CS231N课程中文讲义.pdf-资料库

CS231n 10 Andrej Karpathy ID Python Numpy CS231n Python Numpy Tutorial Flood Sung SunisDown Justin Johnson Python numpy, scipy, matplotlib Python Python Numpy Python Python Python Matlab numpy for Matlab users IPython notebook Volodymyr Kuleshov Isaac Caswell CS 228 Python 课程笔记翻译：教程译者注：本文智能单元首发，翻译自斯坦福课程笔记，由课程教师授权进行翻译。本篇教程由杜客翻译完成，、、巩子嘉和一位不愿透露的知友对本翻译亦有贡献。原文如下这篇教程由创作。我们将使用编程语言来完成本课程的所有作业。是一门伟大的通用编程语言，在一些常用库（）的帮助下，它又会变成一个强大的科学计算环境。我们期望你们中大多数人对于语言和库比较熟悉，而对于没有经验的同学，这篇教程可以帮助你们快速了解编程环境和如何使用作为科学计算工具。一部分同学对于有一定经验。对于这部分同学，我们推荐阅读页面。你们还可以查看本教程的版。该教程是由和为课程创建的。内容列表：基本数据类型容器列表字典杜客个月前

Numpy SciPy MATLAB Matplotlib Python Python Python quicksort Python 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) 集合元组函数类数组访问数组数据类型数组计算广播图像操作文件点之间的距离绘制图形绘制多个图形图像是一种高级的，动态类型的多范型编程语言。很多时候，大家会说看起来简直和伪代码一样，这是因为你能够通过很少行数的代码表达出很有力的思想。举个例子，下面是用实现的经典的算法例子：

print quicksort([3,6,8,10,1,2,1]) # Prints "[1, 1, 2, 3, 6, 8, 10]" Python Python 2.7 3.4 2.7 3.4 3.0 2.7 python --version Python x = 3 print type(x) # Prints "" 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 "" print y, y + 1, y * 2, y ** 2 # Prints "2.5 3.5 5.0 6.25" Python x++ x-- Python Python && || t = True f = False print type(t) # Prints "" print t and f # Logical AND; prints "False" 版本有两个支持的版本，分别是和。这有点让人迷惑，向语言中引入了很多不向后兼容的变化，下的代码有时候在下是行不通的。在这个课程中，我们使用的是版本。如何查看版本呢？使用命令。基本数据类型和大多数编程语言一样，拥有一系列的基本数据类型，比如整型、浮点型、布尔型和字符串等。这些类型的使用方式和在其他语言中的使用方式是类似的。数字：整型和浮点型的使用与其他语言类似。需要注意的是，中没有和的操作符。也有内置的长整型和复杂数字类型，具体细节可以查看文档。布尔型：实现了所有的布尔逻辑，但用的是英语，而不是我们习惯的操作符（比如和等）。

print t or f # Logical OR; prints "True" print not t # Logical NOT; prints "False" print t != f # Logical XOR; prints "True" Python 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" 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" Containers container Python tuples Lists Python lists dictionaries sets 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']" 字符串：对字符串的支持非常棒。字符串对象有一系列有用的方法，比如：如果想详细查看字符串方法，请看文档。容器译者注：有知友建议翻译为复合数据类型，供读者参考。有以下几种容器类型：列表（）、字典（）、集合（）和元组（）。列表列表就是中的数组，但是列表长度可变，且能包含不同类型元素。

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']" Slicing Python nums = 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, 8, 4]" Numpy Loops animals = ['cat', 'dog', 'monkey'] for animal in animals: print animal # Prints "cat", "dog", "monkey", each on its own line. enumerate 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 List comprehensions nums = [0, 1, 2, 3, 4] squares = [] for x in nums: squares.append(x ** 2) print squares # Prints [0, 1, 4, 9, 16] 列表的细节，同样可以查阅文档。切片：为了一次性地获取列表中的元素，提供了一种简洁的语法，这就是切片。在数组的内容中，我们会再次看到切片语法。循环：我们可以这样遍历列表中的每一个元素：如果想要在循环体内访问每个元素的指针，可以使用内置的函数列表推导：在编程的时候，我们常常想要将一种数据类型转换为另一种。下面是一个简单例子，将列表中的每个元素变成它的平方。使用列表推导，你就可以让代码简化很多：

nums = [0, 1, 2, 3, 4] squares = [x ** 2 for x in nums] print squares # Prints [0, 1, 4, 9, 16] 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 , Java Map 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" Loops 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" iteritems d = {'person': 2, 'cat': 4, 'spider': 8} for animal, legs in d.iteritems(): print 'A %s has %d legs' % (animal, legs) # Prints "A person has 2 legs", "A spider has 8 legs", "A cat has 4 legs" Dictionary comprehensions 列表推导还可以包含条件：字典字典用来储存（键值）对，这和中的差不多。你可以这样使用它：想要知道字典的其他特性，请查阅文档。循环：在字典中，用键来迭代更加容易。如果你想要访问键和对应的值，那就使用方法：字典推导：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_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 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" Loops animals = {'cat', 'dog', 'fish'} for idx, animal in enumerate(animals): print '#%d: %s' % (idx + 1, animal) # Prints "#1: fish", "#2: dog", "#3: cat" Set comprehensions from math import sqrt nums = {int(sqrt(x)) for x in range(30)} print nums # Prints "set([0, 1, 2, 3, 4, 5])" Tuples d = {(x, x + 1): x for x in range(10)} # Create a dictionary with tuple keys print d t = (5, 6) # Create a tuple print type(t) # Prints "" 集合集合是独立不同个体的无序集合。示例如下：和前面一样，要知道更详细的，查看文档。循环：在集合中循环的语法和在列表中一样，但是集合是无序的，所以你在访问集合的元素的时候，不能做关于顺序的假设。集合推导：和字典推导一样，可以很方便地构建集合：元组元组是一个值的有序列表（不可改变）。从很多方面来说，元组和列表都很相似。和列表最重要的不同在于，元组可以在字典中用作键，还可以作为集合的元素，而列表不行。例子如下：

print d[t] # Prints "5" print d[(1, 2)] # Prints "1" Functions Python def 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" 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!" Classes Python class Greeter(object): # Constructor def __init__(self, name): self.name = name # Create an instance variable 文档有更多元组的信息。函数函数使用来定义函数：我们常常使用可选参数来定义函数：函数还有很多内容，可以查看文档。类对于类的定义是简单直接的：

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