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# 'Dictionary' in Python
# type 'dict'

# In computer science, a dictionary is a data structure that allows
# us to 'look up' values using names rather than index numbers like
# we did in lists.

# We make a Python dictionary using curly brackets {}
d = {'a': 4, 'b': 5, 'c':6}
d
{'a': 4, 'b': 5, 'c': 6}
type(d)
<class 'dict'>
d['a']
4
d['b']
5

# Dictionaries have 'keys' and 'values'
d.keys()
dict_keys(['a', 'b', 'c'])
d.values()
dict_values([4, 5, 6])

# Each key in the dictionary has a corresponding value
# The value at the key 'a' in d is 4

# We can say that we 'map keys to values'
# A dict is a collection of key-value pairs

d = {'a': 4, 'b': 5, 'c':6, 'a': 9}
d['a']
9
d
{'a': 9, 'b': 5, 'c': 6}
# IMPORTANT: Keys should be unique.
d = {'a': 4, 'b': 5, 'c':4}
d
{'a': 4, 'b': 5, 'c': 4}
# values don't have to be unique
# but key do since we use them to lookup values

d = {1: 4, 'b': 5, 'c':4}
d[1]
4
d[2]
Traceback (most recent call last):
  Python Shell, prompt 29, line 1
builtins.KeyError: 2
# if we don't find a key, we get a KeyError

d = {1: 4, 'b': 1, 'c':4}
d['b']
1
d[1]
4
# we can only look-up with keys. can't put values in square brackets

# Dictionaries don't have a predictable order like lists do
d.keys()
dict_keys([1, 'b', 'c'])
list(d.keys())
[1, 'b', 'c']
# This order usually doesn't matter when you're working with dicts

d = {1: 4, 'b': ['hello'], 'c':6}
d['b']
['hello']

d = {['csc120']: 4, 'b': ['hello'], 'c':6}
Traceback (most recent call last):
  Python Shell, prompt 41, line 1
builtins.TypeError: unhashable type: 'list'
# the word 'hash' comes from the CS term 'hash table' which
# is basically the same as a dictionary
# This error means that we can't use a list as a key

# Using mutable types (like lists) for keys is a bad idea
# since we want to ensure that the keys that we use for looking up
# values don't change
# So Python will just not let you do it - will give you an error

# Ensure keys are immutable.
# Values can be mutable

d
{1: 4, 'b': ['hello'], 'c': 6}
id(d)
4532391800

# we can change values in place
d['c']
6
d['c'] = True
d
{1: 4, 'b': ['hello'], 'c': True}
id(d)
4532391800
# dicts themselves are mutable
id(d['b'])
4534135432
d['b'].append('csc120')
d
{1: 4, 'b': ['hello', 'csc120'], 'c': True}
id(d['b'])
4534135432
# dictionary values that are mutable will continue to be mutable 

d
{1: 4, 'b': ['hello', 'csc120'], 'c': True}
4 in d
False
1 in d
True
# using 'in' looks through the keys of d

4 in d.values()
True
d.values()
dict_values([4, ['hello', 'csc120'], True])
list(d.values())
[4, ['hello', 'csc120'], True]

# To add a key to a dict, simply add it in place
d['e'] = 87
d
{1: 4, 'b': ['hello', 'csc120'], 'c': True, 'e': 87}