MIT公开课： Python 笔记7 列表及可变性，字典，效率

Lecture 7： Lists and mutability，dictionaries，pseudocode，introduction to efficiency 列表及可变性，字典，伪代码，效率

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Lists and mutability 列表及可变性

>>> L1 = [1, 2, 3]>>> L2 = L1>>> print L2[1, 2, 3]>>> L1[0] = 4>>> print L2[4, 2, 3]

def f(L):    L[0] = 4L1 = [1,2,3]L2 = [1,2,3]L3 = L1print L1 == L2f(L1)print L1 == L2print L1print L2print L3#输出TrueFalse[4, 2, 3][1, 2, 3][4, 2, 3]

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dictionaries 字典

3wschool python dictionary

• mutable 可改变的
• not ordered 无序
• generalized indexing 索引
• key ,value pair 键值对
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EtoF = {'one': 'un', 'soccer': 'football'}print EtoF['soccer']print EtoF[0]print EtoFNtoS = {1: 'one', 2: 'two', 'one': 1, 'two': 2}print NtoS.keys()print NtoS.keysdel NtoS['one']print NtoSL = [['un', 'one'], ['deux', 'two']]def keySearch(L, k):    for elem in L:        if elem[0] == k: return elem[1]    return Noneprint keySearch(L, 'deux')# 输出footballprint EtoF[0]KeyError: 0{'soccer': 'football', 'one': 'un'}[1, 2, 'two', 'one']<built-in method keys of dict object at 0x7fc52bf0b1e0>{1: 'one', 2: 'two', 'two': 2}two

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pseudo code 伪代码

求直角三角形的斜边：

• input value for base 输入底边 as float
• input value for height 输入高 as float
• sqrt (base**2 + height **2) 计算斜边
• output value in hypotenuse 输出

import math# Get baseinputOK = Falsewhile not inputOK:    base = input('Enter base: ')    if type(base) == type(1.0):        inputOK = True    else:        print('Error.  Base must be floating point number.')# Get HeightinputOK = Falsewhile not inputOK:    height = input('Enter height: ')    if type(height) == type(1.0):        inputOK = True    else:        print('Error.  Height must be floating point number.')hyp = math.sqrt(base * base + height * height)print 'Base: ' + str(base) + ',height: ' + str(height) + ', hyp: ' + str(hyp)

改进：

def getFloat(requestMsg, errorMsg):    inputOK = False    while not inputOK:        val = input(requestMsg)        if type(val) == type(1.0):            inputOK = True        else:            print(errorMsg)    return valbase = getFloat('Enter base: ', 'Error: base must be a float')height = getFloat('Enter height: ', 'Error: height must be a float')hyp = math.sqrt(base * base + height * height)print 'Base: ' + str(base) + ',height: ' + str(height) + ', hyp: ' + str(hyp)

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Efficiency 效率

Efficiency – orders of growth

• choice of algorithm 算法选择
• map a problem into a class of algorithms of some efficiency 把问题映射为高效的算法

space & time 时间 & 空间

• how much memory does it take 消耗多少存储空间
• what is the number of the basic steps needed as a function of the input size 进行计算的方法有几步

random access model 随机存取模型

• best case – min
• worst case – max
• expected case – avg