Python Chapter 10: 列表 Part2

10.3 实例研究：乐透数

编写程序决定输入数字是否涵盖了1-99之间的所有整数：

# Create a list of 99 Boolean elements with value False
isCovered = 99 * [False]
endOfInput = False
while not endOfInput:
    s = input("Enter a line of numbers separated by spaces: ")
    items = s.split()
    lst = [eval(x) for x in items]

    for number in lst:
        if number == 0:
            endOfInput = True
        else:
            isCovered[number - 1] = True

allCovered = True
for i in range(99):
    if not isCovered[i]:
        allCovered = False
        break
if allCovered:
    print("The tickets cover all numbers")
else:
    print("The tickets don't cover all numbers")

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10.4 实例研究：一副扑克牌

在一副扑克牌中随机抽出四张并显示花色与数字:

# Create a deck of cards
deck = [x for x in range(52)]

# Create suits and ranks lists
suits = ["Spades", "Hearts", "Diamonds", "Clubs"]
ranks = ["Ace", "2", "3", "4", "5", "6", "7", "8", "9", "10", "Jack", "Queen", "King"]

import random
random.shuffle(deck)

for i in range(4):
    suit = suits[deck[i] // 13]
    rank = ranks[deck[i] % 13]
    print("Card number", deck[i], "is the", rank, "of", suit)

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10.5 扑克牌图形用户界面

给出一个图形用户界面，单机shuffle按钮，显示四张随机扑克牌的图像：

from tkinter import *
import random

class DeckOfCardsGUI:
    def __init__(self):
        window = Tk()
        window.title("Pick Four Cards Radnomly")

        self.imageList = []
        for i in range(1, 53):
            self.imageList.append(PhotoImage(file = "image/card/" + str(i) + ".gif"))

        frame = Frame(window);
        frame.pack()

        self.labelList = []
        for i in range(4):
            self.labelList.append(Label(frame, image = self.imageList[i]))
            self.labelList[i].pack(side = LEFT)

        Button(window, text = "Shuffle", command = self.shuffle).pack()

        window.mainloop()

    def shuffle(self):
        random.shuffle(self.imageList)
        for i in range(4):
            self.labelList[i]["image"] = self.imageList[i]

DeckOfCardsGUI()

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10.6 复制列表

想到复制列表时，你可能会尝试这条语句：

list2 = list1

实际上，这样只是让list2指向list1的一个引用，原list2的内容就成了垃圾，被Python自动收集重新利用。因此此时list2与list1实际上会指向同一块内容。
为了达到真正赋值的效果，可以采用：

list2 = [x for x in list1]
list2 = [] + list1

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10.7 将列表传递给函数

）列表传递给函数后值可变

由于列表是一个可变对象，列表的内容可能在函数调用后发生改变：

def main():
    x = 1
    y = [1, 2, 3]

    m(x, y)

    print("x is", x)
    print("y[0] is", y[0])

def m(number, numbers):
    number = 1001
    numbers[0] = 5555

main()

该程序运行结果如图所示：

如图，由于函数对简单变量进行传值调用，x的值在函数调用后并不发生改变。而y这个列表的值就发生了改变

）默认参数

Python中的默认参数只会声明一次，亦即生成函数的默认参数后此参数值将一直保留，在后续调用中保留其值：

def add(x, lst = []):
    if x not in lst:
        lst.append(x)

    return lst

def main():
    list1 = add(1)
    print(list1)
    list2 = add(2)
    print(list2)
    list3 = add(3, [11, 12, 13, 14])
    print(list3)
    list4 = add(4)
    print(list4)

main()

此程序运行结果如下所示：

由此可见，在第二次、第四次调用add()时，原先创建的默认参数lst里的值还存在，故在调用后会累积在列表中。
若想每次调用后默认参数都回到[]，使用如下方法：

def add(x, lst = None):
    if lst == None:
        lst = []
    if x not in lst:
        lst.append(x)

    return lst

def main():
    list1 = add(1)
    print(list1)
    list2 = add(2)
    print(list2)
    list3 = add(3, [11, 12, 13, 14])
    print(list3)
    list4 = add(4)
    print(list4)

main()

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10.8 从函数返回一个列表

函数返回一个列表时，就会返回这个列表的引用值

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10.9 实例研究：统计每个字母的出现次数

随机生成100个小写字母并统计他们出现的频数：

import RandomCharacter

def main():
    chars = createList()

    print("The lowercase letters are:")
    displayList(chars)

    counts = countLetters(chars)

    print("The occurences of each letter are:")
    displayCounts(counts)

def createList():
    chars = []

    for i in range(100):
        chars.append(RandomCharacter.getRandomLowerCaseLetter())

    return chars

def displayList(chars):
    for i in range(len(chars)):
        if (i + 1) % 20 == 0:
            print(chars[i])
        else:
            print(chars[i], end = ' ')

def countLetters(chars):
    counts = 26 * [0]

    for i in range(len(chars)):
        counts[ord(chars[i]) - ord('a')] += 1

    return counts

def displayCounts(counts):
    for i in range(len(counts)):
        if (i + 1) % 10 == 0:
            print(counts[i], chr(i + ord('a')))
        else:
            print(counts[i], chr(i + ord('a')), end = ' ')

main()

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