Dependency Parser

What is dependency parsing

A natural language parser is a program that works out the grammatical structure of sentences, for instance, which groups of words go together (as "phrases") and which words are the subject or object of a verb.

• Source: Stanford Natural Language Processing Group

Each sentence consists of multiple tokens, and those tokens are syntactically/grammatically dependent on each other. For example a sentence like this one: I love you have a nominal subject I, a direct object you and a verbal phrase love. So the verb love can be said to be a parent of I and you, and since it doesn't have a parent, we describe it as the root of the sentence.

A little bit more elaborate example is : The boss gave me a white pen. This sentence also have a root gave and this root has children which are parents of their own children. You can imagine a visual tree-like representation of this sentence as follows:

Dependency Parsing for "The boss gave me a white pen"

You can notice from the above chart that:

• The is a determiner, it's parent is boss.

• Boss is a nominal subject, it's parent is gave.

• gave is a verbal phrase, it has no parent (this it's the master of the sentence).

• me is a direct object, it's parent is gave.

• a is a determiner, it's parent is pen.

• white is an adjective, it's parent is pen.

• pen is an indirect object, it's parent is gave.

Rules

Dependency parsing have a set of rules that every tree adheres to:

• Sentence root has no parents.

• Sentence root is usually a verb that dictates the meaning of the sentence.

• A sentence can only have one root.

• A token can only have one parent.

• A token can have any number of children, or no children at all.

So it really is a tree-like structure.

Annotation Specifications

Each parent-child relationship has an annotation for it. Take the sentence above for example, me is a child of gave and the relationship between the two can be annotated as DOBJ which means Direct Object.

For a full list of the annotation specifications, refer to the Dependency Parsing Annotations Specifications article.