Locomotive Engineer

Decomposition refers to the act of extracting the elements of a collection, such as an Array or Hash. Decomposed values can then be assigned to variables within the same statement.

Multiple assignment is the ability to assign multiple variables to decompose values within one statement. This allows for code to be more concise and readable, and is done by separating the variables to be assigned with a comma such as first, second, third = [1, 2, 3].

The splat operator(*), and double splat operator, (**), are often used in decomposition contexts.

Multiple assignment

Multiple assignment allows you to assign multiple variables in one line. To separate the values, use a comma ,:

>> a, b = 1, 2
=> [1, 2]
>> a
=> 1

Multiple assignment is not limited to one data type:

>> x, y, z = 1, "Hello", true
=> [1, "Hello", true]
>> x
=> 1
>> y
=> 'Hello'
>> z
=> true

Multiple assignment can be used to swap elements in arrays. This practice is pretty common in sorting algorithms. For example:

>> numbers = [1, 2]
=> [1, 2]
>> numbers[0], numbers[1] = numbers[1], numbers[0]
=> [2, 1]
>> numbers
=> [2, 1]

If there are more variables than values, the extra variables will be assigned nil:

>> a, b, c = 1, 2
=> [1, 2]
>> b
=> 2
>> c
=> nil

Decomposition

In Ruby, it is possible to decompose the elements of arrays/hashes into distinct variables. Since values appear within arrays in a index order, they are unpacked into variables in the same order:

>> fruits = ["apple", "banana", "cherry"]
>> x, y, z = fruits
>> x
=> "apple"

If there are values that are not needed then you can use _ to indicate "collected but not used":

>> fruits = ["apple", "banana", "cherry"]
>> _, _, z = fruits
>> z
=> "cherry"

Deep decomposing

Decomposing and assigning values from arrays inside of an array (also known as a nested array), works in the same way a shallow decomposing does, but needs delimited decomposition expression (()) to clarify the values context or position:

>> fruits_vegetables = [["apple", "banana"], ["carrot", "potato"]]
>> (a, b), (c, d) = fruits_vegetables
>> a
=> "apple"
>> d
=> "potato"

You can also deeply unpack just a portion of a nested array:

>> fruits_vegetables = [["apple", "banana"], ["carrot", "potato"]]
>> a, (c, d) = fruits_vegetables
>> a
=> ["apple", "banana"]
>> c
=> "carrot"

If the decomposition has variables with incorrect placement and/or an incorrect number of values, you will get a syntax error:

fruits_vegetables = [["apple", "banana"], ["carrot", "potato"]]

(a, b), (d) = fruits_vegetables
# syntax error, unexpected '=', expecting '.' or &. or :: or '['

((a, b), (d)) = fruits_vegetables
# syntax error, unexpected ')', expecting '.' or &. or :: or '['

Experiment here, and you will notice that the first pattern dictates, not the available values on the right hand side. The syntax error is not tied to the data structure.

Decomposing an array with the single splat operator (*)

When decomposing an array you can use the splat operator (*) to capture the "leftover" values. This is clearer than slicing the array (which in some situations is less readable). For example, we can extract the first element and then assign the remaining values into a new array without the first element:

>> fruits = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
>> x, *last = fruits
>> x
=> "apple"
>> last
=> ["banana", "cherry", "orange", "kiwi", "melon", "mango"]

We can also extract the values at the beginning and end of the array while grouping all the values in the middle:

>> fruits = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
>> x, *middle, y, z = fruits
>> y
=> "melon"
>> middle
=> ["banana", "cherry", "orange", "kiwi"]

We can also use * in deep decomposition:

>> fruits_vegetables = [["apple", "banana", "melon"], ["carrot", "potato", "tomato"]]
>> (a, *rest), b = fruits_vegetables
>> a
=> "apple"
>> rest
=> ["banana", "melon"]

Decomposing a Hash

Decomposing a hash is a bit different than decomposing an array. To be able to unpack a hash you need to convert it to an array first. Otherwise there will be no decomposing:

>> fruits_inventory = {apple: 6, banana: 2, cherry: 3}
>> x, y, z = fruits_inventory
>> x
=> {:apple=>6, :banana=>2, :cherry=>3}
>> y
=> nil

To coerce a Hash to an array you can use the to_a method:

>> fruits_inventory = {apple: 6, banana: 2, cherry: 3}
>> fruits_inventory.to_a
=> [[:apple, 6], [:banana, 2], [:cherry, 3]]
>> x, y, z = fruits_inventory.to_a
>> x
=> [:apple, 6]

If you want to unpack the keys then you can use the keys method:

>> fruits_inventory = {apple: 6, banana: 2, cherry: 3}
>> x, y, z = fruits_inventory.keys
>> x
=> :apple

If you want to unpack the values then you can use the values method:

>> fruits_inventory = {apple: 6, banana: 2, cherry: 3}
>> x, y, z = fruits_inventory.values
>> x
=> 6

Composition

Composing is the ability to group multiple values into one array that is assigned to a variable. This is useful when you want to decomposition values, make changes, and then composition the results back into a variable. It also makes it possible to perform merges on 2 or more arrays/hashes.

Composition an array with splat operator(*)

Composing an array can be done using the splat operator, (*). This will pack all the values into an array.

>> fruits = ["apple", "banana", "cherry"]
>> more_fruits = ["orange", "kiwi", "melon", "mango"]

# fruits and more_fruits are unpacked and then their elements are packed into combined_fruits
>> combined_fruits = *fruits, *more_fruits

>> combined_fruits
=> ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]

Composition a hash with double splat operator(**)

Composing a hash is done by using the double splat operator(**). This will pack all key/value pairs from one hash into another hash, or combine two hashes together.

>> fruits_inventory = {apple: 6, banana: 2, cherry: 3}
>> more_fruits_inventory = {orange: 4, kiwi: 1, melon: 2, mango: 3}

# fruits_inventory and more_fruits_inventory are unpacked into key-values pairs and combined.
>> combined_fruits_inventory = {**fruits_inventory, **more_fruits_inventory}

# then the pairs are packed into combined_fruits_inventory
>> combined_fruits_inventory
=> {:apple=>6, :banana=>2, :cherry=>3, :orange=>4, :kiwi=>1, :melon=>2, :mango=>3}

Usage of splat operator(*) and double splat operator(**) with methods

Composition with method parameters

When you create a method that accepts an arbitrary number of arguments, you can use *arguments or **keyword_arguments in the method definition. *arguments is used to pack an arbitrary number of positional (non-keyworded) arguments and **keyword_arguments is used to pack an arbitrary number of keyword arguments.

Usage of *arguments:

# This method is defined to take any number of positional arguments
# (Using the single line form of the definition of a method.)

>> def my_method(*arguments)= arguments

# Arguments given to the method are packed into an array

>> my_method(1, 2, 3)
=> [1, 2, 3]

>> my_method("Hello")
=> ["Hello"]

>> my_method(1, 2, 3, "Hello", "Mars")
=> [1, 2, 3, "Hello", "Mars"]

Usage of **keyword_arguments:

# This method is defined to take any number of keyword arguments

>> def my_method(**keyword_arguments)= keyword_arguments

# Arguments given to the method are packed into a dictionary

>> my_method(a: 1, b: 2, c: 3)
=> {:a => 1, :b => 2, :c => 3}

If the method defined does not have any defined parameters for keyword arguments(**keyword_arguments or <key_word>: <value>) then the keyword arguments will be packed into a hash and assigned to the last parameter.

>> def my_method(a)= a

>> my_method(a: 1, b: 2, c: 3)
=> {:a => 1, :b => 2, :c => 3}

*arguments and **keyword_arguments can also be used in combination with one another:

def my_method(*arguments, **keyword_arguments)
  p arguments.sum
  for (key, value) in keyword_arguments.to_a
    p key.to_s + " = " + value.to_s
  end
end


my_method(1, 2, 3, a: 1, b: 2, c: 3)
6
"a = 1"
"b = 2"
"c = 3"

You can also write arguments before and after *arguments to allow for specific positional arguments. This works the same way as decomposing an array.

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

my_method(1, 2, 3, 4, 5)
1
2
[3, 4, 5]

You can write positional arguments before and after *arguments:

>> def my_method(a, *middle, b)= middle

>> my_method(1, 2, 3, 4, 5)
=> [2, 3, 4]

You can also combine positional arguments, *arguments, key-word arguments and **keyword_arguments:

>> def my_method(first, *many, last, a:, **keyword_arguments)
     p first
     p many
     p last
     p a
     p keyword_arguments
     end

>> my_method(1, 2, 3, 4, 5, a: 6, b: 7, c: 8)
1
[2, 3, 4]
5
6
{:b => 7, :c => 8}

Writing arguments in an incorrect order will result in an error:

def my_method(a:, **keyword_arguments, first, *arguments, last)
  arguments
end

my_method(1, 2, 3, 4, a: 5)

syntax error, unexpected local variable or method, expecting & or '&'
... my_method(a:, **keyword_arguments, first, *arguments, last)

Decomposing into method calls

You can use splat operator (*) to unpack an array of arguments into a method call:

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

numbers = [1, 2, 3]
my_method(*numbers)
3
2
1

You can also use double splat operator(**) to unpack a hash of arguments into a method call:

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

numbers = {a: 1, b: 2, c: 3}
my_method(**numbers)
3
2
1