✏️ Editing: dig_methods.rdoc Read Only

= Dig Methods

Ruby's +dig+ methods are useful for accessing nested data structures.

Consider this data:
    item = {
      id: "0001",
      type: "donut",
      name: "Cake",
      ppu: 0.55,
      batters: {
        batter: [
          {id: "1001", type: "Regular"},
          {id: "1002", type: "Chocolate"},
          {id: "1003", type: "Blueberry"},
          {id: "1004", type: "Devil's Food"}
        ]
      },
      topping: [
        {id: "5001", type: "None"},
        {id: "5002", type: "Glazed"},
        {id: "5005", type: "Sugar"},
        {id: "5007", type: "Powdered Sugar"},
        {id: "5006", type: "Chocolate with Sprinkles"},
        {id: "5003", type: "Chocolate"},
        {id: "5004", type: "Maple"}
      ]
    }

Without a +dig+ method, you can write:
    item[:batters][:batter][1][:type] # => "Chocolate"

With a +dig+ method, you can write:
    item.dig(:batters, :batter, 1, :type) # => "Chocolate"

Without a +dig+ method, you can write, erroneously
(raises <tt>NoMethodError (undefined method `[]' for nil:NilClass)</tt>):
    item[:batters][:BATTER][1][:type]

With a +dig+ method, you can write (still erroneously, but avoiding the exception):
    item.dig(:batters, :BATTER, 1, :type) # => nil

== Why Is +dig+ Better?

- It has fewer syntactical elements (to get wrong).
- It reads better.
- It does not raise an exception if an item is not found.

== How Does +dig+ Work?

The call sequence is:
  obj.dig(*identifiers)

The +identifiers+ define a "path" into the nested data structures:
- For each identifier in +identifiers+, calls method \#dig on a receiver
  with that identifier.
- The first receiver is +self+.
- Each successive receiver is the value returned by the previous call to +dig+.
- The value finally returned is the value returned by the last call to +dig+.

A +dig+ method raises an exception if any receiver does not respond to \#dig:
  h = { foo: 1 }
  # Raises TypeError (Integer does not have #dig method):
  h.dig(:foo, :bar)

== What Else?

The structure above has \Hash objects and \Array objects,
both of which have instance method +dig+.

Altogether there are six built-in Ruby classes that have method +dig+,
three in the core classes and three in the standard library.

In the core:
- Array#dig: the first argument is an \Integer index.
- Hash#dig: the first argument is a key.
- Struct#dig: the first argument is a key.

In the standard library:
- OpenStruct#dig: the first argument is a \String name.
- CSV::Table#dig: the first argument is an \Integer index or a \String header.
- CSV::Row#dig: the first argument is an \Integer index or a \String header.

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