Python小白的学习之路 Day3

还有一个月要参加美赛了，进度所以会提一些，留时间去学其他的
Loops and Iteration (Chapter 5）
Strings （Chapter 6）
Reading Files (Chapter 7)

Loops and Iteration (Chapter 5）

Repeated Steps

Loops (repeated steps) have iteration variables that change each time through a loop. Often these iteration variables go through a sequence of numbers.
Example:

n = 5
while n > 0 :print(n)n = n – 1
print('Blastoff!')
print(n)

Breaking Out of a Loop

The break statement ends the current loop and jumps to the statement immediately following the loop

while True:line = input('> ')if line == 'done' :breakprint(line)
print('Done!')

Finishing an Iteration with continue

The continue statement ends the current iteration and jumps to the top of the loop and starts the next iteration

Indefinite Loops

While loops are called “indefinite loops” because they keep going until a logical condition becomes False

Definite Loops

Definite loops (for loops) have explicit iteration variables that change each time through a loop. These iteration variables move through the sequence or set.
Example:

for i in [5, 4, 3, 2, 1] :print(i)
print('Blastoff!')

Finding the Largest Value

largest_so_far = -1
print('Before', largest_so_far)
for the_num in [9, 41, 12, 3, 74, 15] :if the_num > largest_so_far :largest_so_far = the_numprint(largest_so_far, the_num)print('After', largest_so_far)

Counting in a Loop

To count how many times we execute a loop, we introduce a counter variable that starts at 0 and we add one to it each time through the loop.

Summing in a Loop

To add up a value we encounter in a loop, we introduce a sum variable that starts at 0 and we add the value to the sum each time through the loop.

Filtering in a Loop

We use an if statement in the loop to catch / filter the values we are looking for.

print('Before')
for value in [9, 41, 12, 3, 74, 15] :if value > 20:print('Large number',value)
print('After')

Search Using a Boolean Variable

If we just want to search and know if a value was found, we use a variable that starts at False and is set to True as soon as we find what we are looking for.

Finding the Smallest Value

We still have a variable that is the smallest so far. The first time through the loop smallest is None, so we take the first value to be the smallest.

smallest = None
print('Before')
for value in [9, 41, 12, 3, 74, 15] :if smallest is None : smallest = valueelif value < smallest : smallest = valueprint(smallest, value)
print('After', smallest)

The is and is not Operators

Similar to, but stronger than ==
Match both type and value
is not also is a logical operator

推荐在Boolean和None type时使用，integer，float等可能会出现问题

Strings （Chapter 6）

String Data Type

A string is a sequence of characters
A string literal uses quotes ‘Hello’ or “Hello”
For strings, + means “concatenate”

Looking Inside Strings

We can get at any single character in a string using an index specified in square brackets
The index value must be an integer and starts at zero

Strings are immutable

>>> greeting = 'Hello, world!'
>>> greeting[0] = 'J'
TypeError: 'str' object does not support item assignment

The reason for the error is that strings are immutable, which means you can’t change an existing string.

Strings Have Length

The built-in function len gives us the length of a string

>>> fruit = 'banana'
>>> print(len(fruit))
6

Looping and Counting

word = 'banana'
count = 0
for letter in word :if letter == 'a' : count = count + 1
print(count)

Slicing Strings

We can also look at any continuous section of a string using a colon operator
The second number is one beyond the end of the slice - “up to but not including”
If the second number is beyond the end of the string, it stops at the end
If we leave off the first number or the last number of the slice, it is assumed to be the beginning or end of the string respectively

>>> s = 'Monty Python'
>>> print(s[0:4])
Mont
>>> print(s[6:7])
P
>>> print(s[6:20])
Python
>>> print(s[:2])
Mo
>>> print(s[8:])
thon
>>> print(s[:])
Monty Python

Using in as a Logical Operator

The in keyword can also be used to check to see if one string is “in” another string
The in expression is a logical expression that returns True or False and can be used in an if statement

>>> fruit = 'banana'
>>> 'n' in fruit
True
>>> if 'a' in fruit :
...     print('Found it!')
Found it!

String Library

Python has a number of string functions which are in the string library
These functions are already built into every string - we invoke them by appending the function to the string variable
These functions do not modify the original string, instead they return a new string that has been altered

Searching a String

find() finds the first occurrence of the substring
If the substring is not found, find() returns -1
Often when we are searching for a string using find() we first convert the string to lower case so we can search a string regardless of case
find(a,b) a is the target string and b is the start index number

>>> fruit = 'banana'
>>> pos = fruit.find('na')
>>> print(pos)
2
>>> aa = fruit.find('z')
>>> print(aa)
-1

Search and Replace

The replace() function is like a “search and replace” operation in a word processor
It replaces all occurrences of the search string with the replacement string

>>> greet = 'Hello Bob'
>>> nstr = greet.replace('o','X')
>>> print(nstr)
HellX BXb

Stripping Whitespace

lstrip() and rstrip() remove whitespace at the left or right
strip() removes both beginning and ending whitespace

Prefixes

>>> line = 'Please have a nice day'
>>> line.startswith('Please')
True
>>> line.startswith('p')
False

Parsing and Extracting

>>> data = 'From stephen.marquard@uct.ac.za Sat Jan  5 09:14:16 2008'
>>> atpos = data.find('@')
>>> print(atpos)
21
>>> sppos = data.find(' ',atpos)
>>> print(sppos)
31
>>> host = data[atpos+1 : sppos]
>>> print(host)
uct.ac.za

Reading Files (Chapter 7)

File Processing

A text file can be thought of as a sequence of lines
A text file has newlines at the end of each line (talk about later)

Opening a File

handle = open(filename, mode)
returns a handle use to manipulate the file
filename is a string
mode is optional and should be ‘r’ if we are planning to read the file and ‘w’ if we are going to write to the file

handle = open('mbox.txt', 'r')

Meaning of handle: Handle is a thing allow you to get the file instead of file itself or the data in the file. Just like a wrapper.

The handle is a connection to the file’s data

The newline Character

We use a special character called the “newline” to indicate when a line ends
We represent it as \n in strings
Newline is still one character - not two

>>> stuff = 'X\nY'
>>> print(stuff)
X
Y

File Handle as a Sequence

We can use the for statement to iterate through a sequence
Remember - a sequence is an ordered set

xfile = open('mbox.txt')
for cheese in xfile:print(cheese)

Reading the Whole File

-We can read the whole file (newlines and all) into a single string

>>> fhand = open('mbox-short.txt')
>>> inp = fhand.read()
>>> print(len(inp))
94626
>>> print(inp[:20])
From stephen.marquar

Searching Through a File (fixed)

We can put an if statement in our for loop to only print lines that meet some criteria
We can strip the whitespace from the right-hand side of the string using rstrip() from the string library
The newline is considered “white space” and is stripped

fhand = open('mbox-short.txt')
for line in fhand:line = line.rstrip()if line.startswith('From:') :print(line)

Skipping with continue

Do the same things with continue

fhand = open('mbox-short.txt')
for line in fhand:line = line.rstrip()if not line.startswith('From:') :continueprint(line)

Using in to Select Lines

fhand = open('mbox-short.txt')
for line in fhand:line = line.rstrip()if not '@uct.ac.za' in line : continueprint(line)

Dealing with bad files

  fname = input('Enter the file name:  ')try:fhand = open(fname)except:print('File cannot be opened:', fname)quit()count = 0for line in fhand:if line.startswith('Subject:') :count = count + 1print('There were', count, 'subject lines in', fname)

三章学完脑子有点乱…

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