Introduction to Python Dictionaries - Brown...

Introduction to Python Dictionaries

CSCI 0931 - Intro. to Comp. for the Humanities and Social Sciences 1

Warmup

• Brief discussion of list sorting • Discussion of types in Python


Sorting Lists


Preloaded Functions

Name Inputs Outputs CHANGES sort List Original List!

>>> myList = [0,4,1,5,-1,6] >>> myList.sort() >>> myList [-1, 0, 1, 4, 5, 6]

Sorting Lists


Preloaded Functions

Name Inputs Outputs CHANGES sort List Original List!

>>> myList = [0,4,1,5,-1,6] >>> myList.sort() >>> myList [-1, 0, 1, 4, 5, 6] >>> myList = ['b','d','c','a','z','i'] >>> myList.sort() >>> myList ['a', 'b', 'c', 'd', 'i', 'z']

Types • When we write > x = 4 The number 4 is an integer, and it cannot be changed: 4 always means 4. • We say that “integers are not mutable” • Some other Python types are mutable: > lst = [1, 2, 3] > lst[0]=5 > lst [5, 2, 3] • The thing that lst refers to has changed!


Subtleties

• Subscripted list assignment is not a reassignment: it’s a MUTATION

>>> lst = [1, 2, 3]

>>> lst2 = lst

>>> lst[0] = 5

>>> lst2

[5, 2, 3] (!)


>>> lst = [1, 2, 3]

>>> lst2 = lst

>>> lst[0] = 5

>>> lst2

[5, 2, 3]


vars

Name Value lst [1 2 3]

vars

Name Value lst

lst2

[1 2 3]

vars

Name Value lst

lst2

[5 2 3]

What just happened? (v1)

>>> num = 7

>>> num2 = num

>>> num = 4 #reassignment!

>>> num2

7


vars

Name Value num 7

vars

Name Value num

num2

7

vars

Name Value num

num2

7

Compare with integer values 4

>>> lst = [1, 2, 3]

>>> lst[0] = 7


vars

Name Value lst

[ , , ]

What just happened? (v2)

3 1 2

7

General Rules

• “Primitive types” like int, float, string are created and stored somewhere

• Variables store “arrows” pointing to these • Assignments like “x = 4” create new arrows to

new stored values • Subscripted assignments like “lst[3] = 2” replace

the arrow at “lst[3]” with a new stored value • Slice assignments like lst[0:2] = [5] replace

multiple arrows (more details later)


Disclaimer

• This story is one that’s consistent with the behavior of the part of Python that you’ll see in CS0931

• The whole story is somewhat deeper...


HW 2-3

• removePunctuation() – Look at the output you get and see if you need to

catch more types of punctuation – How do we check for quotation marks?


HW 2-3

• removePunctuation() – Look at the output you get and see if you need to

catch more types of punctuation – How do we check for quotation marks?

• Remember how return works – You can return at any point in the function; the

function will just stop executing at that point.


HW 2-3

• testRemovePunctuation() – What is this testing thing all about?


HW 2-3


def testRemovePunctuation(): return True # All the tests must have passed!

HW 2-3


def testRemovePunctuation(): if removePunctuation(‘foo!’) != ???? : return True # All the tests must have passed!

I know what this should look like after removing the punctuation

This is the output I expect

HW 2-3


def testRemovePunctuation(): if removePunctuation(‘foo!’) != ‘foo ‘: return False return True # All the tests must have passed!

HW 2-3


def testRemovePunctuation(): if removePunctuation(‘foo!’) != ‘foo ‘: return False if removePunctuation(‘(hello.?)’ != ‘ hello ‘: return False return True # All the tests must have passed!

HW 2-3


def testRemovePunctuation(): if removePunctuation(‘foo!’) != ‘foo ‘: return False if removePunctuation(‘(hello.?)’ != ‘ hello ‘: return False <another test here!> return True # All the tests must have passed!

Reminders

• Don’t forget the Homework Policy you signed – You can only discuss problems with others, not

copy code, and you must add a comment saying who helped you.

– The CS department enforces this strictly.


The Big Picture


Overall Goal Build a Concordance of a text

• Locations of words • Frequency of words

Today: Get Word Frequencies • Define the inputs and the outputs • Learn a new data structure • Write a function to get word frequencies • Go from word frequencies to a concordance (finally!)

Word Frequency: Inputs and Outputs

I want to write a wordFreq function


The cat had a hat. The cat sat on the hat.



• What is the input to wordFreq?





• What is the input to wordFreq? • What is the output of wordFreq?








Word Freq.

the 3

cat 2

had 1

a 1

hat 2

sat 1

on 1



• What is the input to wordFreq? • What is the output of wordFreq? • We could do this with a list. How?



Word Freq.

the 3

cat 2

had 1

a 1

hat 2

sat 1

on 1

The Big Picture





A New Data Structure

• A Data Structure is simply a way to store information. – Lists are a type of data structure

• We can have lists of integers, floats, strings, booleans, or any combination.

• Organized linearly (indexed by a range of integers)





Word Frequency

the 3

cat 2

had 1

a 1

hat 2

sat 1

on 1




Word Frequency

the 3

cat 2

had 1

a 1

hat 2

sat 1

on 1

Associate each word with the frequency.




Word Frequency

the 3

cat 2

had 1

a 1

hat 2

sat 1

on 1


Keys Values




Word Frequency

the 3

cat 2

had 1

a 1

hat 2

sat 1

on 1


Keys Values

Key-Value Pairs Key Value

‘the’ 3

‘cat’ 2

A New Data Structure: Dictionaries


Key Type Value Type Example Key Example Value

Keys can be almost any type or data structure. Values can be any type or data structure.



Key Type Value Type Example Key Example Value String Integer 'the' 3

String Integer 'cat' 2






String String 'Geisel' '078-05-1120'

String String 'Whitcher' '552-38-5014'








Float String 1.0 'one point oh'

Float String 2.8 'two point eight'








Float String 1.0 'one point oh'

Float String 2.8 'two point eight'

Integer List 1638 [2, 3, 3, 7, 13]





Word Frequency

the 3

cat 2

had 1

a 1

hat 2

sat 1

on 1

>>> freq = {} >>> freq {} >>>

Initialize a Dictionary




Word Frequency

the 3

cat 2

had 1

a 1

hat 2

sat 1

on 1

>>> freq = {} >>> freq {} >>> freq['the'] = 3 >>> freq {'the': 3} >>>


Key = ‘the’ Value = 3




Word Frequency

the 3

cat 2

had 1

a 1

hat 2

sat 1

on 1

>>> freq = {} >>> freq {} >>> freq['the'] = 3 >>> freq {'the': 3} >>> freq['cat'] = 2 >>> freq {'the': 3, 'cat': 2} >>>


Key = ‘the’ Value = 3

Key = ‘cat’ Value = 2




Word Frequency

the 3

cat 2

had 1

a 1

hat 2

sat 1

on 1

>>> freq2 = {'the':3,'cat':2} >>> freq2 {'the': 3, 'cat': 2} >>>

Initialize a dictionary with two key-value pairs




Word Frequency

the 3

cat 2

had 1

a 1

hat 2

sat 1

on 1

>>> freq2 = {'the':3,'cat':2} >>> freq2 {'the': 3, 'cat': 2} >>> >>> freq2['cat'] 2 >>> freq2['the'] 3

Retrieve a value using the key

Redefining things in the dictionary



Word Frequency

the 3

cat 2

had 1

a 1

hat 2

sat 1

on 1

>>> freq2 = {'the':3,'cat':2} >>> freq2 {'the': 3, 'cat': 2} >>> >>> freq2['cat'] = 7 >>> freq2['a'] = freq2['a']+1

Assign a new value to a key!

7

2

The Big Picture





Python Dictionaries


Function (All operate on Dictionaries)

Input Output Example

keys() None List of keys

>>> freq2.keys() ['the', 'cat']

values() None List of values

>>> freq2.values() [3, 2]

has_key(<key>) Key Boolean >>> freq2.has_key('missing') False

<key> in <dict> (means same as above)

>>> 'the' in freq2 True

del(<dict>[<key>]) Dict. Entry

None >>> del(freq2['cat'])

• Keys Are Unique! • Assigning/getting any value is very fast

Python Dictionaries








has_key(<key>) Key Boolean >>> freq2.has_key('missing') False




None >>> del(freq2['cat'])

Break


http://xkcd.com/1007/

The Big Picture





Python Dictionaries








has_key(<key>) Key True or False

>>> freq2.has_key('missing') False




None >>> del(freq2[‘cat'])

Python Dictionaries





Word Freq.

the 3

cat 2

had 1

a 1

hat 2

sat 1

on 1

The Big Picture





Building a Concordance


The cat had a hat. The cat sat on the hat. 0 1 2 3 4 5 6 7 8 9 10

Word List of Positions Frequency

the [0,5,9] 3

cat [1,6] 2

had [2] 1

a [3] 1

hat [4,10] 2

sat [7] 1

on [8] 1

The Big Picture





This will be part of your next HW

Introduction to Python Dictionaries - Brown...

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