This document discusses dictionaries in Python. It describes dictionaries as mappings between keys and values where keys can be any immutable data type and values can be any data type. It covers creating dictionaries using the dict() function and literal notation, accessing values using keys or getting default values, and various dictionary methods like keys(), values(), and items(). Examples demonstrate using dictionaries to count word frequencies and invert a dictionary to map values to a list of keys. The document also discusses using tuples as dictionary keys.

1.  Dictionary overview Creating
 Dictionaries
 Dictionary operations
 Dictionaries Data structure
Python
Dictionaries
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2. objectives
1. Describe the characteristics of the
dictionary data structure in Python
2. Perform basic operations with
dictionaries including creation,
copying, updating, and traversing
3. Use dictionaries in functions
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3. The dictionary data structure
In Python, a dictionary is
mapping between a set of indices
(keys) and a set of values
The items in a dictionary are key-
value pairs
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4. The dictionary data structure
Keys can be any Python data
type
Because keys are used for indexing,
they should be immutable
Values can be any Python data
type
Values can be mutable or
immutable
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5. Creating a dictionary
eng2sp = dict()
print eng2sp
eng2sp['one'] = 'uno'
print eng2sp
eng2sp['two'] = 'dos'
print eng2sp
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6. Creating a dictionary
eng2sp = {'one': 'uno', 'two': 'dos',
'three': 'tres'}
print eng2sp
• In general, the order of items in a
dictionary is unpredictable
• Dictionaries are indexed by keys, not
integers
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7. Dictionary indexing
print eng2sp['three']
print eng2sp['five']
* If the index is not a key in the
dictionary, Python raises an exception
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8. Dictionary indexing
if 'five' in eng2sp:
print eng2sp['five']
print eng2sp.get('five')
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9. The in operator
• Note that the in operator works differently
for dictionaries than for other sequences
• For offset indexed sequences (strings, lists,
tuples), x in y checks to see whether x is an
item in the sequence
• For dictionaries, x in y checks to see whether
x is a key in the dictionary
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10. Keys and values
The keys method returns a list of the
keys in a dictionary
print eng2sp.keys()
The values method returns a list of
the values
print eng2sp.values()
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11. Keys and values
The items method returns a list of
tuple pairs of the key-value pairs in a
dictionary
print eng2sp.items()
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12. Example: histogram.py
def histogram(seq):
d = dict()
for element in seq:
if element not in d:
d[element] = 1
else:
d[element] += 1
return d
h = histogram('brontosaurus')
print h
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13. Example: histogram2.py
Add the following code to histogram.py:
def print_hist(hist):
for key in hist:
print key, hist[key]
h = histogram('brontosaurus')
print_hist(h)
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14. Example: histogram2.py
Change the print_hist function:
def print_hist(hist):
for key, value in hist:
print key, value
h = histogram('brontosaurus')
print_hist(h)
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15. Sorting the keys
Change the print_hist function:
def print_hist(hist):
keys = hist.keys()
keys.sort()
for key in keys:
print key, hist[key]
h = histogram('brontosaurus')
print_hist(h)
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16. Using lists as values: invert.py
Add the following code to histogram.py:
def invert_dict(d):
inv = dict()
for key in d:
val = d[key]
if val not in inv:
inv[val] = [key]
else:
inv[val].append(key)
return inv
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17. Using lists as values: invert.py
Add the following code to histogram.py:
hist = histogram('parrot')
print hist
inverted = invert_dict(hist)
print inverted
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18. Using tuples as keys: troupe.py
troupe = {('Cleese', 'John'): [1,2,3],
('Chapman', 'Graham'): [4,5,6],
('Idle', 'Eric'): [7,8,9],
('Jones', 'Terry'): [10,11,12],
('Gilliam', 'Terry'): [13,14,15,16,17,18],
('Palin', 'Michael'): [19,20]}
for last, first in troupe:
print first, last, troupe[last, first]
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19. Thank You…
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