Python
讲述点⬇
基本数据类型-Basic data types
同c++明显不同的是，python无需在变量前声明变量的数据类型，python会自动根据数据的类型进行归类。
python无自增(++)和自减符(–)。
python中进行与或等操作的操作符是其英文单词，而非c++之类的标识符(||、&&等)
# 计算操作
x = 3
print(type(x)) # Prints "<class 'int'>"
print(x) # Prints "3"
print(x + 1) # Addition; prints "4"
print(x - 1) # Subtraction; prints "2"
print(x * 2) # Multiplication; prints "6"
print(x ** 2) # Exponentiation; prints "9"
x += 1
print(x) # Prints "4"
x *= 2
print(x) # Prints "8"
y = 2.5
print(type(y)) # Prints "<class 'float'>"
print(y, y + 1, y * 2, y ** 2) # Prints "2.5 3.5 5.0 6.25"
#逻辑判断
t = True
f = False
print(type(t)) # Prints "<class 'bool'>"
print(t and f) # Logical AND; prints "False"
print(t or f) # Logical OR; prints "True"
print(not t) # Logical NOT; prints "False"
print(t != f) # Logical XOR; prints "True"
#字符串操作
hello = 'hello' # String literals can use single quotes
world = "world" # or double quotes; it does not matter.
print(hello) # Prints "hello"
print(len(hello)) # String length; prints "5"
hw = hello + ' ' + world # String concatenation
print(hw) # prints "hello world"
hw12 = '%s %s %d' % (hello, world, 12) # sprintf style string formatting
print(hw12) # prints "hello world 12"
#字符串转换
s = "hello"
print(s.capitalize()) # Capitalize a string; prints "Hello"
print(s.upper()) # Convert a string to uppercase; prints "HELLO"
print(s.rjust(7)) # Right-justify a string, padding with spaces; prints " hello"
print(s.center(7)) # Center a string, padding with spaces; prints " hello "
print(s.replace('l', '(ell)')) # Replace all instances of one substring with another;
# prints "he(ell)(ell)o"
print(' world '.strip()) # Strip leading and trailing whitespace; prints "world"
容器-Containers
Lists ~~ 列表
[-1]表示指向列表的最后一个元素。
可变长的。
虽然可变长，但不能直接通过x[len(x)]=10来增加超出范围的元素。只能通过x.append(10)来增加！
可容纳不同类型的数据。
访问数组元素方式更加简洁。
循环的in表达。
支持[ ]索引中进行循环操作和判断操作！
所谓Lists在python中就是数组的含义，但是是可变长的及可容纳不同类型的。
#单个数组元素的操作
xs = [3, 1, 2] # Create a list
print(xs, xs[2]) # Prints "[3, 1, 2] 2"
print(xs[-1]) # Negative indices count from the end of the list; prints "2"
xs[2] = 'foo' # Lists can contain elements of different types
print(xs) # Prints "[3, 1, 'foo']"
xs.append('bar') # Add a new element to the end of the list
print(xs) # Prints "[3, 1, 'foo', 'bar']"
x = xs.pop() # Remove and return the last element of the list
print(x, xs) # Prints "bar [3, 1, 'foo']"
#对数组连续的块操作
nums = list(range(5)) # range is a built-in function that creates a list of integers
print(nums) # Prints "[0, 1, 2, 3, 4]"
print(nums[2:4]) # Get a slice from index 2 to 4 (exclusive); prints "[2, 3]"
print(nums[2:]) # Get a slice from index 2 to the end; prints "[2, 3, 4]"
print(nums[:2]) # Get a slice from the start to index 2 (exclusive); prints "[0, 1]"
print(nums[:]) # Get a slice of the whole list; prints "[0, 1, 2, 3, 4]"
print(nums[:-1]) # Slice indices can be negative; prints "[0, 1, 2, 3]"
nums[2:4] = [8, 9] # Assign a new sublist to a slice
print(nums) # Prints "[0, 1, 8, 9, 4]"
#基于数组循环之直接取元素
animals1 = ['cat', 'dog', 'monkey']
for animal in animals1:
print(animal)
# Prints "cat", "dog", "monkey", each on its own line.
#基于数组循环之下标索引
animals2 = ['cat', 'dog', 'monkey']
for idx, animal in enumerate(animals2): #Function enumerate()
print('#%d: %s' % (idx + 1, animal))
# Prints "#1: cat", "#2: dog", "#3: monkey", each on its own line
#一般计算方法
nums = [0, 1, 2, 3, 4]
squares = []
for x in nums:
squares.append(x ** 2)
print(squares) # Prints [0, 1, 4, 9, 16]
#[]中进行循环操作
nums = [0, 1, 2, 3, 4]
squares = [x ** 2 for x in nums]
print(squares) # Prints [0, 1, 4, 9, 16]
#[]中进行循环操作和判断操作
nums = [0, 1, 2, 3, 4]
even_squares = [x ** 2 for x in nums if x % 2 == 0]
print(even_squares) # Prints "[0, 4, 16]"
Dictionaries ~~ 字典
存放(Key,Value)的容器。即类是由关键码Key来找到键值Value的一个关系。
通过 Key:Value 的形式表示关系。
也是可变长的，但是可直接Dic[Key]=Value来增加。该语句表示有该Key则修改其对应的Value值，无则增加Key-Value值。
循环关系类似Lists。
#字典基本操作
d = {'cat': 'cute', 'dog': 'furry'} # Create a new dictionary with some data
print(d['cat']) # Get an entry from a dictionary; prints "cute"
print('cat' in d) # Check if a dictionary has a given key; prints "True"
d['fish'] = 'wet' # Set an entry in a dictionary
print(d['fish']) # Prints "wet"
# print(d['monkey']) # KeyError: 'monkey' not a key of d
print(d.get('monkey', 'N/A')) # Get an element with a default; prints "N/A"
print(d.get('fish', 'N/A')) # Get an element with a default; prints "wet"
del d['fish'] # Remove an element from a dictionary
print(d.get('fish', 'N/A')) # "fish" is no longer a key; prints "N/A"
#对字典的循环取值的方法一
d = {'person': 2, 'cat': 4, 'spider': 8}
for animal in d:
legs = d[animal]
print('A %s has %d legs' % (animal, legs))
# Prints "A person has 2 legs", "A cat has 4 legs", "A spider has 8 legs"
#对字典的循环取值的方法二--用到item()函数
d = {'person': 2, 'cat': 4, 'spider': 8}
for animal, legs in d.items():
print('A %s has %d legs' % (animal, legs))
# Prints "A person has 2 legs", "A cat has 4 legs", "A spider has 8 legs"
#方法类似Lists，允许更简单的构造字典。
nums = [0, 1, 2, 3, 4]
even_num_to_square = {x: x ** 2 for x in nums if x % 2 == 0}
print(even_num_to_square) # Prints "{0: 0, 2: 4, 4: 16}"
Sets ~~ 集合
集合是不同元素的一个无序的集合。通俗讲，即不能通过下标索引的方式找到之对应的元素值，
是可变长的及可移除的
循环关系类似Lists。
#Sets的基本操作
animals = {'cat', 'dog'}
print('cat' in animals) # Check if an element is in a set; prints "True"
print('fish' in animals) # prints "False"
animals.add('fish') # Add an element to a set
print('fish' in animals) # Prints "True"
print(len(animals)) # Number of elements in a set; prints "3"
animals.add('cat') # Adding an element that is already in the set does nothing
print(len(animals)) # Prints "3"
animals.remove('cat') # Remove an element from a set
print(len(animals)) # Prints "2"
#循环取值的方式
animals = {'cat', 'dog', 'fish'}
for idx, animal in enumerate(animals):
print('#%d: %s' % (idx + 1, animal))
# Prints "#1: fish", "#2: dog", "#3: cat"
#对元素的操作性循环
from math import sqrt
nums = {int(sqrt(x)) for x in range(30)}
print(nums) # Prints "{0, 1, 2, 3, 4, 5}"
Tuples ~~ 元组
元组是不可变的一个的列表(不能有添加，删除，和修改的操作)。
固定的，一旦生成就不能变(除了特殊情况–元素是一个对象的引用时，引用的对象是可以被修改的)
循环关系类似Lists。
元组可以用作字典中的Key和Sets的元素，而列表不能。
#该元组用于字典中的Key与Set的元素
d = {(x, x + 1): x for x in range(10)} # Create a dictionary with tuple keys
#{(0, 1): 0, (1, 2): 1, (2, 3): 2, (3, 4): 3, (4, 5): 4, (5, 6): 5, (6, 7): 6, (7, 8): 7, (8, 9): 8, (9, 10): 9}
t = (5, 6) # Create a tuple
print(type(t)) # Prints "<class 'tuple'>"
print(d[t]) # Prints "5"
print(d[(1, 2)]) # Prints "1"
函数-Functions
使用关键字 def 来定义函数。
函数名后接符号 ： 开启函数体，靠缩进判断何时结束(事实上python就是靠缩进来判断，如if,for的有效操作域等)。
函数的参数也类似C++，也可以有默认参数。
#定义一个判断正负的函数
#注意缩进量！
def sign(x):
if x > 0:
return 'positive'
elif x < 0:
return 'negative'
else:
return 'zero'
for x in [-1, 0, 1]:
print(sign(x))
# Prints "negative", "zero", "positive"
#定义一个带默认形参的函数
def hello(name, loud=False):
if loud:
print('HELLO, %s!' % name.upper())
else:
print('Hello, %s' % name)
hello('Bob') # Prints "Hello, Bob"
hello('Fred', loud=True) # Prints "HELLO, FRED!"
类-Classes
有C++或Java基础的应该很好理解什么是类。我就不详细说了。
#类的构造方法
class Greeter(object):
# 构造函数
def __init__(self, name):
self.name = name # Create an instance variable
# 示例函数
def greet(self, loud=False):
if loud:
print('HELLO, %s!' % self.name.upper())
else:
print('Hello, %s' % self.name)
#使用上述定义的类
g = Greeter('Fred') # Construct an instance of the Greeter class
g.greet() # Call an instance method; prints "Hello, Fred"
g.greet(loud=True) # Call an instance method; prints "HELLO, FRED!"
上述示例代码均来自 CS231n-Python Numpy Tutorial