Transcript for today.

# IPython log file

# create an empty dictionary
d = {}

# create a dictionary with 3 key:value pairs
d = { 1:"hi there", 2:"ho ho", 42:"the answer"}
d
#[Out]# {1: 'hi there', 2: 'ho ho', 42: 'the answer'}

# ask for the value associated with the key 1
d[1]
#[Out]# 'hi there'
# or key 42
d[42]
#[Out]# 'the answer'
# but we get an error if we ask for key 3 which isn't present
d[3]

# we can test to see if a key is present
42 in d
#[Out]# True
3 in d
#[Out]# False

# btw, in also works for lists. But in the case of lists it has to compare to each item
# for a dictionary it takes constant time regardless of the number of entries
L = [ 1, 2, 42 ]
1 in L
#[Out]# True
3 in L
#[Out]# False

# we can get just the keys
d.keys()
#[Out]# [1, 2, 42]

# we can add a new key
d[17] = True
d
#[Out]# {1: 'hi there', 2: 'ho ho', 17: True, 42: 'the answer'}
d.keys()
#[Out]# [1, 2, 42, 17]

# we can get just the values
d.values()
#[Out]# ['hi there', 'ho ho', 'the answer', True]

# we can get both the keys and the values
d.items()
#[Out]# [(1, 'hi there'), (2, 'ho ho'), (42, 'the answer'), (17, True)]

# keys don't have to be numbers
d['gary'] = 58
d.keys()
#[Out]# [1, 2, 'gary', 42, 17]

# we can print the contents in sorted order by sorting the keys
for key in sorted(d.keys()):
    print key, d[key]

# at this point we wrote a function to count items using a dictionalry
# I'll include the source here but we wrote it in the editor and used run to load it.
def count(input):
    d = {}  # initialize an empty dictionary
    for i in input:  # iterate over the items in the input
        if i not in d:  # if the item is not in the dictionary
            d[i] = 0    #    add it with a count of 0
        d[i] = d[i] + 1  # add 1 to the number of times we've seen this item
    return d  # return the dictionary

# use it to count the occurances of letters in a string
count('now is the time for all good men')
#[Out]# {' ': 7,
#[Out]#  'a': 1,
#[Out]#  'd': 1,
#[Out]#  'e': 3,
#[Out]#  'f': 1,
#[Out]#  'g': 1,
#[Out]#  'h': 1,
#[Out]#  'i': 2,
#[Out]#  'l': 2,
#[Out]#  'm': 2,
#[Out]#  'n': 2,
#[Out]#  'o': 4,
#[Out]#  'r': 1,
#[Out]#  's': 1,
#[Out]#  't': 2,
#[Out]#  'w': 1}

# we can use split on a string to break it into a list of words
s = 'now is the time for all good men'
s.split()
#[Out]# ['now', 'is', 'the', 'time', 'for', 'all', 'good', 'men']

# we can count integers, for example we could roll a die
r = np.random.random_integers(1, 6, 1000)
r
#[Out]# array([3, 2, 6, 2, 4, 1, 4, 3, 4, 2, 1, 6, 6, 6, 3, 4, 1, 2, 6, 3, 1, 3, 4,
#[Out]#        6, 3, 1, 1, 5, 6, 5, 4, 6, 1, 2, 6, 2, 2, 4, 2, 6, 3, 2, 3, 2, 5, 6,
#[Out]#        5, 4, 6, 3, 2, 5, 1, 4, 3, 1, 3, 3, 2, 6, 5, 4, 2, 6, 5, 3, 5, 5, 4,
#[Out]#        2, 3, 1, 5, 5, 3, 4, 2, 3, 2, 5, 2, 3, 2, 2, 5, 4, 3, 1, 4, 4, 4, 1,
#[Out]#        1, 1, 3, 4, 3, 5, 3, 4, 5, 3, 4, 5, 5, 6, 1, 6, 5, 2, 4, 3, 4, 5, 5,
#[Out]#        1, 6, 5, 2, 1, 3, 6, 1, 4, 4, 2, 3, 6, 4, 3, 4, 2, 3, 1, 1, 1, 4, 5,
#[Out]#        3, 3, 6, 1, 5, 2, 3, 3, 5, 1, 2, 5, 5, 4, 6, 3, 3, 3, 6, 5, 5, 1, 1,
#[Out]#        3, 5, 2, 2, 4, 5, 3, 6, 3, 2, 6, 5, 6, 3, 6, 6, 2, 3, 2, 2, 5, 5, 6,
#[Out]#        3, 4, 1, 1, 3, 6, 4, 6, 3, 6, 4, 2, 4, 1, 3, 2, 6, 6, 3, 6, 1, 1, 4,
#[Out]#        2, 1, 4, 1, 4, 1, 6, 5, 1, 5, 5, 3, 6, 1, 2, 4, 3, 3, 2, 3, 6, 2, 6,
#[Out]#        3, 5, 6, 1, 1, 5, 5, 1, 2, 6, 3, 3, 3, 2, 1, 3, 5, 6, 1, 4, 3, 4, 5,
#[Out]#        3, 5, 6, 5, 5, 4, 4, 3, 2, 3, 2, 4, 3, 3, 1, 4, 1, 2, 5, 1, 4, 3, 1,
#[Out]#        2, 2, 6, 3, 4, 2, 1, 3, 2, 6, 4, 2, 5, 1, 3, 4, 5, 2, 1, 4, 5, 5, 1,
#[Out]#        3, 4, 3, 2, 6, 4, 5, 2, 6, 2, 1, 4, 4, 2, 6, 1, 4, 3, 1, 5, 4, 6, 4,
#[Out]#        5, 4, 4, 6, 1, 5, 5, 1, 5, 6, 6, 5, 1, 3, 5, 2, 2, 6, 2, 5, 5, 5, 3,
#[Out]#        4, 4, 5, 4, 2, 5, 1, 1, 4, 4, 1, 2, 2, 6, 1, 2, 3, 3, 4, 2, 1, 5, 1,
#[Out]#        3, 2, 2, 5, 4, 2, 5, 6, 2, 3, 6, 6, 1, 2, 3, 3, 4, 4, 1, 3, 1, 1, 3,
#[Out]#        6, 6, 2, 6, 3, 5, 6, 2, 1, 1, 3, 5, 3, 5, 4, 3, 5, 3, 2, 6, 4, 4, 6,
#[Out]#        4, 5, 4, 3, 5, 3, 5, 1, 1, 4, 1, 1, 6, 5, 2, 4, 6, 4, 3, 4, 6, 2, 5,
#[Out]#        6, 3, 5, 6, 3, 3, 3, 5, 4, 5, 3, 2, 2, 2, 2, 4, 2, 2, 4, 3, 6, 5, 4,
#[Out]#        2, 6, 4, 1, 1, 6, 2, 3, 4, 1, 5, 4, 5, 3, 3, 3, 4, 3, 2, 6, 6, 3, 2,
#[Out]#        6, 6, 1, 6, 1, 2, 1, 4, 3, 1, 6, 6, 4, 1, 3, 3, 3, 5, 1, 6, 6, 5, 6,
#[Out]#        6, 2, 4, 5, 5, 6, 1, 6, 4, 3, 5, 4, 6, 5, 1, 1, 4, 2, 1, 6, 3, 4, 3,
#[Out]#        3, 2, 3, 5, 5, 3, 6, 5, 6, 2, 4, 5, 3, 6, 6, 6, 5, 3, 4, 2, 2, 5, 1,
#[Out]#        4, 4, 6, 6, 6, 3, 5, 5, 2, 4, 1, 6, 6, 6, 5, 4, 2, 3, 6, 6, 4, 4, 2,
#[Out]#        4, 1, 4, 1, 5, 2, 2, 6, 6, 4, 1, 4, 4, 4, 4, 4, 4, 2, 4, 6, 6, 1, 3,
#[Out]#        3, 1, 3, 6, 4, 2, 2, 5, 3, 6, 2, 6, 4, 4, 3, 3, 3, 3, 3, 4, 1, 2, 6,
#[Out]#        6, 3, 3, 4, 5, 6, 1, 6, 6, 1, 3, 2, 1, 3, 1, 2, 3, 1, 6, 6, 3, 5, 1,
#[Out]#        5, 1, 1, 6, 5, 1, 6, 6, 2, 3, 1, 6, 1, 1, 4, 3, 4, 2, 1, 6, 4, 4, 1,
#[Out]#        6, 4, 3, 4, 5, 6, 2, 1, 4, 2, 3, 1, 2, 2, 3, 2, 5, 1, 3, 1, 5, 5, 6,
#[Out]#        2, 1, 1, 5, 4, 4, 4, 2, 6, 1, 5, 5, 1, 2, 5, 3, 4, 2, 5, 1, 6, 5, 5,
#[Out]#        4, 3, 2, 3, 2, 3, 5, 5, 6, 5, 2, 4, 1, 6, 3, 1, 2, 5, 2, 1, 3, 1, 3,
#[Out]#        4, 3, 2, 6, 4, 1, 6, 4, 6, 4, 1, 4, 1, 6, 4, 4, 4, 3, 6, 4, 1, 5, 4,
#[Out]#        1, 5, 3, 5, 2, 1, 1, 1, 1, 5, 1, 2, 3, 2, 6, 2, 6, 1, 4, 4, 1, 5, 1,
#[Out]#        2, 5, 5, 2, 1, 6, 4, 5, 5, 6, 3, 1, 3, 3, 6, 1, 3, 4, 5, 4, 3, 2, 2,
#[Out]#        6, 2, 5, 4, 6, 5, 2, 3, 6, 3, 4, 2, 3, 5, 5, 1, 3, 1, 5, 5, 6, 1, 4,
#[Out]#        4, 6, 3, 1, 5, 6, 6, 6, 6, 3, 4, 5, 1, 6, 3, 2, 4, 5, 6, 2, 1, 1, 5,
#[Out]#        4, 4, 5, 4, 2, 3, 1, 1, 3, 4, 5, 1, 6, 4, 4, 6, 5, 2, 6, 5, 2, 5, 3,
#[Out]#        5, 6, 2, 6, 5, 3, 4, 3, 1, 1, 6, 3, 4, 2, 2, 1, 5, 3, 5, 4, 4, 3, 3,
#[Out]#        2, 6, 6, 4, 4, 1, 6, 3, 5, 1, 5, 6, 1, 6, 4, 5, 3, 1, 2, 2, 1, 1, 1,
#[Out]#        3, 3, 1, 6, 2, 6, 3, 1, 1, 5, 6, 3, 5, 2, 3, 4, 4, 2, 1, 2, 2, 6, 5,
#[Out]#        3, 4, 2, 6, 4, 5, 1, 1, 4, 5, 4, 6, 3, 4, 2, 5, 5, 6, 6, 3, 2, 1, 2,
#[Out]#        2, 6, 3, 5, 3, 1, 4, 4, 5, 6, 3, 1, 2, 2, 3, 5, 1, 6, 2, 1, 3, 4, 1,
#[Out]#        5, 1, 2, 3, 1, 4, 5, 1, 3, 6, 4])
count(r)
#[Out]# {1: 169, 2: 152, 3: 180, 4: 171, 5: 160, 6: 168}

# remember our original dictionary?
d
#[Out]# {1: 'hi there', 2: 'ho ho', 17: True, 42: 'the answer', 'gary': 58}

# we can delete a key
del d['gary']

# now gary is gone.
d
#[Out]# {1: 'hi there', 2: 'ho ho', 17: True, 42: 'the answer'}