CS108 Lecture 19: Data Collections: Dictionaries Aaron Stevens 4 - - PDF document

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CS108 Lecture 19: Data Collections: Dictionaries Aaron Stevens 4 March 2008 1 Overview/Questions Review: lists and list operations Associative data relationships, key-value pairs, examples. The Python dictionary ADT. 2 1

SLIDE 1

1

Aaron Stevens

4 March 2008

CS108 Lecture 19: Data Collections: Dictionaries

Overview/Questions

– Review: lists and list operations – Associative data relationships, key-value pairs, examples. – The Python dictionary ADT.

SLIDE 2

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Review: Python lists

The Python list is a linear data

collection. Elements are stored in the
rder they are added.

The list support typical sequence

perations, including indexing, slicing,

and iteration.

Associative Data

Some data is not well suited to list storage/manipulation. Example: stock prices Huh? Prices for which stocks?

39.73 40.62 47.83 71.41

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Example: Stock Quotes

Common Stocks (shares of ownership in publicly traded companies) are often referred to by the ticker symbol, and a share has a price in dollars. Neither element has much meaning by itself.

Symbols: Prices:

“BAC” “EQR” “MO” “PCL” 32.51 38.24 23.24 19.32

Example: Stock Quotes

Data is given meaning by a relationship called a mapping, where an key from one domain is associated with a value from another domain.

Symbols: Prices:

32.51 38.24 23.24 19.32 “BAC” “EQR” “MO” “PCL”

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Associative Data Relationship

Data is stored by a relationship called a mapping, where an key from one domain is associated with a value from another domain. Sometimes data is called a key-value pair. Other examples:

– Name : Phone Number – Course : Grade

Python Dictionaries

A map or dictionary is an associative collection of key-value pairs. Python provides a built-in type dict, which is a dictionary.

# creating an empty dictionary stockPrices = {}

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Key-value Indexing

Elements in the dictionary are accessed by key-value indexing. For inserting/updating:

<dict>[<key>] = <value>

For retrieving:

<value> = <dict>[<key>] <key>,<value> can be of any type.

Key-value Indexing

stockPrices = {} # an empty dictionary stockPrices['BAC'] = 23.24 stockPrices['EQR'] = 32.51 stockPrices['MO'] = 19.32 stockPrices['PCL'] = 38.24 print stockPrices['BAC'] print stockPrices

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dicts are Sequences

Python dictionaries are sequences, which means usual sequence operations apply:

Reads a <value> out of a <dict>, assigns to var.

<value> = <dict>[<key>]

x takes on each tuple (key-value pair) in the sequence.

for x in <dict>.items()

x takes on each value in the sequence.

for x in <dict>.values()

Assigns a <value> to be mapped to a <key>.

Meaning Operation

x takes on each key in the sequence.

for x in <dict> for x in <dict>.keys()

dicts are Objects

Python dictionaries are objects, which supply some useful methods:

Deletes the pair corresponding to <key>. del <dict>[<key>] Deletes the entire collection of <key>-<value> pairs. <dict>.clear() Returns the <value> corresponding to <key>. <dict>.get(<key>)

Meaning Method

Returns a list of tuple s (<key>, <value>). <dict>.items() Returns a list of all <value> s. <dict>.values() Returns a list of all <key>s. <dict>.keys() Returns True if dictionary contains this key. <dict>.has_key(<key>) <key> in <dict>

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Data Constraints

Note: duplicate keys are not allowed.

– What happens if we try to add another mapping with an existing key? – What about duplicate values? The python dict is a uni-map. Python does not provide a multi-map, but we could write our own.

Example: wordcount.py

Let’s write a program which counts the

ccurrences of words in a text file.

– Create a mapping of _ to _ – Open a text file for reading – For each line in the file, create a list of words

For each word in the list, add to its count in mapping

– For all keys in mapping, print values (counts)

The work we write to process a dict of counts will be general – they will work an any data – not just words!

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Wordcount.py

Checking for existing key

Another approach, same effect:

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Ordering by Values

Normally the sort works by key. This function helps count by value:

Print out Word Counts:

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Take-Away Points

– Aassociative data relationship – Python’s built-in type dict – Key-value relationships – Uni-map – Multi-map

Student To Dos

– Reading: chapter 11.6 (today) – Lab 08 will be on on list/dict operations and data processing. – HW08 is posted now – Readings for after spring break: SQL Tutorial http://www.firstsql.com/tutor.htm

1

Aaron Stevens

4 March 2008

CS108 Lecture 19: Data Collections: Dictionaries

Overview/Questions

– Review: lists and list operations – Associative data relationships, key-value pairs, examples. – The Python dictionary ADT.

2

Review: Python lists

The Python list is a linear data

The list support typical sequence

and iteration.

Associative Data

Some data is not well suited to list storage/manipulation. Example: stock prices Huh? Prices for which stocks?

3

Example: Stock Quotes

Common Stocks (shares of ownership in publicly traded companies) are often referred to by the ticker symbol, and a share has a price in dollars. Neither element has much meaning by itself.

Symbols: Prices:

Example: Stock Quotes

Data is given meaning by a relationship called a mapping, where an key from one domain is associated with a value from another domain.

4

Associative Data Relationship

Data is stored by a relationship called a mapping, where an key from one domain is associated with a value from another domain. Sometimes data is called a key-value pair. Other examples:

– Name : Phone Number – Course : Grade

Python Dictionaries

A map or dictionary is an associative collection of key-value pairs. Python provides a built-in type dict, which is a dictionary.

# creating an empty dictionary stockPrices = {}

5

Key-value Indexing

Elements in the dictionary are accessed by key-value indexing. For inserting/updating:

<dict>[<key>] = <value>

For retrieving:

<value> = <dict>[<key>] <key>,<value> can be of any type.

Key-value Indexing

stockPrices = {} # an empty dictionary stockPrices['BAC'] = 23.24 stockPrices['EQR'] = 32.51 stockPrices['MO'] = 19.32 stockPrices['PCL'] = 38.24 print stockPrices['BAC'] print stockPrices

6

dicts are Sequences

Python dictionaries are sequences, which means usual sequence operations apply:

Meaning Operation

dicts are Objects

Python dictionaries are objects, which supply some useful methods:

Meaning Method

7

Data Constraints

Note: duplicate keys are not allowed.

– What happens if we try to add another mapping with an existing key? – What about duplicate values? The python dict is a uni-map. Python does not provide a multi-map, but we could write our own.

Example: wordcount.py

Let’s write a program which counts the

– Create a mapping of _______ to _______ – Open a text file for reading – For each line in the file, create a list of words

– For all keys in mapping, print values (counts)

The work we write to process a dict of counts will be general – they will work an any data – not just words!

8

Wordcount.py

Checking for existing key

Another approach, same effect:

9

Ordering by Values

Normally the sort works by key. This function helps count by value:

Print out Word Counts:

10

Take-Away Points

– Aassociative data relationship – Python’s built-in type dict – Key-value relationships – Uni-map – Multi-map

Student To Dos

– Reading: chapter 11.6 (today) – Lab 08 will be on on list/dict operations and data processing. – HW08 is posted now – Readings for after spring break: SQL Tutorial http://www.firstsql.com/tutor.htm

– Create a mapping of _ to _ – Open a text file for reading – For each line in the file, create a list of words