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Sets and DictionariesDictionaries Back to the data from our summer counting birds in a mosquito-infested swamp in northern Ontario How many birds of each kind did we see?
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Dictionaries
Copyright © Software Carpentry 2010This work is licensed under the Creative Commons Attribution LicenseSee http://software-carpentry.org/license.html for more information.
Sets and Dictionaries
Sets and Dictionaries Dictionaries
Back to the data from our summer counting birds ina mosquito-infested swamp in northern Ontario
Sets and Dictionaries Dictionaries
Back to the data from our summer counting birds ina mosquito-infested swamp in northern OntarioHow many birds of each kind did we see?
Sets and Dictionaries Dictionaries
Back to the data from our summer counting birds ina mosquito-infested swamp in northern OntarioHow many birds of each kind did we see?Input is a list of several thousand bird names
Sets and Dictionaries Dictionaries
Back to the data from our summer counting birds ina mosquito-infested swamp in northern OntarioHow many birds of each kind did we see?Input is a list of several thousand bird namesOutput is a list of names and counts
Sets and Dictionaries Dictionaries
Could use a list of [name, count] pairs
Sets and Dictionaries Dictionaries
Could use a list of [name, count] pairs def another_bird(counts, bird_name): for i in range(len(counts)): if counts[i][0] == bird_name: counts[i][1] += 1 return counts.append([bird_name, 1])
Sets and Dictionaries Dictionaries
Could use a list of [name, count] pairs def another_bird(counts, bird_name): for i in range(len(counts)): if counts[i][0] == bird_name: counts[i][1] += 1 return counts.append([bird_name, 1])
List of pairs
Sets and Dictionaries Dictionaries
Could use a list of [name, count] pairs def another_bird(counts, bird_name): for i in range(len(counts)): if counts[i][0] == bird_name: counts[i][1] += 1 return counts.append([bird_name, 1])
Name to add
Sets and Dictionaries Dictionaries
Could use a list of [name, count] pairs def another_bird(counts, bird_name): for i in range(len(counts)): if counts[i][0] == bird_name: counts[i][1] += 1 return counts.append([bird_name, 1)
Look at each pairalready in the list
Sets and Dictionaries Dictionaries
Could use a list of [name, count] pairs def another_bird(counts, bird_name): for i in range(len(counts)): if counts[i][0] == bird_name: counts[i][1] += 1 return counts.append([bird_name, 1])
If this is the birdwe're looking for…
Sets and Dictionaries Dictionaries
Could use a list of [name, count] pairs def another_bird(counts, bird_name): for i in range(len(counts)): if counts[i][0] == bird_name: counts[i][1] += 1 return counts.append([bird_name, 1])
…add 1 to itscount and finish
Sets and Dictionaries Dictionaries
Could use a list of [name, count] pairs def another_bird(counts, bird_name): for i in range(len(counts)): if counts[i][0] == bird_name: counts[i][1] += 1 return counts.append([bird_name, 1])Otherwise, add
a new pair tothe list
Sets and Dictionaries Dictionaries
Could use a list of [name, count] pairs def another_bird(counts, bird_name): for i in range(len(counts)): if counts[i][0] == bird_name: counts[i][1] += 1 return counts.append([bird_name, 1])
Pattern: handle an existing case and return in loop,or take default action if we exit the loop normally
Sets and Dictionaries Dictionaries
Could use a list of [name, count] pairs def another_bird(counts, bird_name): for i in range(len(counts)): if counts[i][0] == bird_name: counts[i][1] += 1 return counts.append([bird_name, 1])
start []
Sets and Dictionaries Dictionaries
Could use a list of [name, count] pairs def another_bird(counts, bird_name): for i in range(len(counts)): if counts[i][0] == bird_name: counts[i][1] += 1 return counts.append([bird_name, 1])
start []loon [['loon', 1]]
Sets and Dictionaries Dictionaries
Could use a list of [name, count] pairs def another_bird(counts, bird_name): for i in range(len(counts)): if counts[i][0] == bird_name: counts[i][1] += 1 return counts.append([bird_name, 1])
start []loon [['loon', 1]]goose [['loon', 1], ['goose', 1]]
Sets and Dictionaries Dictionaries
Could use a list of [name, count] pairs def another_bird(counts, bird_name): for i in range(len(counts)): if counts[i][0] == bird_name: counts[i][1] += 1 return counts.append([bird_name, 1])
start []loon [['loon', 1]]goose [['loon', 1], ['goose', 1]]loon [['loon', 2], ['goose', 1]]
Sets and Dictionaries Dictionaries
There's a better way
Sets and Dictionaries Dictionaries
There's a better wayUse a dictionary
Sets and Dictionaries Dictionaries
There's a better wayUse a dictionaryAn unordered collection of key/value pairs
Sets and Dictionaries Dictionaries
There's a better wayUse a dictionaryAn unordered collection of key/value pairsLike set elements, keys are:
Sets and Dictionaries Dictionaries
There's a better wayUse a dictionaryAn unordered collection of key/value pairsLike set elements, keys are:- Immutable
Sets and Dictionaries Dictionaries
There's a better wayUse a dictionaryAn unordered collection of key/value pairsLike set elements, keys are:- Immutable- Unique
Sets and Dictionaries Dictionaries
There's a better wayUse a dictionaryAn unordered collection of key/value pairsLike set elements, keys are:- Immutable- Unique- Not stored in any particular order
Sets and Dictionaries Dictionaries
There's a better wayUse a dictionaryAn unordered collection of key/value pairsLike set elements, keys are:- Immutable- Unique- Not stored in any particular orderNo restrictions on values
Sets and Dictionaries Dictionaries
There's a better wayUse a dictionaryAn unordered collection of key/value pairsLike set elements, keys are:- Immutable- Unique- Not stored in any particular orderNo restrictions on values- Don't have to be immutable or unique
Sets and Dictionaries Dictionaries
Create a dictionary by putting key:value pairs in {}
Sets and Dictionaries Dictionaries
>>> birthdays = {'Newton' : 1642, 'Darwin' : 1809}
Create a dictionary by putting key:value pairs in {}
Sets and Dictionaries Dictionaries
>>> birthdays = {'Newton' : 1642, 'Darwin' : 1809}
Create a dictionary by putting key:value pairs in {}
Retrieve values by putting key in []
Sets and Dictionaries Dictionaries
>>> birthdays = {'Newton' : 1642, 'Darwin' : 1809}
Create a dictionary by putting key:value pairs in {}
Retrieve values by putting key in []Just like indexing strings and lists
Sets and Dictionaries Dictionaries
>>> birthdays = {'Newton' : 1642, 'Darwin' : 1809}
Create a dictionary by putting key:value pairs in {}
>>> print birthdays['Newton']1642
Retrieve values by putting key in []Just like indexing strings and lists
Sets and Dictionaries Dictionaries
>>> birthdays = {'Newton' : 1642, 'Darwin' : 1809}
Create a dictionary by putting key:value pairs in {}
>>> print birthdays['Newton']1642
Retrieve values by putting key in []Just like indexing strings and lists
Just like using a phonebook or dictionary
Sets and Dictionaries Dictionaries
Add another value by assigning to it
Sets and Dictionaries Dictionaries
>>> birthdays['Turing'] = 1612 # that's not right
Add another value by assigning to it
Sets and Dictionaries Dictionaries
>>> birthdays['Turing'] = 1612 # that's not right
Add another value by assigning to it
Overwrite value by assigning to it as well
Sets and Dictionaries Dictionaries
>>> birthdays['Turing'] = 1612 # that's not right
Add another value by assigning to it
>>> birthdays['Turing'] = 1912>>> print birthdays{'Turing' : 1912, 'Newton' : 1642, 'Darwin' : 1809}
Overwrite value by assigning to it as well
Sets and Dictionaries Dictionaries
Note: entries are not in any particular order
Sets and Dictionaries Dictionaries
'Turing'
'Newton'
'Darwin'1912
1642
1809
Note: entries are not in any particular order
Sets and Dictionaries Dictionaries
Key must be in dictionary before use
Sets and Dictionaries Dictionaries
>>> birthdays['Nightingale']KeyError: 'Nightingale'
Key must be in dictionary before use
Sets and Dictionaries Dictionaries
>>> birthdays['Nightingale']KeyError: 'Nightingale'
Key must be in dictionary before use
Test whether key is present using in
Sets and Dictionaries Dictionaries
>>> birthdays['Nightingale']KeyError: 'Nightingale'
Key must be in dictionary before use
>>> 'Nightingale' in birthdaysFalse>>> 'Darwin' in birthdaysTrue
Test whether key is present using in
Sets and Dictionaries Dictionaries
Use for to loop over keys
Sets and Dictionaries Dictionaries
Use for to loop over keysUnlike lists, where for loops over values
Sets and Dictionaries Dictionaries
>>> for name in birthdays:... print name, birthdays[name]
Turing 1912Newton 1642Darwin 1809
Use for to loop over keysUnlike lists, where for loops over values
Sets and Dictionaries Dictionaries
Let's count those birds
Sets and Dictionaries Dictionaries
import sys
if __name__ == '__main__': reader = open(sys.argv[1], 'r') lines = reader.readlines() reader.close() count = count_names(lines) for name in count: print name, count[name]
Let's count those birds
Sets and Dictionaries Dictionaries
import sys
if __name__ == '__main__': reader = open(sys.argv[1], 'r') lines = reader.readlines() reader.close() count = count_names(lines) for name in count: print name, count[name]
Let's count those birds
Read all the data
Sets and Dictionaries Dictionaries
import sys
if __name__ == '__main__': reader = open(sys.argv[1], 'r') lines = reader.readlines() reader.close() count = count_names(lines) for name in count: print name, count[name]
Let's count those birds
Count distinct values
Sets and Dictionaries Dictionaries
import sys
if __name__ == '__main__': reader = open(sys.argv[1], 'r') lines = reader.readlines() reader.close() count = count_names(lines) for name in count: print name, count[name]
Let's count those birds
Show results
Sets and Dictionaries Dictionaries
def count_names(lines): '''Count unique lines of text, returning dictionary.'''
result = {} for name in lines: name = name.strip() if name in result: result[name] = result[name] + 1 else: result[name] = 1
return result
Sets and Dictionaries Dictionaries
def count_names(lines): '''Count unique lines of text, returning dictionary.'''
result = {} for name in lines: name = name.strip() if name in result: result[name] = result[name] + 1 else: result[name] = 1
return result
Explain what we're doingto the next reader
Sets and Dictionaries Dictionaries
def count_names(lines): '''Count unique lines of text, returning dictionary.'''
result = {} for name in lines: name = name.strip() if name in result: result[name] = result[name] + 1 else: result[name] = 1
return result
Create an emptydictionary to fill
Sets and Dictionaries Dictionaries
def count_names(lines): '''Count unique lines of text, returning dictionary.'''
result = {} for name in lines: name = name.strip() if name in result: result[name] = result[name] + 1 else: result[name] = 1
return result
Handle input valuesone at a time
Sets and Dictionaries Dictionaries
def count_names(lines): '''Count unique lines of text, returning dictionary.'''
result = {} for name in lines: name = name.strip() if name in result: result[name] = result[name] + 1 else: result[name] = 1
return result
Clean up beforeprocessing
Sets and Dictionaries Dictionaries
def count_names(lines): '''Count unique lines of text, returning dictionary.'''
result = {} for name in lines: name = name.strip() if name in result: result[name] = result[name] + 1 else: result[name] = 1
return result
If we haveseen this valuebefore…
Sets and Dictionaries Dictionaries
def count_names(lines): '''Count unique lines of text, returning dictionary.'''
result = {} for name in lines: name = name.strip() if name in result: result[name] = result[name] + 1 else: result[name] = 1
return result
…add one toits count
Sets and Dictionaries Dictionaries
def count_names(lines): '''Count unique lines of text, returning dictionary.'''
result = {} for name in lines: name = name.strip() if name in result: result[name] = result[name] + 1 else: result[name] = 1
return result
But if it's the first timewe have seen this name,store it with a count of 1
Sets and Dictionaries Dictionaries
def count_names(lines): '''Count unique lines of text, returning dictionary.'''
result = {} for name in lines: name = name.strip() if name in result: result[name] = result[name] + 1 else: result[name] = 1
return result Return the result
Sets and Dictionaries Dictionaries
Counter in action
Sets and Dictionaries Dictionaries
Counter in actionstart {}
Sets and Dictionaries Dictionaries
Counter in actionstart {}loon {'loon' : 1}
Sets and Dictionaries Dictionaries
Counter in actionstart {}loon {'loon' : 1}goose {'loon' : 1, 'goose' : 1}
Sets and Dictionaries Dictionaries
Counter in actionstart {}loon {'loon' : 1}goose {'loon' : 1, 'goose' : 1}loon {'loon' : 2, 'goose' : 1}
Sets and Dictionaries Dictionaries
Counter in actionstart {}loon {'loon' : 1}goose {'loon' : 1, 'goose' : 1}loon {'loon' : 2, 'goose' : 1}
But like sets, dictionaries are much more efficientthan lookup lists
July 2010
created by
Greg Wilson
Copyright © Software Carpentry 2010This work is licensed under the Creative Commons Attribution LicenseSee http://software-carpentry.org/license.html for more information.
