CS231n:Python Numpy教程

• Python
  • 基本数据类型
  • 容器
    • 列表
    • 字典
    • 集合
    • 元组
  • 函数　　
  • 类
• Numpy
  • 数组　　
  • 访问数组
  • 数据类型
  • 数组计算
  • 广播
• SciPy
  • 图像操作
  • MATLAB文件
  • 点之间的距离
• Matplotlib
  • 绘制图形
  • 绘制多个图形
  • 图像

Python

python实现的经典的quicksort算法：

def quicksort(arr):
    if len(arr)<=1:
        return arr
    pivot = arr[len(arr)/2]
    left = [x for x in arr if x < pivot]
    middle = [x for x in arr if x == pivot]
    right = [x for x in arr if x > pivot]
    return quicksort(left) + middle + quicksort(right)

print quicksort([3,6,8,10,1,2,1])
#prints "[1,1,2,3,6,8,10]"

数字：整型和浮点型的使用与其他语言类似。

x = 3
print(type(x))#printd "<type '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) #Expoentiaton; prints "9"
x += 1
print(x)# prints "4"
x *= 2
print(x) #prints "8"
y = 2.5
print(type(y))# prints "<type 'float'>"
print(y,y + 1,y * 2,y ** 2) #prints "2.5 3.5 5.0 6.25"

注意！！！！ Python 中没有x++和x--的操作符。

布尔型：Python实现了所有的布尔逻辑。

t = True
f = False

print(type(t)) #prints "<type'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：

hello = 'hello' #String literals  can use single quotes
world = 'world' # pr double quotes; it does not matter,
print(hello) #prints "hello"
print(len(hello)) #String length; prints "5"
hw = hello + ' '+ world #String concatention
print(hw)# prints "hello world "
hw12 = '%s %s %d' % (hello,world,12) #sprintf style string formatting
print(hw12)#prints "hello world 12"

字符串对象的方法：

s = "hello"
print(s.capitalize()) #Capitalize a string prints "hello"
print(s.upper()) #Convert a stirng to upercase; prints "HELLO"
print(s.rjust(7))#Righr-justify a string,padding with space; prints "  hello"
print(s.center(7))#Center a string,padding with space;prints " hello "
print(s.replace('l','(ell)')) #Replace all instance of one substring with another;
                                #prints "he(ell)(ell)o"
print(' world'.strip())#Strip leading and trailing whitespace; prints "world"

容器 Containers

容器类型：列表（lists） 字典（dictionaries） 集合（sets）和元组（tuples）

列表Lists

列表就是python 中的数组，但是列表长度可变，且能包含不同类型元素

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
x = xs.pop()#Remove and return the last element of the list
print(x,xs)#Prints "bar [3,1,'foo']"

切片Slicing : 为了一次性地获取列表中的元素，python 提供了一种简洁的语法，这就是切片。

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]"

在Numoy数组的内容中，再次看到切片语法。

循环LOOPS：

animals =['cat','dog','monkey']
for animal in animals:
    print(animal)
#prints "cat", "dog","monkey",each on its own line.

如果想要在循环体内访问每个元素的指针，可以使用内置的enumerate函数

animals = ['cat ','dog','monkey']
for idx,animal in enumerate(animals):
    print('#%d: %s'%(idx + 1,animal))

#prints "#1: cat","#2: dog","#3: monkey",each on its own line

列表推导List comprehensions :在编程的时候，我们常常想要将一个数据类型转换为另一种。

将列表中的每个元素变成它的平方。

nums = [0,1,2,3,4]
squares = []
for x in nums:
    squares.append(x ** 2)
print(squares) #Prints [0,1,4,9,16]

使用列表推导：

nums  = [0,1,2,3,4]
squares = [x ** 2 for x in nums]
print(squares) # prints [0,1,4,9,16]

列表推导还可以包含条件：

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]"

字典Dicionaries

字典用来存储（键，值）对，

d = {'cat':'cute','dog':'furry'}#Create a new dictionary with come 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 "Ture"
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"

循环LOOPS：在字典中，用键来迭代。

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 spider has 8 legs","A cat has 4 legs"

访问键和对应的值，使用items方法：

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 spider has 8 legs","A cat has 4 legs"

字典推导Dictionary comprehensions :和列表推导类似，但是允许你方便地构建字典。

nums = {0,1,2,3,4}
even_num_square = {x:x**2 for x in nums if x % 2 == 0}
print(even_num_square) #  prints "{0:0,2:4,4:16}"