代入¶

An assignment expression assigns a value to a named identifier (usually a variable). The assignment operator is the equals sign (=).

The target of an assignment can be:

a local variable
an instance variable
a class variable
a constant
an assignment method

# Assigns to a local variable
local = 1

# Assigns to an instance variable
@instance = 2

# Assigns to a class variable
@@class = 3

# Assigns to a constant
CONST = 4

# Assigns to a setter method
foo.method = 5
foo[0] = 6

Method as assignment target¶

A method ending with an equals sign (=) is called a setter method. It can be used as the target of an assignment. The semantics of the assignment operator apply as a form of syntax sugar to the method call.

Calling setter methods requires an explicit receiver. The receiver-less syntax x = y is always parsed as an assignment to a local variable, never a call to a method x=. Even adding parentheses does not force a method call, as it would when reading from a local variable.

The following example shows two calls to a setter method in typical method notation and with assignment operator. Both assignment expressions are equivalent.

class Thing
  def name=(value); end
end

thing = Thing.new

thing.name=("John")
thing.name = "John"

The following example shows two calls to an indexed assignment method in typical method notation and with index assignment operator. Both assignment expressions are equivalent.

class List
  def []=(key, value); end
end

list = List.new

list.[]=(2, 3)
list[2] = 3

複合代入¶

Combined assignments are a combination of an assignment operator and another operator. This works with any target type except constants.

= を使った代入のためのシンタックスシュガーがいくつか用意されています。

local += 1  # same as: local = local + 1

This assumes that the corresponding target local is assignable, either as a variable or via the respective getter and setter methods.

The = operator syntax sugar is also available to setter and index assignment methods. このとき || と && は、 []? メソッドをキーの存在のチェックに使うことに注意してください。

person.age += 1 # person.age = person.age + 1 と同じ

person.name ||= "John" # person.name || (person.name = "John") と同じ
person.name &&= "John" # person.name && (person.name = "John") と同じ

objects[1] += 2 # objects[1] = objects[1] + 2 と同じ

objects[1] ||= 2 # objects[1]? || (objects[1] = 2) と同じ
objects[1] &&= 2 # objects[1]? && (objects[1] = 2) と同じ

連続した代入¶

The same value can be assigned to multiple targets using chained assignment. This works with any target type except constants.

a = b = c = 123

# このとき a, b, c は同じ値になる
a # => 123
b # => 123
c # => 123

多重代入¶

You can declare/assign multiple variables at the same time by separating expressions with a comma (,). This works with any target type except constants.

name, age = "Crystal", 1

# 上記は以下と同じ
temp1 = "Crystal"
temp2 = 1
name = temp1
age = temp2

また、式は一時的な変数に代入される仕組みになっているため、次のように1行で変数の値を交換することも可能です。

a = 1
b = 2
a, b = b, a
a # => 2
b # => 1

多重代入は名前が=で終わるメソッドに対しても有効です。

person.name, person.age = "John", 32

# 以下に同じ
temp1 = "John"
temp2 = 32
person.name = temp1
person.age = temp2

そして、インデックス指定の代入 ([]=) の場合にも同様に有効です。

objects[1], objects[2] = 3, 4

#以下に同じ
temp1 = 3
temp2 = 4
objects[1] = temp1
objects[2] = temp2

One-to-many assignment¶

代入式の右辺に1つしか式が無かった場合、次の例のように各変数に順にインデックスして代入していきます。

name, age, source = "Crystal, 123, GitHub".split(", ")

# 上記は以下に同じ
temp = "Crystal, 123, GitHub".split(", ")
name = temp[0]
age = temp[1]
source = temp[2]

Additionally, if the strict_multi_assign flag is provided, the number of elements must match the number of targets, and the right-hand side must be an Indexable:

name, age, source = "Crystal, 123, GitHub".split(", ")

# The above is the same as this:
temp = "Crystal, 123, GitHub".split(", ")
if temp.size != 3 # number of targets
  raise IndexError.new("Multiple assignment count mismatch")
end
name = temp[0]
age = temp[1]
source = temp[2]

a, b = {0 => "x", 1 => "y"} # Error: right-hand side of one-to-many assignment must be an Indexable, not Hash(Int32, String)

Splat assignment¶

The left-hand side of an assignment may contain one splat, which collects any values not assigned to the other targets. A range index is used if the right-hand side has one expression:

head, *rest = [1, 2, 3, 4, 5]

# Same as:
temp = [1, 2, 3, 4, 5]
head = temp[0]
rest = temp[1..]

Negative indices are used for targets after the splat:

*rest, tail1, tail2 = [1, 2, 3, 4, 5]

# Same as:
temp = [1, 2, 3, 4, 5]
rest = temp[..-3]
tail1 = temp[-2]
tail2 = temp[-1]

If the expression does not have enough elements and the splat appears in the middle of the targets, IndexError is raised:

a, b, *c, d, e, f = [1, 2, 3, 4]

# Same as:
temp = [1, 2, 3, 4]
if temp.size < 5 # number of non-splat assignment targets
  raise IndexError.new("Multiple assignment count mismatch")
end
# note that the following assignments would incorrectly not raise if the above check is absent
a = temp[0]
b = temp[1]
c = temp[2..-4]
d = temp[-3]
e = temp[-2]
f = temp[-1]

The right-hand side expression must be an Indexable. Both the size check and the Indexable check occur even without the strict_multi_assign flag (see One-to-many assignment above).

A Tuple is formed if there are multiple values:

*a, b, c = 3, 4, 5, 6, 7

# Same as:
temp1 = {3, 4, 5}
temp2 = 6
temp3 = 7
a = temp1
b = temp2
c = temp3

アンダースコア¶

アンダースコアを任意の代入の左辺に置くことができます。この代入には副作用がなく、代入した値を読み出すこともできません。

_ = 1     # 副作用なし
_ = "123" # 副作用なし
puts _    # Error: can't read from _

これは多重代入で、右辺のいくつかの値は重要ではないようなときに役に立ちます。

before, _, after = "main.cr".partition(".")

# 上記は以下と同じ
temp = "main.cr".partition(".")
before = temp[0]
_ = temp[1] # この行は副作用がない
after = temp[2]

Assignments to *_ are dropped altogether, so multiple assignments can be used to extract the first and last elements in a value efficiently, without creating an intermediate object for the elements in the middle:

first, *_, last = "127.0.0.1".split(".")

# Same as:
temp = "127.0.0.1".split(".")
if temp.size < 2
  raise IndexError.new("Multiple assignment count mismatch")
end
first = temp[0]
last = temp[-1]