Sets and Dictionaries Examples Copyright © Software Carpentry 2010 This work is licensed under the Creative Commons Attribution License See http://software-carpentry.org/license.html for more information.

How early was each kind of bird seen? 2

How early was each kind of bird seen? 2010-07-03 05:38 loon 2010-07-03 06:02 goose 2010-07-03 06:07 loon 2010-07-04 05:09 ostrich # hallucinating? 2010-07-04 05:29 loon ⋮ ⋮ ⋮ 3

How early was each kind of bird seen? 2010-07-03 05:38 loon 2010-07-03 06:02 goose 2010-07-03 06:07 loon 2010-07-04 05:09 ostrich # hallucinating? 2010-07-04 05:29 loon ⋮ ⋮ ⋮ Want the minimum of all times associated with a bird 4

How early was each kind of bird seen? 2010-07-03 05:38 loon 2010-07-03 06:02 goose 2010-07-03 06:07 loon 2010-07-04 05:09 ostrich # hallucinating? 2010-07-04 05:29 loon ⋮ ⋮ ⋮ Want the minimum of all times associated with a bird Use bird name as dictionary key 5

How early was each kind of bird seen? 2010-07-03 05:38 loon 2010-07-03 06:02 goose 2010-07-03 06:07 loon 2010-07-04 05:09 ostrich # hallucinating? 2010-07-04 05:29 loon ⋮ ⋮ ⋮ Want the minimum of all times associated with a bird Use bird name as dictionary key And earliest observation time as value 6

def read_observations(filename): '''Read data, return [(date, time, bird)...].''' reader = open(filename, 'r') result = [] for line in reader: fields = line.split('#')[0].strip().split() assert len(fields) == 3, 'Bad line "%s"' % line result.append(fields) return result 7

Setup def read_observations(filename): '''Read data, return [(date, time, bird)...].''' reader = open(filename, 'r') result = [] for line in reader: fields = line.split('#')[0].strip().split() assert len(fields) == 3, 'Bad line "%s"' % line result.append(fields) return result Setup 8

Get data from each line of the file def read_observations(filename): '''Read data, return [(date, time, bird)...].''' reader = open(filename, 'r') result = [] for line in reader: fields = line.split('#')[0].strip().split() assert len(fields) == 3, 'Bad line "%s"' % line result.append(fields) return result Get data from each line of the file 9

Check that the data might be right def read_observations(filename): '''Read data, return [(date, time, bird)...].''' reader = open(filename, 'r') result = [] for line in reader: fields = line.split('#')[0].strip().split() assert len(fields) == 3, 'Bad line "%s"' % line result.append(fields) return result Check that the data might be right 10

Store it def read_observations(filename): '''Read data, return [(date, time, bird)...].''' reader = open(filename, 'r') result = [] for line in reader: fields = line.split('#')[0].strip().split() assert len(fields) == 3, 'Bad line "%s"' % line result.append(fields) return result Store it 11

def earliest_observation(data): '''How early did we see each bird?''' result = {} for (date, time, bird) in data: if bird not in result: result[bird] = time else: result[bird] = min(result[bird], time) return result 12

Setup def earliest_observation(data): '''How early did we see each bird?''' result = {} for (date, time, bird) in data: if bird not in result: result[bird] = time else: result[bird] = min(result[bird], time) return result Setup 13

Process each tuple in turn def earliest_observation(data): '''How early did we see each bird?''' result = {} for (date, time, bird) in data: if bird not in result: result[bird] = time else: result[bird] = min(result[bird], time) return result Process each tuple in turn 14

First sighting, so this must be earliest time def earliest_observation(data): '''How early did we see each bird?''' result = {} for (date, time, bird) in data: if bird not in result: result[bird] = time else: result[bird] = min(result[bird], time) return result First sighting, so this must be earliest time 15

Subsequent sighting, so take minimum def earliest_observation(data): '''How early did we see each bird?''' result = {} for (date, time, bird) in data: if bird not in result: result[bird] = time else: result[bird] = min(result[bird], time) return result Subsequent sighting, so take minimum 16

What birds were seen on each day?

What birds were seen on each day? Very similar structure...

What birds were seen on each day? Very similar structure... ...but use a set to record one or more birds, rather than taking the minimum time

def birds_by_date(data): '''Which birds were seen on each day?''' result = {} for (date, time, bird) in data: if date not in result: result[date] = set() result[date].add(bird) return result 20

Setup def birds_by_date(data): '''Which birds were seen on each day?''' result = {} for (date, time, bird) in data: if date not in result: result[date] = set() result[date].add(bird) return result Setup 21

Process each tuple in turn def birds_by_date(data): '''Which birds were seen on each day?''' result = {} for (date, time, bird) in data: if date not in result: result[date] = set() result[date].add(bird) return result Process each tuple in turn 22

Make sure there is space to record this bird def birds_by_date(data): '''Which birds were seen on each day?''' result = {} for (date, time, bird) in data: if date not in result: result[date] = set() result[date].add(bird) return result Make sure there is space to record this bird 23

Record this bird def birds_by_date(data): '''Which birds were seen on each day?''' result = {} for (date, time, bird) in data: if date not in result: result[date] = set() result[date].add(bird) return result Record this bird 24

2010-07-03 05:38 loon '2010-07-03' 'loon'

2010-07-03 05:38 loon 2010-07-03 06:02 goose '2010-07-03' 'goose' 'loon'

2010-07-03 05:38 loon 2010-07-03 06:02 goose 2010-07-03 06:07 loon '2010-07-03' 'goose' 'loon'

2010-07-03 05:38 loon 2010-07-03 06:02 goose 2010-07-03 06:07 loon 2010-07-04 05:09 ostrich # hallucinating? '2010-07-03' '2010-07-04' 'goose' 'ostrich' 'loon'

2010-07-03 05:38 loon 2010-07-03 06:02 goose 2010-07-03 06:07 loon 2010-07-04 05:09 ostrich # hallucinating? 2010-07-04 05:29 loon '2010-07-03' '2010-07-04' 'loon' 'goose' 'ostrich' 'loon'

Which bird did we see least frequently? 31

Which bird did we see least frequently? Actually, which birds, since two or more could be tied for the low score 32

Which bird did we see least frequently? Actually, which birds, since two or more could be tied for the low score Two-pass algorithm 33

Which bird did we see least frequently? Actually, which birds, since two or more could be tied for the low score Two-pass algorithm - Find the minimum value in the dictionary 34

Which bird did we see least frequently? Actually, which birds, since two or more could be tied for the low score Two-pass algorithm - Find the minimum value in the dictionary - Find all keys with that value 35

Which bird did we see least frequently? Actually, which birds, since two or more could be tied for the low score Two-pass algorithm - Find the minimum value in the dictionary - Find all keys with that value Combine these calculations in a one-pass algorithm 36

Which bird did we see least frequently? Actually, which birds, since two or more could be tied for the low score Two-pass algorithm - Find the minimum value in the dictionary - Find all keys with that value Combine these calculations in a one-pass algorithm Assume we already have a dictionary counts recording how often each kind of bird was seen 37

def least_frequently_seen(counts): '''Which bird(s) were least frequently seen?''' result = set() number = 0 for bird in counts: …handle this bird… return result 38

def least_frequently_seen(counts): '''Which bird(s) were least frequently seen?''' result = set() number = 0 for bird in counts: …handle this bird… return result if len(result) == 0: result = {bird} number = counts[bird] elif counts[bird] < number: elif counts[bird] == number: result.add(bird) 39

Case 1: first bird (initializing data structures) def least_frequently_seen(counts): '''Which bird(s) were least frequently seen?''' result = set() number = 0 for bird in counts: …handle this bird… return result if len(result) == 0: result = {bird} number = counts[bird] elif counts[bird] < number: elif counts[bird] == number: result.add(bird) Case 1: first bird (initializing data structures) 40

Case 2: new minimum, so replace everything def least_frequently_seen(counts): '''Which bird(s) were least frequently seen?''' result = set() number = 0 for bird in counts: …handle this bird… return result if len(result) == 0: result = {bird} number = counts[bird] elif counts[bird] < number: elif counts[bird] == number: result.add(bird) Case 2: new minimum, so replace everything 41

Case 3: tied equal for minimum def least_frequently_seen(counts): '''Which bird(s) were least frequently seen?''' result = set() number = 0 for bird in counts: …handle this bird… return result if len(result) == 0: result = {bird} number = counts[bird] elif counts[bird] < number: elif counts[bird] == number: result.add(bird) Case 3: tied equal for minimum 42

Before the loop { 'loon' : 3, 'goose' : 1, 'ostrich' : 1 } number result Before the loop

Case 1: first bird (initializing data structures) { 'loon' : 3, 'goose' : 1, 'ostrich' : 1 } number 3 'loon' result Case 1: first bird (initializing data structures)

Case 2: new minimum, so replace everything { 'loon' : 3, 'goose' : 1, 'ostrich' : 1 } number 1 result 'goose' Case 2: new minimum, so replace everything

Case 3: tied equal for minimum { 'loon' : 3, 'goose' : 1, 'ostrich' : 1 } number 1 'ostrich' result 'goose' Case 3: tied equal for minimum

Greg Wilson created by July 2010 Copyright © Software Carpentry 2010 This work is licensed under the Creative Commons Attribution License See http://software-carpentry.org/license.html for more information.