((a, b), c))
      # ...
    end

### Array/Hash Argument¶ ↑

Prefixing an argument with `*` causes any remaining arguments to be converted to an Array:

    def gather_arguments(*arguments)
      p arguments
    end

    gather_arguments 1, 2, 3 # prints [1, 2, 3]

The array argument must be the last positional argument, it must appear before any keyword arguments.

The array argument will capture a `Hash` as the last entry if a hash was sent by the caller after all positional arguments.

    gather_arguments 1, a: 2 # prints [1, {:a=>2}]

However, this only occurs if the method does not declare any keyword arguments.

    def gather_arguments_keyword(*positional, keyword: nil)
     p positional: positional, keyword: keyword
    end

    gather_arguments_keyword 1, 2, three: 3
    #=> raises: unknown keyword: three (ArgumentError)

Also, note that a bare `*` can be used to ignore arguments:

    def ignore_arguments(*)
    end

### Keyword Arguments¶ ↑

Keyword arguments are similar to positional arguments with default values:

    def add_values(first: 1, second: 2)
      first + second
    end

Arbitrary keyword arguments will be accepted with `**`:

    def gather_arguments(first: nil, **rest)
      p first, rest
    end

    gather_arguments first: 1, second: 2, third: 3
    # prints 1 then {:second=>2, :third=>3}

When calling a method with keyword arguments the arguments may appear in any order. If an unknown keyword argument is sent by the caller an `ArgumentError` is raised.

When mixing keyword arguments and positional arguments, all positional arguments must appear before any keyword arguments.

## Block Argument¶ ↑

The block argument is indicated by `&` and must come last:

    def my_method(&my_block)
      my_block.call(self)
    end

Most frequently the block argument is used to pass a block to another method:

    def each_item(&block)
      @items.each(&block)
    end

If you are only going to call the block and will not otherwise manipulate it or send it to another method using `yield` without an explicit block parameter is preferred. This method is equivalent to the first method in this section:

    def my_method
      yield self
    end

There is also a performance benefit to using yield over a calling a block parameter. When a block argument is assigned to a variable a `Proc` object is created which holds the block. When using yield this `Proc` object is not created.

If you only need to use the block sometimes you can use `Proc.new` to create a proc from the block that was passed to your method. See `Proc.new` for further details.

## `Exception` Handling¶ ↑

Methods have an implied exception handling block so you do not need to use `begin` or `end` to handle exceptions. This:

    def my_method
      begin
        # code that may raise an exception
      rescue
        # handle exception
      end
    end

May be written as:

    def my_method
      # code that may raise an exception
    rescue
      # handle exception
    end

If you wish to rescue an exception for only part of your method, use `begin` and `end`. For more details see the page on exception handling.