09-Dictionaries

Dictionaries
money
tissue
calculator
perfume
candy
http://en.wikipedia.org/wiki/Associative_array
Dictionaries
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Dictionaries are Python’s most powerful data collection
•
Dictionaries allow us to do fast database-like operations in Python
•
Dictionaries have different names in different languages
•
Associative Arrays - Perl / Php
•
Properties or Map or HashMap - Java
•
Property Bag - C# / .Net
http://en.wikipedia.org/wiki/Associative_array
Dictionaries
•
Lists index their entries
based on the position in the
list
•
Dictionaries are like bags -
no order
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So we index the things we
put in the dictionary with a
“lookup tag”
>>> purse = dict()
>>> purse['money'] = 12
>>> purse['candy'] = 3
>>> purse['tissues'] = 75
>>> print purse
{'money': 12, 'tissues': 75, 'candy': 3}
>>> print purse['candy']
3
>>> purse['candy'] = purse['candy'] + 2
>>> print purse
{'money': 12, 'tissues': 75, 'candy': 5}
money
12
tissues
75
>>> purse = dict()
>>> purse['money'] = 12
>>> purse['candy'] = 3
>>> purse['tissues'] = 75
>>> print purse
{'money': 12, 'tissues': 75, 'candy': 3}
>>> print purse['candy']
3
>>> purse['candy'] = purse['candy'] + 2
>>> print purse
{'money': 12, 'tissues': 75, 'candy': 5}
candy
3
candy
5
Comparing Lists and Dictionaries
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Dictionaries are like Lists except that they use keys instead of
numbers to look up values
>>> lst = list()
>>> lst.append(21)
>>> lst.append(183)
>>> print lst
[21, 183]
>>> lst[0] = 23
>>> print lst
[23, 183]
>>> ddd = dict()
>>> ddd['age'] = 21
>>> ddd['course'] = 182
>>> print ddd
{'course': 182, 'age': 21}
>>> ddd['age'] = 23
>>> print ddd
{'course': 182, 'age': 23}
>>> lst = list()
>>> lst.append(21)
>>> lst.append(183)
>>> print lst
[21, 183]
>>> lst[0] = 23
>>> print lst
[23, 183]
>>> ddd = dict()
>>> ddd['age'] = 21
>>> ddd['course'] = 182
>>> print ddd
{'course': 182, 'age': 21}
>>> ddd['age'] = 23
>>> print ddd
{'course': 182, 'age': 23}
[0]
21
[1]
183
lll
Key Value
[course]
183
[age]
21
ddd
Key Value
List
Dictionary
23
23
Dictionary Literals (Constants)
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Dictionary literals use curly braces and have a list of key : value pairs
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You can make an empty dictionary using empty curly braces
>>> jjj = { 'chuck' : 1 , 'fred' : 42, 'jan': 100}
>>> print jjj
{'jan': 100, 'chuck': 1, 'fred': 42}
>>> ooo = { }
>>> print ooo
{}
>>>
Most Common Name?
csev
zhen
zhen
marquard
zhen
cwen
csev
marquard
zhen
marquard
csev
cwen
zhen
Most Common Name?
csev
zhen
zhen
marquard
zhen
cwen
csev
marquard
zhen
marquard
csev
cwen
zhen
Many Counters with a Dictionary
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One common use of dictionary is
counting how often we “see” something
Key Value
>>> ccc = dict()
>>> ccc['csev'] = 1
>>> ccc['cwen'] = 1
>>> print ccc
{'csev': 1, 'cwen': 1}
>>> ccc['cwen'] = ccc['cwen'] + 1
>>> print ccc
{'csev': 1, 'cwen': 2}

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