1. Sets and Dictionaries Examples Copyright © Software Carpentry 2010
This work is licensed under the Creative Commons Attribution License
See for more information.

2. How early was each kind of bird seen?2

3. How early was each kind of bird seen?
:38 loon
:02 goose
:07 loon
:09 ostrich # hallucinating?
:29 loon
⋮ ⋮ ⋮ 3

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

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

6. How early was each kind of bird seen?
:38 loon
:02 goose
:07 loon
:09 ostrich # hallucinating?
: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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. What birds were seen on each day?

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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

26. :38 loon
' '
'loon'

27. :38 loon
:02 goose
' '
'goose'
'loon'

28. :38 loon
:02 goose
:07 loon
' '
'goose'
'loon'

29. :38 loon
:02 goose
:07 loon
:09 ostrich # hallucinating?
' '
' '
'goose'
'ostrich'
'loon'

30. :38 loon
:02 goose
:07 loon
:09 ostrich # hallucinating?
:29 loon
' '
' '
'loon'
'goose'
'ostrich'
'loon'

31. Which bird did we see least frequently?31

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

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 33

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 34

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 35

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 36

37. 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

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 38

39. 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

40. 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

41. 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

42. 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

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

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

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

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

47. Greg Wilson created by July 2010 Copyright © Software Carpentry 2010
This work is licensed under the Creative Commons Attribution License
See for more information.