7.1 What is a dictionary?

>>> x = []        # create a new, empty list and assign it to x.
>>> y = {}        # create a new, empty DICTIONARY, and assign it to y.
>>> y[0] = 'Hello'
>>> y[1] = 'Goodbye'
>>> x[0] = 'Hello'
Traceback (innermost last):
 File "<stdin>", line 1, in ?
IndexError: list assignment index out of range

>>> print(y[0])
Hello
>>> y[1] + ", Friend."
'Goodbye, Friend.'

>>> y["two"] = 2
>>> y["pi"] = 3.14
>>> y["two"] * y["pi"]
6.2800000000000002


7.1.1 

>>> english_to_french = {}            # Create an empty dictionary. 
>>> english_to_french['red'] = 'rouge'    # Store three words in it. 
>>> english_to_french['blue'] = 'bleu'
>>> english_to_french['green'] = 'vert'
>>> print("red is", english_to_french['red']) # Obtain the value for 'red'.
red is rouge


7.2 Other operations

>>> english_to_french = {'red':'rouge','blue':'bleu','green':'vert'}
>>> len(english_to_french)
3

>>> list(english_to_french.keys())
['green', 'blue', 'red']

>>> list(english_to_french.values())
 ['vert', 'bleu', 'rouge']

>>> list(english_to_french.items())
[('green', 'vert'), ('blue', 'bleu'), ('red', 'rouge')]

>>> list(english_to_french.items())
[('green', 'vert'), ('blue', 'bleu'), ('red', 'rouge')]

>>> del english_to_french['green']
>>> list(english_to_french.items())
 [('blue', 'bleu'), ('red', 'rouge')]

>>> 'red' in english_to_french
True
>>> 'orange' in english_to_french
False

>>> print(english_to_french.get('blue', 'No translation'))
bleu
>>> print(english_to_french.get('chartreuse', 'No translation'))
No translation

>>> print(english_to_french.setdefault('chartreuse', 'No translation'))
No translation

>>> x = {0 :'zero', 1 :'one'}
>>> y = x.copy()
>>> y
{0: 'zero', 1: 'one'}

>>> z = { 1: 'One', 2: 'Two'}
>>> x = {0 :'zero', 1 :'one'}
>>> x.update(z)
>>> x
{0: 'zero', 1: 'One', 2: 'Two'}



7.3 Word Counting

>>> sample_string = "To be or not to be"
>>> occurrences = {}
>>> for word in sample_string.split():
...     occurrences[word] = occurrences.get(word, 0) + 1
>>> for word in occurrences:
...     print("The word", word, "occurs", occurrences[word], \
...            "time(s) in the string")


7.5 Sparse matrices

matrix = {(0, 0):3, (0, 2):-2, (0, 3):11, 
        (1, 1):9, (2, 1):7, (3, 3):-5}

if (rownum, colnum) in matrix:
    element = matrix[(rownum, colnum)]
else:
    element = 0


7.6 Dictionaries as caches

sole_cache = {}
def sole(m, n, t):
    if (m, n, t) in sole_cache:
        return sole_cache[(m, n, t)]
    else:
        # . . . do some time-consuming calculations . . .
        sole_cache[(m, n, t)] = result
        return result