Notes of Py for informatics 2

Chapter 7 Files

File Processing

www.py4inf.com/code/mbox-short.txt

open() returns a "file handle" - a variable used to perform operations on the file

handle is a connection. doesn't actually have data

kind of like "File -> Open" in a Word Processor

handle = open(filename, mode) #filename is a string
fhand = open('mbox.txt','r') #'r':reading,'w':writing

The newline Character

stuff = 'Hello\nWorld'  #\n means new line, and it is counted as a character
print stuff
print len(stuff)
output:

Hello
World
11

Counting Lines in a File

xfile = open('/Users/huyifan/documents/mbox.txt')
count = 0
for line in xfile:
    count = count + 1

print 'Line Count:',count
important! A file handle ope for read can be treated as a sequence of strings where each line in the file is a string in the sequence

Reading the *Whole* File

Use read() to read the whole file (newlines and all) into a single string.

xfile = open('/Users/huyifan/documents/mbox.txt')
inp = xfile.read()
print len(inp)
print inp[:20]
output:

94625
From stephen.marquar

Searching Through a File
for line in xfile:
    if line.startswith('Author'):
        print line
output:

Author: stephen.marquard@uct.ac.za
                                     #why there are black lines? because of \n !
Author: louis@media.berkeley.edu

Author: zqian@umich.edu

Author: rjlowe@iupui.edu

Author: zqian@umich.edu

Author: rjlowe@iupui.edu
We can use the rstrip() or lstrip().

for line in xfile:
    if line.startswith('A'):
        print line.rstrip()

output:

Author: david.horwitz@uct.ac.za
Author: stephen.marquard@uct.ac.za
Author: louis@media.berkeley.edu
Author: louis@media.berkeley.edu
Author: ray@media.berkeley.edu
Author: cwen@iupui.edu
Author: cwen@iupui.edu
Author: cwen@iupui.edu

Skipping with continue

for line in xfile:
    if not line.startswith('Author'):  #or: if not '@media.berkeley.edu' in line:
        continue   #process our 'interesting lines'
    print line.rstrip()
The output will be the same.

Bad File Names

fname = raw_input('Enter the file name:')
try:
    fhand = open(fname)
except:
    print 'File cannot be openned:', fname
    exit()
count = 0
for line in fhand:
    if line.startswith('Author:'):
        count = count + 1
print 'There were', count, 'pieces of important information'

Chapter 8 Lists
8.1 basic understanding and operations
A list is a kind of collection.

A collection allows us to put many values in a single "variable"

like: 
friends = ['Joe','Joey','Joseph']

A list element can be any Python object - even another list

print [[1,2],3,4]

A list can be empty

print []

Strings are "immutable" - we cannot change change the contents of a string - we must make a new string to make any change

Lists are "mutable" - we can change an element of a list using the index operator

len()

print len([[1,2],3,4])

Using the range function: generate a list

print range(4)  # often be used as: for i in range(len(friends))    friend[i]
output:

[0, 1, 2, 3]

Concatenating list using +

print range(4)+[2,3]
output:

[0, 1, 2, 3, 2, 3]

Slice lists: like 'strings'

8.2 Build-in Functions

Building a list from scratch: xxx.append()

stuff = list()
stuff.append('book')
stuff.append(99)
print stuff

output:

['book', 99]

xxx in xxxx?

Order the list

friends = ['Joey','Joe','Joseph','Mike','Ellen']
friends.sort()
print friends

output:

['Ellen', 'Joe', 'Joey', 'Joseph', 'Mike']   #important: it has changed itself!

The sort method (unlike in strings) means "sort yourself"

other useful build-in functions

len(numbers), max(numbers), min(numbers), sum(numbers)

Strings -> Lists: Use split()/split(';')/... to break a string into parts and produces a list of strings. (leave out all the comma/space..)

abc = 'With three words'
stuff = abc.split()
print stuff
print len(stuff)

output:

['With', 'three', 'words']
3

abc = 'With;three;words'
stuff1 = abc.split()
stuff2 = abc.split(';')   #important!
print stuff1
print stuff2
print len(stuff1)
print len(stuff2)

output:

['With;three;words']
['With', 'three', 'words']
1
3