recursion - pace university

2007 OOPSLA Educators’ Symposium Reinforcing Structural Recursion with Lists – 1 of 40

Teaching Strategies for ReinforcingStructural Recursion with Lists

Michael H. Goldwasser David Letscher

Saint Louis University

Active Learning

Overview

Active Learning

Structural Recursion

Beyond Graphics

Python’s List Class

Role Playing

Variants

Mutators

Implementation

Conclusions


I need somevolunteersfor today


Overview

Active Learning


Beyond Graphics


Role Playing

Variants

Mutators

Implementation

Conclusions


For an object-oriented CS1, structural recursion canbe more natural than functional recursion.

An object is composed of a basic shape and a(recursive) instance of the same class.

Beyond Graphics

Overview

Active Learning


Beyond Graphics


Role Playing

Variants

Mutators

Implementation

Conclusions


Pro: Graphics are fun and tangible.

Beyond Graphics

Overview

Active Learning


Beyond Graphics


Role Playing

Variants

Mutators

Implementation

Conclusions



Con: recursive patterns are generally limited(“draw outer, draw rest”; “move outer, move rest”)

Beyond Graphics

Overview

Active Learning


Beyond Graphics


Role Playing

Variants

Mutators

Implementation

Conclusions



Con: recursive patterns are generally limited(“draw outer, draw rest”; “move outer, move rest”)

Our goal is to provide a tangible presentation for anon-graphical example of structural recursion(namelypurely-recursive lists).


Overview

Active Learning


Beyond Graphics


Role Playing

Variants

Mutators

Implementation

Conclusions


Python supports alist class as a standard container.


Overview

Active Learning


Beyond Graphics


Role Playing

Variants

Mutators

Implementation

Conclusions



Disclaimer:the internal implementation is not trulyrecursive; its an expandible array akin to Java’sArrayList or C++’svector.


Overview

Active Learning


Beyond Graphics


Role Playing

Variants

Mutators

Implementation

Conclusions




Public Interface:Our students are very familiarwith use of this class and its menu of behaviors(we use lists from the opening weeks of CS1).


Overview

Active Learning


Beyond Graphics


Role Playing

Variants

Mutators

Implementation

Conclusions




Public Interface:Our students are very familiarwith use of this class and its menu of behaviors(we use lists from the opening weeks of CS1).

This allows us todecoupletwo potentiallyintertwined concepts:

1. the use of recursion2. the abstraction of a container class

Emulating Python’s List Class

Overview

Active Learning


Beyond Graphics


Role Playing

Variants

Mutators

Implementation

Conclusions


We rely on the familiar public interface byprecisely emulatingPython’slist class, includingbehaviors such as:

count(value) len ( )index(value) contains (value)append(value) getitem (index)insert(index, value) setitem (index, value)remove(value) repr ( )

This allows us to sidestep the design issue ofparameterizing the recursion.

Role Playing

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


Role Playing

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


Classic activity for teaching object orientation.

Role Playing

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions



Classic activity for teaching (functional) recursion.

Role Playing

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions



Classic activity for teaching (functional) recursion.

Limited history for the combination of these ideas.

Ground Rules for Students

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


OurList Class: an instance will be representedrecursively using two attributes:

■ head: a reference to the first element (if any)

■ rest: a reference to a secondary list with allremaining elements (if any)

Our base case is anempty list, represented withboth head and rest set to theNone reference.

An empty list is a natural concept for our studentsbecause Python’s default list instance is empty.

State Information

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


Each actor is given a slip of paper that representshis/her state information.

head:rest:

: OurList

State Information

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions



head:rest:

: OurList

Per'E'Sharon

Example: here is the slip currently held by Sharon .

State Information

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions



head:rest:

: OurList

NoneNone

Matthew

Example: here is the slip currently held byMatthew .

Message Passing

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


We enforce strict “message passing” for allcommunication.

Activation records are sent inside a tennis ball.

ACTIVATION RECORD

Sent to:Method:Parameters (if any):Please return to:

Return Value (if any):

Message Passing

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


We enforce strict “message passing” for allcommunication.

Activation records are sent inside a tennis ball.

ACTIVATION RECORD



count'E'Errol

Michael

Let’s get started with a callErrol .count('E')

Errol ’s Point of View

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


head:rest:

: OurList

Sharon

'F'Errol ACTIVATION RECORD



count'E'Errol

Michael


Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


head:rest:

: OurList

Sharon




count'E'Errol

Michael

ACTIVATION RECORD



count'E'Sharon

Errol

Sharon ’s Point of View

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


head:rest:

: OurList

Per

'E'Sharon ACTIVATION RECORD



count'E'Sharon

Errol


Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


head:rest:

: OurList

Per




count'E'Sharon

Errol

ACTIVATION RECORD



count'E'Per

Sharon


Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


head:rest:

: OurList

Per




count'E'Sharon

Errol

ACTIVATION RECORD



count'E'

1

Per

Sharon


Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


head:rest:

: OurList

Per




count'E'2

Sharon

Errol

ACTIVATION RECORD



count'E'

1

Per

Sharon


Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


head:rest:

: OurList

Sharon




count'E'Errol

Michael

ACTIVATION RECORD



count'E'

2

Sharon

Errol


Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


head:rest:

: OurList

Sharon




count'E'2

Errol

Michael

ACTIVATION RECORD



count'E'

2

Sharon

Errol

Sequence Diagram

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Sharon

'F'Errol

head:rest:

: OurList

Per

'E'Sharon

head:rest:

Sequence Diagram

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Sharon

'F'Errol

count('E')

head:rest:

: OurList

Per

'E'Sharon

head:rest:

Sequence Diagram

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Sharon

'F'Errol

count('E')count('E')

head:rest:

: OurList

Per

'E'Sharon

head:rest:

Sequence Diagram

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Sharon

'F'Errol


count('E')

head:rest:

: OurList

Per

'E'Sharon

head:rest:

Sequence Diagram

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Sharon

'F'Errol



head:rest:

: OurList

Per

'E'Sharon

head:rest:

Sequence Diagram

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Sharon

'F'Errol



count('E')

head:rest:

: OurList

Per

'E'Sharon

head:rest:

Sequence Diagram

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Sharon

'F'Errol



count('E')

0

head:rest:

: OurList

Per

'E'Sharon

head:rest:

Sequence Diagram

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Sharon

'F'Errol



count('E')

10

head:rest:

: OurList

Per

'E'Sharon

head:rest:

Sequence Diagram

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Sharon

'F'Errol



count('E')

11

0

head:rest:

: OurList

Per

'E'Sharon

head:rest:

Sequence Diagram

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Sharon

'F'Errol



count('E')

21

10

head:rest:

: OurList

Per

'E'Sharon

head:rest:

Sequence Diagram

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Sharon

'F'Errol



count('E')

22

11

0

head:rest:

: OurList

Per

'E'Sharon

head:rest:

Local View

Overview

Role Playing

Role Playing

Ground Rules

State Information

Message Passing

Point of View

Sequence Diagram

Local View

Variants

Mutators

Implementation

Conclusions


rest:

: OurList

Per

'E'Sharon


21

head:

Variants

Overview

Role Playing

Variants

The index methodThe getitemmethod

Recursive Patterns

Mutators

Implementation

Conclusions


The index method

Overview

Role Playing

Variants


Recursive Patterns

Mutators

Implementation

Conclusions


head:rest:

: OurList

Sharon

'F'Errol

index('T')

The index method

Overview

Role Playing

Variants


Recursive Patterns

Mutators

Implementation

Conclusions


head:rest:

: OurList

Sharon

'F'Errol

index('T')index('T')

The index method

Overview

Role Playing

Variants


Recursive Patterns

Mutators

Implementation

Conclusions


head:rest:

: OurList

Sharon

'F'Errol

index('T')

1

index('T')

The index method

Overview

Role Playing

Variants


Recursive Patterns

Mutators

Implementation

Conclusions


head:rest:

: OurList

Sharon

'F'Errol

index('T')

1

index('T')

2

The index method

Overview

Role Playing

Variants


Recursive Patterns

Mutators

Implementation

Conclusions


rest:

: OurList

Dale

'T'Per

index('T')

head:

The index method

Overview

Role Playing

Variants


Recursive Patterns

Mutators

Implementation

Conclusions


rest:

: OurList

Dale

'T'Per

index('T')

0

head:

The index method

Overview

Role Playing

Variants


Recursive Patterns

Mutators

Implementation

Conclusions


: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Sharon

'F'Errol

index('T')

head:rest:

: OurList

Per

'E'Sharon

head:rest:

The index method

Overview

Role Playing

Variants


Recursive Patterns

Mutators

Implementation

Conclusions


: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Sharon

'F'Errol


head:rest:

: OurList

Per

'E'Sharon

head:rest:

The index method

Overview

Role Playing

Variants


Recursive Patterns

Mutators

Implementation

Conclusions


: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Sharon

'F'Errol


index('T')

head:rest:

: OurList

Per

'E'Sharon

head:rest:

The index method

Overview

Role Playing

Variants


Recursive Patterns

Mutators

Implementation

Conclusions


: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Sharon

'F'Errol

index('T')

0


head:rest:

: OurList

Per

'E'Sharon

head:rest:

The index method

Overview

Role Playing

Variants


Recursive Patterns

Mutators

Implementation

Conclusions


: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Sharon

'F'Errol

index('T')

0

index('T')

1

index('T')

head:rest:

: OurList

Per

'E'Sharon

head:rest:

The index method

Overview

Role Playing

Variants


Recursive Patterns

Mutators

Implementation

Conclusions


: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Sharon

'F'Errol

index('T')

0

index('T')

12

index('T')

head:rest:

: OurList

Per

'E'Sharon

head:rest:

The index method

Overview

Role Playing

Variants


Recursive Patterns

Mutators

Implementation

Conclusions


: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Sharon

'F'Errol

index('T')

0

index('T')

12

index('T')

head:rest:

: OurList

Per

'E'Sharon

head:rest:

This differs fromcount because the recursion doesnot necessarily proceed to an empty list.

The getitem method

Overview

Role Playing

Variants


Recursive Patterns

Mutators

Implementation

Conclusions


In Python, the operator syntaxdata[2] isimplemented with a call todata. getitem (2).

head:rest:

: OurList

Sharon

'F'Errol

getitem (2)

The getitem method

Overview

Role Playing

Variants


Recursive Patterns

Mutators

Implementation

Conclusions



head:rest:

: OurList

Sharon

'F'Errol

getitem (1)getitem (2)

The getitem method

Overview

Role Playing

Variants


Recursive Patterns

Mutators

Implementation

Conclusions



head:rest:

: OurList

Sharon

'F'Errol

getitem (1)

'T'getitem (2)

The getitem method

Overview

Role Playing

Variants


Recursive Patterns

Mutators

Implementation

Conclusions



head:rest:

: OurList

Sharon

'F'Errol

getitem (1)

'T'getitem (2)

'T'

The getitem method

Overview

Role Playing

Variants


Recursive Patterns

Mutators

Implementation

Conclusions



head:rest:

: OurList

Sharon

'F'Errol

getitem (1)

'T'getitem (2)

'T'Note: the parameter value changes during therecursion; the return value does not change.

Recursive Patterns


base case parameters return valuemethod empty head index same vary none same vary none

len ✓ ✓ ✓contains ✓ ✓ ✓ ✓getitem ✓ ✓ ✓ ✓setitem ✓ ✓ ✓ ✓

repr ✓ ✓ ✓count ✓ ✓ ✓index ✓ ✓ ✓ ✓

append ✓ ✓ ✓insert ✓ ✓ ✓ ✓

remove ✓ ✓ ✓ ✓

Mutators

Overview

Role Playing

Variants

Mutators

Mutators

Theappend method

insert, remove, pop

Implementation

Conclusions


Easiest: setitem

It is a one-for-one change of data,without any structural change on the list.

(very similar pattern to getitem )

The append method


: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Matthew

'E'Dale

head:rest:

: OurList

Sharon

'F'Errol

append('S')

head:rest:

: OurList

Per

'E'Sharon

head:rest:

The append method


: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Matthew

'E'Dale

head:rest:

: OurList

Sharon

'F'Errol

append('S')append('S')

head:rest:

: OurList

Per

'E'Sharon

head:rest:

The append method


: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Matthew

'E'Dale

head:rest:

: OurList

Sharon

'F'Errol


append('S')

head:rest:

: OurList

Per

'E'Sharon

head:rest:

The append method


: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Matthew

'E'Dale

head:rest:

: OurList

Sharon

'F'Errol



head:rest:

: OurList

Per

'E'Sharon

head:rest:

The append method


: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Matthew

'E'Dale

head:rest:

: OurList

Sharon

'F'Errol



append('S')

head:rest:

: OurList

Per

'E'Sharon

head:rest:

The append method


: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Matthew

'E'Dale

head:rest:

: OurList

Sharon

'F'Errol



append('S')

'S'head:rest:

: OurList

Per

'E'Sharon

head:rest:

The append method


: OurList

NoneNone

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Matthew

'E'Dale

head:rest:

: OurList

Sharon

'F'Errol



append('S')

'S'OurList( )

head:rest:

: OurList

Per

'E'Sharon

head:rest:

The append method


None

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Judy

head:rest:

: OurList

Sharon

'F'Errol



append('S')OurList( )

'S'

Judy

head:rest:

: OurList

Per

'E'Sharon

head:rest:

: OurList

None

The append method


None

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Judy

head:rest:

: OurList

Sharon

'F'Errol




Judy

'S'

Judy

head:rest:

: OurList

Per

'E'Sharon

head:rest:

: OurList

None

The append method


None

Matthew

head:rest:

: OurList

Dale

'T'Per

head:rest:

: OurList

Matthew

'E'Dale

head:rest:

: OurList

NoneNone

Judy

head:rest:

: OurList

Sharon

'F'Errol




Judy

'S'

Judy

head:rest:

: OurList

Per

'E'Sharon

head:rest:

: OurList

None

Instructor can highlight the system’s memory management.

insert, remove, pop

Overview

Role Playing

Variants

Mutators

Mutators

Theappend method

insert, remove, pop

Implementation

Conclusions


Arbitrary insertions and deletions can be performed(more on this in the conclusion...)

Implementation

Overview

Role Playing

Variants

Mutators

Implementation

Getting Started

appendcount

contains

Conclusions


Getting Started

Overview

Role Playing

Variants

Mutators

Implementation

Getting Started

appendcount

contains

Conclusions


class OurList:def init (self):

self. head = Noneself. rest = None

def isEmpty(self): # a private utilityreturn self. rest is None

The append method

Overview

Role Playing

Variants

Mutators

Implementation

Getting Started

appendcount

contains

Conclusions


Has a base case and a simple recursion

def append(self, value):if self. isEmpty( ):

self. head = value # we have one itemself. rest = OurList( ) # followed by empty list

else:self. rest.append(value) # recurse

The count method

Overview

Role Playing

Variants

Mutators

Implementation

Getting Started

appendcount

contains

Conclusions


Has a base case and a non-trivial recursion

def count(self, value):if self. isEmpty( ):

return 0else:

answer = self. rest.count(value)if self. head == value: # additional match

answer += 1return answer

The contains method

Overview

Role Playing

Variants

Mutators

Implementation

Getting Started

appendcount

contains

Conclusions


Has two distinct base cases

def contains (self, value):if self. isEmpty( ):

return Falseelif self. head == value:

return Trueelse:

return value in self. rest # implicit recursion

Conclusions

Overview

Role Playing

Variants

Mutators

Implementation

Conclusions

Conclusions

Advanced Lessons

Error handling

insert andremove

Default parameters

reversesort


■ Lots of fun

■ Strategic challenges

■ Varying recursive patterns

■ Instills a local perspective

■ Coherent transition to source code

■ We have really used functional recursion aswell as structural recursion.

Advanced Lessons

Overview

Role Playing

Variants

Mutators

Implementation

Conclusions

Conclusions

Advanced Lessons

Error handling

insert andremove

Default parameters

reversesort


■ Ample opportunities for advanced lessons(time permitting)

◆ Error handling

◆ Default parameter values

◆ More complex recursive patterns

Error handling

Overview

Role Playing

Variants

Mutators

Implementation

Conclusions

Conclusions

Advanced Lessons

Error handling

insert andremove

Default parameters

reversesort


def getitem (self, i):if self. isEmpty( ):

raise IndexError('index out of range')elif i == 0:

return self. headelse:

return self. rest. getitem (i−1)

Error handling

Overview

Role Playing

Variants

Mutators

Implementation

Conclusions

Conclusions

Advanced Lessons

Error handling

insert andremove

Default parameters

reversesort


def getitem (self, i):if self. isEmpty( ):

raise IndexError('index out of range')elif i == 0:

return self. headelse:

try:return self. rest. getitem (i−1)

except IndexError:raise IndexError('index out of range')

insert and remove

Overview

Role Playing

Variants

Mutators

Implementation

Conclusions

Conclusions

Advanced Lessons

Error handling

insert andremove

Default parameters

reversesort


def insert(self, index, value):if self. isEmpty( ): # ”append” to end

self. head = valueself. rest = OurList( )

elif index > 0: # insert recursivelyself. rest.insert(index−1, value)

else:# reinsert our head as the front of the restself. rest.insert(0, self. head)# and then store the new value hereself. head = value

The insert method (alternative)

Overview

Role Playing

Variants

Mutators

Implementation

Conclusions

Conclusions

Advanced Lessons

Error handling

insert andremove

Default parameters

reversesort


def insert(self, index, value):if self. isEmpty( ): # ”append” to end

self. head = valueself. rest = OurList( )

elif index > 0: # insert recursivelyself. rest.insert(index−1, value)

else: # new item goes here!shift = OurList( )shift. head = self. headshift. rest = self. restself. head = valueself. rest = shift

The remove method

Overview

Role Playing

Variants

Mutators

Implementation

Conclusions

Conclusions

Advanced Lessons

Error handling

insert andremove

Default parameters

reversesort


def remove(self, value):if self. isEmpty( ):

raise ValueError('value not in list')elif self. head == value:

self. head = self. rest. head # privateself. rest = self. rest. rest # private

else:self. rest.remove(value)

Default parameters

Overview

Role Playing

Variants

Mutators

Implementation

Conclusions

Conclusions

Advanced Lessons

Error handling

insert andremove

Default parameters

reversesort


def pop(self, index=None):if self. isEmpty( ):

raise IndexError('pop from empty list')else:

if index is None:index = len(self) − 1

if index == 0:answer = self. headself. head = self. rest. headself. rest = self. rest. restreturn answer

else:return self. rest.pop(index−1)

reverse

Overview

Role Playing

Variants

Mutators

Implementation

Conclusions

Conclusions

Advanced Lessons

Error handling

insert andremove

Default parameters

reversesort


Make use of other existing methods together withone recursive call.

def reverse(self):if not self. isEmpty( ):

self. rest.reverse( )self. rest.append(self. head)self.remove(self. head)

sort

Overview

Role Playing

Variants

Mutators

Implementation

Conclusions

Conclusions

Advanced Lessons

Error handling

insert andremove

Default parameters

reversesort


head:rest:

: OurList

Sharon

'F'Errol

sort( )

recursion - pace university

Documents