Assignment

Assignment

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:

# 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

Combined assignments

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

Some syntax sugar that contains the = character is available:

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. Note that || and && use the []? method to check for key presence.

person.age += 1 # same as: person.age = person.age + 1
person.name ||= "John" # same as: person.name || (person.name = "John")
person.name &&= "John" # same as: person.name && (person.name = "John")
objects[1] += 2 # same as: objects[1] = objects[1] + 2
objects[1] ||= 2 # same as: objects[1]? || (objects[1] = 2)
objects[1] &&= 2 # same as: objects[1]? && (objects[1] = 2)

Chained assignment

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

a = b = c = 123
# Now a, b and c have the same value:
a # => 123
b # => 123
c # => 123

Multiple assignment

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
# The above is the same as this:
temp1 = "Crystal"
temp2 = 1
name = temp1
age = temp2

Note that because expressions are assigned to temporary variables it is possible to exchange variables’ contents in a single line:

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

Multiple assignment is also available to methods that end with =:

person.name, person.age = "John", 32
# Same as:
temp1 = "John"
temp2 = 32
person.name = temp1
person.age = temp2

And it is also available to index assignments ([]=):

objects[1], objects[2] = 3, 4
# Same as:
temp1 = 3
temp2 = 4
objects[1] = temp1
objects[2] = temp2

One-to-many assignment

If the right-hand side contains just one expression, the type is indexed for each variable on the left-hand side like so:

name, age, source = "Crystal, 123, GitHub".split(", ")
# The above is the same as this:
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

Underscore

The underscore can appear on the left-hand side of any assignment. Assigning a value to it has no effect and the underscore cannot be read from:

_ = 1     # no effect
_ = "123" # no effect
puts _    # Error: can't read from _

It is useful in multiple assignment when some of the values returned by the right-hand side are unimportant:

before, _, after = "main.cr".partition(".")
# The above is the same as this:
temp = "main.cr".partition(".")
before = temp[0]
_ = temp[1] # this line has no effect
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]