COSC 1306 COMPUTER SCIENCE AND PROGRAMMING

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COSC 1306COMPUTER SCIENCE AND PROGRAMMING

• Jehan-François Pâris
• jfparis_at_uh.edu

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CHAPTER VIIDICTIONARIES, TUPLES AND SETS
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DICTIONARIES
VERY USEFUL
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Dictionaries (I)

• Store pairs of entries called items 'CS'
  '743-713-3350', 'UHPD' '713-743-3333'
• Each pair of entries contains
• A key
• A value
• Key and values are separated by a colon
• Pairs of entries are separated by commas
• Dictionary is enclosed within curly braces

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Usage

• Keys must be unique within a dictionary
• No duplicates
• If we have age 'Alice' 25, 'Bob'
  28then age'Alice' is 25and ageBob' is
  28

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Dictionaries are mutable

• gtgtgt age 'Alice' 25, 'Bob' 28
• gtgtgt saved age
• gtgtgt age'Bob' 29
• gtgtgt age'Bob' 29, 'Alice' 25
• gtgtgt saved'Bob' 29, 'Alice' 25

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Keys must be unique

• gtgtgt age 'Alice' 25, 'Bob' 28, 'Alice'
  26
• gtgtgt age'Bob' 28, 'Alice' 26

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Displaying contents

• gtgtgt age 'Alice' 25, 'Carol' 'twenty-two'
• gtgtgt age.items()dict_items( ('Alice', 25),
  ('Carol', 'twenty-two'))
• gtgtgt age.keys()dict_keys( 'Alice', 'Carol')
• age.values()dict_values(25, 'twenty-two')

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Updating directories

• gtgtgt age 'Alice' 26 , 'Carol' 22
• gtgtgt age.update('Bob' 29)
• gtgtgt age'Bob' 29, 'Carol' 22, 'Alice' 26
• gtgtgt age.update('Carol' 23)
• gtgtgt age'Bob' 29, 'Carol' 23, 'Alice' 26

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Returning a value

• gtgtgt age 'Bob' 29, 'Carol' 23, 'Alice' 26
• gtgtgt age.get('Bob')29
• gtgtgt age'Bob'29

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Removing a specific item (I)

• gtgtgt a 'Alice' 26, 'Carol' 'twenty-two'
• gtgtgt a'Carol' 'twenty-two', 'Alice' 26
• gtgtgt a.pop('Carol)'twenty-two'
• gtgtgt a'Alice' 26

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Removing a specific item (II)

• gtgtgt a.pop('Alice')26
• gtgtgt a
• gtgtgt

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Remove a random item

• gtgtgt age 'Bob' 29, 'Carol' 23, 'Alice' 26
• gtgtgt age.popitem()('Bob', 29)
• gtgtgt age
• 'Carol' 23, 'Alice' 26
• gtgtgt age.popitem()('Carol', 23)
• gtgtgt age'Alice' 26

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TUPLES
NOT COVERED
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Tuples

• Same as lists but
• Immutable
• Enclosed in parentheses
• A tuple with a single element must have a comma
  inside the parentheses
• a (11,)

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Examples

• gtgtgt mytuple (11, 22, 33)
• gtgtgt mytuple011
• gtgtgt mytuple-133
• gtgtgt mytuple01(11,)
• The comma is required!

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Why?

• No confusion possible between 11 and 11
• (11) is a perfectly acceptable expression
• (11) without the comma is the integer 11
• (11, ) with the comma is a list containing the
  integer 11
• Sole dirty trick played on us by tuples!

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Tuples are immutable

• gtgtgt mytuple (11, 22, 33)
• gtgtgt saved mytuple
• gtgtgt mytuple (44,)
• gtgtgt mytuple(11, 22, 33, 44)
• gtgtgt saved(11, 22, 33)

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Things that do not work

• mytuple 55Traceback (most recent call
  last)ZTypeError can only concatenate tuple
  (not "int") to tuple
• Can understand that!

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Sorting tuples

• gtgtgt atuple (33, 22, 11)
• gtgtgt atuple.sort()Traceback (most recent call
  last)AttributeError'tuple' object has no
  attribute 'sort'
• gtgtgt atuple sorted(atuple)
• gtgtgt atuple11, 22, 33

Tuples are immutable!
sorted( ) returns a list!
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Most other things work!

• gtgtgt atuple (11, 22, 33)
• gtgtgt len(atuple)3
• gtgtgt 44 in atupleFalse
• gtgtgt i for i for i in atuple11, 22, 33

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The reverse does not work

• gtgtgt alist 11, 22, 33
• gtgtgt (i for i in alist)ltgenerator object
  ltgenexprgt at 0x02855DA0gt
• Does not work!

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Converting sequences into tuples

• gtgtgt alist 11, 22, 33
• gtgtgt atuple tuple(alist)
• gtgtgt atuple(11, 22, 33)
• gtgtgt newtuple tuple('Hello World!')
• gtgtgt newtuple('H', 'e', 'l', 'l', 'o', ' ', 'W',
  'o', 'r', 'l', 'd', '!')

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SETS
NOT COVERED A SPECIALIZED TOOL
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Sets

• Indentified by curly braces
• 'Alice', 'Bob', 'Carol'
• 'Dean' is a singleton
• Can only contain unique elements
• Duplicates are eliminated
• Immutable like tuples and strings

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Sets do not contain duplicates

• gtgtgt cset 11, 11, 22
• gtgtgt cset11, 22

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Sets are immutable

• gtgtgt aset 11, 22, 33
• gtgtgt bset aset
• gtgtgt aset aset 55
• gtgtgt aset33, 11, 22, 55
• gtgtgt bset33, 11, 22

Union of two sets
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Sets have no order

• gtgtgt 1, 2, 3, 4, 5, 6, 71, 2, 3, 4, 5, 6, 7
• gtgtgt 11, 22, 3333, 11, 22

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Sets do not support indexing

• gtgtgt myset 'Apples', 'Bananas', 'Oranges'
• gtgtgt myset'Bananas', 'Oranges', 'Apples'
• gtgtgt myset0Traceback (most recent call last)
  File "ltpyshell2gt", line 1, in ltmodulegt
  myset0TypeError 'set' object does not support
  indexing

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Examples

• gtgtgt alist 11, 22, 33, 22, 44
• gtgtgt aset set(alist)
• gtgtgt aset33, 11, 44, 22
• gtgtgt aset aset 55SyntaxError invalid syntax

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Boolean operations on sets (I)

• Union of two sets
• Contains all elements that are in set A or in set
  B

A
B
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Boolean operations on sets (II)

• Intersection of two sets
• Contains all elements that are in both sets A and
  B

A
B
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Boolean operations on sets (III)

• Difference of two sets
• Contains all elements that are in A but not in B

A
B
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Boolean operations on sets (IV)

• Symmetric difference of two sets
• Contains all elements that are either
• in set A but not in set B or
• in set B but not in set A

A
B
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Boolean operations on sets (V)

• gtgtgt aset 11, 22, 33
• gtgtgt bset 12, 23, 33
• Union of two sets
• gtgtgt aset bset 33, 22, 23, 11, 12
• Intersection of two sets
• gtgtgt aset bset33

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Boolean operations on sets (VI)

• gtgtgt aset 11, 22, 33
• gtgtgt bset 12, 23, 33
• Difference
• gtgtgt aset bset11, 22
• Symmetric difference
• gtgtgt aset bset11, 12, 22, 23

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Summary

• Strings, lists, tuples, sets and dictionaries all
  deal with aggregates
• Two big differences
• Lists and dictionaries are mutable
• Unlike strings, tuples and sets
• Strings, lists and tuples are ordered
• Unlike sets and dictionaries

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Mutable aggregates

• Can modify individual items
• x 11, 22, 33x0 44will work
• Cannot save current value
• x 11,22, 33y xwill not work

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Immutable aggregates

• Cannot modify individual items
• s 'hello!'s0 'H'is an ERROR
• Can save current value
• s 'hello!'t swill work

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Ordered aggregates

• Entities in the collection can be accessed
  through a numerical index
• s 'Hello!'s0
• x 'Alice', 'Bob', 'Carol'x-1
• t (11, 22)t1

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Other aggregates

• Cannot index sets
• myset 'Apples', 'Bananas', 'Oranges'
  myset0 is WRONG
• Can only index dictionaries through their keys
• age 'Bob' 29, 'Carol' 23, 'Alice'
  26age'Alice' worksage0 is WRONG