1. Files and Dictionaries

2. Files >>> f = open( ‘spam.txt‘, ‘r’ )
In Python reading files is no problem…
>>> f = open( ‘spam.txt‘, ‘r’ )
>>> text = f.read()
alternative… >>> text = f.read(50)
>>> f.close()
>>> text
'I like poptarts and 42 and spam.\nWill I ...
>>> word_list = text.split()
[ 'I', 'like', 'poptarts', ... ]
opens the file for reading and calls it f
reads the whole file and calls it f
reads the next 50 bytes from the file
Point out that open is a method on file objects (technically, TextIOStream objects, but stay away from that complexity). And split is a method on string objects.
Might want to demonstrate this on the spam.txt file.
closes the file (optional)
all the text (as one big string)
text.split() returns a list of each "word"

3. Try file_ops.py >>> f = open( ‘greeting.txt‘, ‘w’ )
>>> text = “Hello, world!”
>>> f.write(text)
>>> f.close()
opens the file for writing and calls it f
Writes string text to f
Try file_ops.py

4. Count how many words are in a given file
def wc( filename ):
""" word-counting program """
f = open( filename, “r”)
text = f.read()
f.close()
words = text.split()
print("There are", len(words), "words.")
file handling
word counting
This is in cha6_2.py, but it is the complete function, so you might want to go to the next slide first
Try wc.py

5. Dictionaries In Python a dictionary is a set of key - value pairs.
1999 Feb 16
2000 Feb 4
Rabbit
1998 Jan 28
1999 Feb 15
Tiger
1997 Feb 7
1998 Jan 27 Ox
1996 Feb 19
1997 Feb 6
Rat
Dictionaries
In Python a dictionary is a set of key - value pairs.
>>> d = {}
>>> d[1996] = ‘Rat'
>>> d[1997] = ‘Ox'
>>> d
{1996:‘Rat', 1997:‘Ox'}
>>> d[1996]
‘Rat'
>>> d[1969]
key error
creates an empty dictionary, d
1996 is the key
‘Rat' is the value
1997 is the key
‘Ox' is the value
Curly! And colony!
Retrieve data like lists…
It's a sequence where the index can be any immutable key.

6. More on dictionaries
Dictionaries have lots of built-in methods, or functions:
>>> d = {1996:‘Rat', 1997:‘Ox'}
>>> 1996 in d >>> 1969 in d
True False
>>> len(d) 2
>>> list(d.keys())
[ 1996, 1997 ]
>>> list(d.values())
[ ‘Rat', ‘Ox' ]
>>> list(d.items())
[ (1996, ‘Rat'), (1997, ‘Ok') ]
in checks if a key is present
len returns the # of keys
d.keys returns a list of all keys
d.values returns a list of all keys
d.items returns a list of all key, value pairs

7. """ word-counting program """ f = open( filename ) text = f.read()
What if we wanted the number of different words in the file?
This would be the author's vocabulary size, instead of the total word count.
def wc( filename ):
""" word-counting program """
f = open( filename )
text = f.read()
f.close()
words = text.split()
print("There are", len(words), "words.")
d = {}
for w in words:
if w not in d:
d[w] = 1
else:
d[w] += 1
print ("There are", len(d), "distinct words.\n“)
return len(d) # return the number of distict words
file handling
word counting
Tracking the number of occurences of each word with a dictionary, d.
Demo wc() using spam.txt.

8. Vocabularists?
Shakespeare used 31,534 different words and a grand total of 884,647 words counting repetitions (across his works)
Active vocabulary estimates range from 10,000-60,000.
Passive vocabulary estimates are much higher.
Many Shakespearean contributions:
One contemporary author in the Oxford Eng. Dictionary…
Shakespeare
which word?

9. Vocabularists?
Shakespeare used 31,534 different words and an estimated vocabulary of 66,534 words (across his works)
Average English speaker knows 10,000 to 20,000 words.
Many Shakespearean contributions:
Shakespeare
J. K. Rowling

10. Consider a game of guessing all 50 states…
A state challenge…
Consider a game of guessing all 50 states…
STATES = { 'AL':0, 'AK':0, 'AZ':0, 'AR':0, 'CA':0, 'CO':0, 'CT':0,
'DE':0, 'DC':0, 'FL':0, 'GA':0, 'HI':0, 'ID':0, 'IL':0,
'IN':0, 'IA':0, 'KS':0, 'KY':0, 'LA':0, 'ME':0, 'MD':0,
'MA':0, 'MI':0, 'MN':0, 'MS':0, 'MO':0, 'MT':0, 'NE':0,
'NV':0, 'NH':0, 'NJ':0, 'NM':0, 'NY':0, 'NC':0, 'ND':0,
'OH':0, 'OK':0, 'OR':0, 'PA':0, 'RI':0, 'SC':0, 'SD':0,
'TN':0, 'TX':0, 'UT':0, 'VT':0, 'VA':0, 'WA':0, 'WV':0,
'WI':0, 'WY':0 }

11. A dictionary challenge…
state_dict = { 'AL': 0, ‘AK’: 0,… }
All US state abbreviations are in this dictionary…
key
value
def state_challenge( state_dict ):
while 0 in state_dict.values():
print(list(state_dict.values()).count(0), \
"states left to guess")
guess = input("Name a state: ")
if guess not in state_dict:
print 'Try again...'
elif state_dict[guess] == 0:
print 'Yes!'
state_dict[guess] += 1
else:
print('Already guessed...')
print('Phew!')
Tell user how many
left to guess
wasn't a key at all
Run stateChallenge in the script…
newly guessed key: value was 0
repeated guess

12. File exercise

13. Write a function remove_duplicates(somelist) that removes duplicates from the list. Example, alist = [‘hello’, ‘world’, ‘hello’] >>> remove_duplicates(alist) [‘hello’, ‘world’] >>> alist = [‘how’, ‘are’, ‘you’] >>> remove_duplicates[alist) [‘how’, ‘are’, ‘you’]