Methods in Computational Linguistics II

Queens College

Lecture 7: Structuring Things

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Today

• Functions• Object Oriented Programming• Trees• Graphs• Recursion

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Functions in Python

• Functions are a way to structure code for reuse.

• Return Values

• Arguments

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Multiple return values

x, y = swapTwoValues(x, y)

def swapTwoValues(x, y): return y, x

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Arguments

• The information you give to a method is called an argument

def swapTwoValues(x, y): return y, x

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Default Arguments

• Arguments can be optional

def myFunction(text, offset=0, prefix=‘hi ’): print ‘ ‘*offset + prefix + text

myFunction(“andrew”)myFunction(“andrew”, 5)

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Named Arguments

• The specific argument can be named in the call to the function

def myFunction(text, offset=0, prefix=‘hi ’): print ‘ ‘*offset + prefix + text

myFunction(“andrew”)myFunction(“andrew”, prefix=“hello ”)myFunction(text=“andrew”, offset=2)

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Object Oriented Programming

• aka Object Oriented Design.

• Structure functionality into “objects” with associated “member variables” and “methods”.

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Objects vs. Types

• str and int are special classes• They have methods defined specifically for

them• However, these are called types because

they are built-in to the language.• The distinction is rather blurry.

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Classes and Instances

• int is a class• 1 is an instance of the class “int”

• fd = FreqDist()

• FreqDist is a class• fd is an instance of FreqDist.

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Classes in Python

class MyClass: def __init__(self): self.text = ‘hello’

def __repr__(self): return self.text

also __len__ __str__ and __add__

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Importing a class

from filename import class

The filename specifies the package name of the class. This can be used like any other package.

from filename import *

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Inheritance

• Objects can define relationships between objects.

• Membership operations allow for the representation of “has-a”, “has-many”, and arbitrary property relationships.

• Inheritance relationships allow for the representation of “is-a” relationships.

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Shape Example

Shape

Rectangle Triangle Circle

Square

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Trees

• Binary Trees are a commonly used data structure.

• The core element of a tree is a node

• These nodes can contain values, and include pointers to one or more children, that are differentiated as “left” and “right”

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Tree Example

5

a

xyz15

The

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Binary Search Trees

• Binary Search trees have the properties– the value (or key), of any node is greater than

the value of its left child– the value of any node is less than the value of

its right child.

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Binary Search Tree Example

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2

41

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What does this have to do with Binary Search?

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Tree class

• What does a tree class require?

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Graphs

• Graphs are similar to Trees

• Graphs have:– Nodes– Edges

• Nodes can contain values• Edges connect two notes, and can also

have a value.

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Graph Example

5

2

41

9

a

a

b

c

cd

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What does a Graph class require?

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In class coding

• Using an object in a program• Initializing data in an object• Objects that you can iterate over

• Write a Shape class• Write a Tree class• Write a Graph class

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Recursion

• A function that calls itself is called a recursive function.

• A good recursive function must include – A stopping condition– Modification

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Example of Recursion

• Print every element of a tree.

• Search for an entry in a tree.

• Print every element of a graph.

• Search in a graph.

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Next Time

• Machine Learning– A primer on machine learning and its role in

Computational Linguistics.– Classification in NLTK (after break)