Lec16.

Class:

• template for data type
• cluster data & method
• modularity/abstraction
• data hiding:only acess the parts though a method
• class used to make instances

Encapsulation:(封装)

• data hide
• inheritance:继承
• shadow/override methods
• hierarchy of classes

Lec17.&Lec18.

• Informal problem description to a formal problem statement:把一个非正式的问题转化到一个更加通用的模型上来
• Inventing computational models:创造计算模型
• dealing with & exploiting randomness:探索随机性
• making sense of data:让数据变得有意义,如何理解数据
• evaluating quality of answers:如何来判断和分析程序给出的答案是不是我们所想要的答案

随机走动:

• simolate(模拟) random walk(无规则运动)
• 数据抽象
  • Location
  • compass Pt
  • Field
  • Drunk
• pseudo random(伪随机)

课程使用的代码:

import mathimport randomimport pylab

class Location(object):    
    def __init__(self, x, y):        
        self.x = float(x)        
        self.y = float(y)    
    
    def move(self, xc, yc):        
        return Location(self.x + float(xc), self.y+float(yc))    
    
    def getCoords(self):        
        return self.x, self.y    
    
    def getDist(self, other):        
        ox, oy = other.getCoords()        
        xDist = self.x - ox        
        yDist = self.y - oy        
        return math.sqrt(xDist**2 + yDist ** 2)


class CompassPt(object):    
    possibles = ('N', 'S', 'E', 'W')    
    def __init__(self, pt):        
        if pt in self.possibles:            
            self.pt = pt        
        else:            
            raise ValueError('in CompassPt.__init__')    
    
    def move(self, dist):        
        if self.pt == 'N':            
            return (0, dist)        
        elif self.pt == 'S':            
            return (0, -dist)        
        elif self.pt == 'E':            
            return (dist, 0)        
        elif self.pt == 'W':            
            return (-dist, 0)        
        else:            
            raise ValueError('in compassPt.move')


class Field(object):    
    def __init__(self, drunk, loc):        
        self.drunk = drunk        
        self.loc = loc    
        
    def move(self, cp, dist):        
        oldLoc = self.loc        
        xc, yc = cp.move(dist)        
        self.loc = oldLoc.move(xc, yc)    
    
    def getLoc(self):        
        return self.loc    
    
    def getDrunk(self):        
        return self.drunk


class Drunk(object):    
    def __init__(self, name):        
        self.name = name    
    
    def move(self, field, time=1):        
        if field.getDrunk() != self:            
            raise ValueError('Drunk.move called with drunk not in field')        
        for i in range(time):            
            pt = CompassPt(random.choice(CompassPt.possibles))            
            field.move(pt, 1)
    
def performTrial(time, f):    
    start = f.getLoc()    
    distances = [0.0]    
    for t in range(1, time+1):        
        f.getDrunk().move(f)        
        newLoc = f.getLoc()        
        distance = newLoc.getDist(start)        
        distances.append(distance)    
    return distances

# assert False
drunk = Drunk('a man')
for i in range(3):    
    f = Field(drunk, Location(0, 0))    
    distances = performTrial(500, f)    
    pylab.plot(distances)
pylab.title('random walk')
pylab.xlabel('time')
pylab.ylabel('distance from origin')

pylab.show()

assert False

perform trial 的解释:

1. inner loop that simolates 1 trial
2. ‘enclose’ inner loop in a loop that conducts appropriate of trials
3. calculate and present statics

随机运动的应用:

• Browning motion:布朗运动
• stock market:预测股票
• kinetics
• evolution