Task 06--For、If以及While

Task 06–For、If以及While

一、If语句

1.1代码实例

people=20
cats=30
dogs=15if people < cats:print('Too many cats! The world is doomed!')if people >cats:print('Not many cats! The world is saved!')if people < dogs:print('The world is drooled on !')if people >dogs:print('The world is dry.')dogs +=5if people >=dogs:print('People are greater than or equal to dogs.')if people <= dogs:print('People are less than or equal to dogs.')if people ==dogs:print('People are dogs.')

Too many cats! The world is doomed!
The world is dry.
People are greater than or equal to dogs.
People are less than or equal to dogs.
People are dogs.

1.2 附加练习

在附加练习中，试着猜猜 if 语句是什么以及它是干什么的。在继续进行下个练习之前，试着用自己的话回答以下这些问题，

1、你认为 if 对它下面的代码起什么作用？
if用来判断某个条件满足时的情况,当满足这个条件时,执行下面代码的操作,而不满足该条件时,什么都不做。
2、为什么 if 下面的代码要缩进 4 个空格？
Python 是以缩进来标记代码块的，代码块一定要有缩进，没有缩进的不是代码块。另外，同一个代码块的缩进量要相同，缩进量不同的不属于同一个代码块。if下面的代码缩进表示此代码属于这个if语句的。
3、如果没有缩进会发生什么？
该语句就不受if的影响，无论是否满足条件，该语句都会被执行。
4、你能把一些布尔表达式放进 if 语句吗？试试看。

if  not (True and False)==True :print ("True")if  True or 1 == 1==True:print('True')

True
True

5、如果你改变 people，cats 和 dogs 的初始值会发生什么？
输出的内容根据if条件变化。

people=30
cars=40
trucks=15if cars > people:print('We should take the cars.')
elif cars < people:print('We should not take the cars. ')
else:print("We can't decide.")if trucks > cars:print("That's too many trucks.")
elif trucks < cars:print("Maybe we could take the trucks.")
else:print("We still can't decide.")if people > trucks:print("Alright,let's just take the trucks.")
else:print("Fine,let's stay home then.")

We should take the cars.
Maybe we could take the trucks.
Alright,let's just take the trucks.

1.3 常见问题

+= 是什么意思？ x += 1 就相当于 x = x + 1 ，但是输入的内容更少。你可以把它叫做“累加”（increment by）运算符。之后你还会学到 -= 这样类似的表达。

1.4 附加练习2

1、试着猜猜 elif 和 else 的作用是什么。
两者都是出现在if、for、while语句内部的,else表示可以增加一种选择；而elif则是需要检查更多条件时会被使用，与if和else一同使用，elif是else if 的简写。当if的条件语句为真，执行if下的语句，当if的条件语句为假，执行else下的语句，如果还需继续判断则使用elif。
2、改变 cars，people，和 trucks 的数值，然后追溯每一个 if 语句，看看什么会被打印出来。
3、试试一些更复杂的布尔表达式，比如cars > people 或者 trucks < cars。

people=30
cars=40
trucks=15if cars > people or cars<trucks:print('We should take the cars.')
elif cars > people or cars<trucks:print('We should not take the cars. ')
else:print("We can't decide.")

We should take the cars.

4、在每一行上面加上注释。

people = 50
cars = 40
trucks = 60 #如果cars大于people执行print否则执行elif
if cars > people: print("We should take the cars.")
#如果cars小于people执行print否则执行else
elif cars < people: print("We should not take the cars.")
else: print("We can't decide.")
#如果trucks > cars执行print否则执行elif
if trucks > cars: print("That's too many trucks.")
#如果trucks < cars执行print否则执行else
elif trucks < cars: print("Maybe we could take the trucks.")
else: print("We still can't decide.")
#如果people > trucks执行print否则执行else
if people > trucks: print("Alright, let's just take the trucks.")
else: print("Fine, let's stay home then.")

We should not take the cars.
That's too many trucks.
Fine, let's stay home then.

1.5 常见问题2

如果多个 elif 块都是 True 会发生什么？ Python 从顶部开始，然后运行第一个是 True 的代码块，也就是说，它只会运行第一个。

1.6 IF嵌套使用

前面主要学习了调用函数、打印东西，但是这些基本都是直线运行下来的。你的脚本从上面开始运行，然后到底部结束。如果你用了一个函数，你可以随后再运行它，但是仍然不会有分叉需要你做决定的情况。现在你学习了 if，else，以及 elif，你就可以让脚本来做决定了。

1.6.1 代码实例

print("""You enter a dark room with two doors.Do you go through door #1 or door #2?""")door = input("> ")if door == "1":print("There's a giant bear here eating a cheese cake.")print("What do you do?")print("1. Take the cake.")print("2. Scream at the bear.")     bear = input("> ")if bear == "1":print("The bear eats your face off. Good job!")elif bear == "2":print("The bear eats your legs off. Good job!")else:print(f"Well, doing {bear} is probably better.")print("Bear runs away.")elif door == "2":print("You stare into the endless abyss at Cthulhu's retina.")print("1. Blueberries.")print("2. Yellow jacket clothespins.")print("3. Understanding revolvers yelling melodies.")insanity = input("> ")if insanity == "1" or insanity == "2":print("Your body survives powered by a mind of jello.")print("Good job!")else:print("The insanity rots your eyes into a pool of muck.")print("Good job!")else:print("You stumble around and fall on a knife and die. Good job!")

You enter a dark room with two doors.Do you go through door #1 or door #2?
> 1
There's a giant bear here eating a cheese cake.
What do you do?
1. Take the cake.
2. Scream at the bear.
> 3
Well, doing 3 is probably better.
Bear runs away.

1.6.2 附加练习

给这个游戏加一些新内容，同时改变用户可以做的决定。尽可能地扩展这个游戏，直到它变得很搞笑。
写一个完全不同的新游戏。可能你不喜欢我的这个，你可以做一个你自己的。

print("""You enter a dark room with two doors.Do you go through door #1 、 door #2 or door #3?""")door = input("> ")if door == "1":print("There's a giant bear here eating a cheese cake.")print("What do you do?")print("1. Take the cake.")print("2. Scream at the bear.")     bear = input("> ")if bear == "1":print("The bear eats your face off. Good job!")elif bear == "2":print("The bear eats your legs off. Good job!")else:print(f"Well, doing {bear} is probably better.")print("Bear runs away.")elif door == "2":print("You stare into the endless abyss at Cthulhu's retina.")print("1. Blueberries.")print("2. Yellow jacket clothespins.")print("3. Understanding revolvers yelling melodies.")insanity = input("> ")if insanity == "1" or insanity == "2":print("Your body survives powered by a mind of jello.")print("Good job!")else:print("The insanity rots your eyes into a pool of muck.")print("Good job!")
#以下是补充内容
elif door == "3":print("In front of you stands a man dressed in black.")print("1.Give him a slap in the face.")print("2.Shout: Who are you.")print("3.Begged him.")insanity = input("> ")if insanity == "1" or insanity == "3":print("Unfortunately, you got shot.")print("You fell down")else:print("He said, I'm Killer Bill.")print("You are a sinner!")else:print("You stumble around and fall on a knife and die. Good job!")

You enter a dark room with two doors.Do you go through door #1 or door #2?
> 3
In front of you stands a man dressed in black.
1.Give him a slap in the face.
2.Shout: Who are you.
3.Begged him.
> 2
He said, I'm Killer Bill.
You are a sinner!

1.6.3 常见问题

我能用一系列的 if 语句来代替 elif 吗？在某些情况下可以，但是取决于每个 if/else 是怎么写的。如果这样的话还意味着 Python 将会检查每一个 if-else 组合，而不是像 if-elif-else 组合那样只会检查第一个是 false 的。你可以多试几次，感受一下区别。

我如何表示一个数字的区间？有两种方式：一种是 0 < x < 10 或者 1 <= x < 10 这种传统表示方法，另一种是 x 的区间是 (1, 10)。

如果我想在 if-elif-else 代码块中放更多的选择怎么办？为每种可能的选择增加更多的 elif 块。

二、For

2.1代码实例

the_count=[0,1,2,3,4]#list[],dict{},tuple元组()
fruits=['apple','oranges','pears','apricots']
change=[1,'pennies',2,'dimes',3,'quarters']
for number in the_count:print("This is count",number)
for fruit in fruits:print(f"A fruit of type:{fruit}")
for i in change:print("I got",i)
#build list
elements=[]
for i in range(0,6):#n-1print(f"adding {i} to the list.")
# append is a function that lists understand
elements.append(i)

This is count 0
This is count 1
This is count 2
This is count 3
This is count 4
A fruit of type:apple
A fruit of type:oranges
A fruit of type:pears
A fruit of type:apricots
I got 1
I got pennies
I got 2
I got dimes
I got 3
I got quarters
adding 0 to the list.
adding 1 to the list.
adding 2 to the list.
adding 3 to the list.
adding 4 to the list.
adding 5 to the list.

2.2附加练习

1、看看你是如何使用 range 的。查阅上面的 range 函数并理解掌握。
Python3 range() 函数返回的是一个可迭代对象（类型是对象），而不是列表类型，所以打印的时候不会打印列表。

Python3 list() 函数是对象迭代器，可以把range()返回的可迭代对象转为一个列表，返回的变量类型为列表。

Python2 range() 函数返回的是列表。
函数语法
range(stop)
range(start, stop[, step])
参数说明
start: 计数从 start 开始。默认是从 0 开始。例如range（5）等价于range（0， 5）;
stop: 计数到 stop 结束，但不包括 stop。例如：range（0， 5）是[0, 1, 2, 3, 4]没有5
step：步长，默认为1。例如：range（0， 5）等价于 range(0, 5, 1)
参考链接：https://www.runoob.com/python/python-func-range.html
2、你能在第 22 行不使用 for-loop，而是直接把 range(0, 6) 赋给 elements 吗？

elements = []
elements = list(range(0,6))
print(elements)

[0, 1, 2, 3, 4, 5]

3、找到 Python 文档关于列表的部分，然后读一读。看看除了 append，你还能对列表做哪些操作？
详见Task2，我的博客：https://blog.csdn.net/weixin_57878519

2.3 常见问题

append() 方法
描述
append() 方法用于在列表末尾添加新的对象。

语法
append()方法语法：

list.append(obj)
参数
obj – 添加到列表末尾的对象。
返回值
该方法无返回值，但是会修改原来的列表。

实例
以下实例展示了 append()函数的使用方法：

list1 = ['Google', 'Runoob', 'Taobao']
list1.append('Baidu')
print ("更新后的列表 : ", list1)

更新后的列表 :  ['Google', 'Runoob', 'Taobao', 'Baidu']

参考链接：https://www.runoob.com/python3/python3-att-list-append.html
其他问题详见教程：https://linklearner.com/datawhale-homepage/index.html#/learn/detail/6

三、While语句

3.1 While语句的基本认识及练习

现在我们来看一个新的循环： while-loop。只要一个布尔表达式是 True，while-loop 就会一直执行它下面的代码块。

回到 while-loop，它所做的只是像 if 语句一样的测试，但是它不是只运行一次代码块，而是在 while 是对的地方回到顶部再重复，直到表达式为 False。

但是 while-loop 有个问题：有时候它们停不下来。如果你的目的是让程序一直运行直到宇宙的终结，那这样的确很屌。但大多数情况下，你肯定是需要你的循环最终能停下来的。

为了避免这些问题，你得遵守一些规则：
1、保守使用 while-loop，通常用 for-loop 更好一些。
2、检查一下你的 while 语句，确保布尔测试最终会在某个点结果为 False。
3、当遇到问题的时候，把你的 while-loop 开头和结尾的测试变量打印出来，看看它们在做什么。
在这个练习中，你要通过以下三个检查来学习 while-loop：

i=0
numbers=[]
while i<6:print("at the top i is",i)numbers.append(i)i=i+1print("numbers now:",numbers)print("at the bottom i is",i)
for num in numbers:print(num)

at the top i is 0
numbers now: [0]
at the bottom i is 1
at the top i is 1
numbers now: [0, 1]
at the bottom i is 2
at the top i is 2
numbers now: [0, 1, 2]
at the bottom i is 3
at the top i is 3
numbers now: [0, 1, 2, 3]
at the bottom i is 4
at the top i is 4
numbers now: [0, 1, 2, 3, 4]
at the bottom i is 5
at the top i is 5
numbers now: [0, 1, 2, 3, 4, 5]
at the bottom i is 6
0
1
2
3
4
5

3.2 附加练习

附加练习
1、把这个 while-loop 转换成一个你可以调用的函数，然后用一个变量替代 i < 6 里面的 6。

def while_loop(number):i = 0numbers = []while i < number:print(f"At the top i is {i}")numbers.append(i)i = i + 1print("Numbers now: ",numbers)print(f"At the bottom i is {i}")print("The numbers: ")for num in numbers:print(num)while_loop(6)

At the top i is 0
Numbers now:  [0]
At the bottom i is 1
At the top i is 1
Numbers now:  [0, 1]
At the bottom i is 2
At the top i is 2
Numbers now:  [0, 1, 2]
At the bottom i is 3
At the top i is 3
Numbers now:  [0, 1, 2, 3]
At the bottom i is 4
At the top i is 4
Numbers now:  [0, 1, 2, 3, 4]
At the bottom i is 5
At the top i is 5
Numbers now:  [0, 1, 2, 3, 4, 5]
At the bottom i is 6
The numbers:
0
1
2
3
4
5

2、用这个函数重新写一下这个脚本，试试不同的数值。

def while_loop(number):i = 0numbers = []while i < number:print(f"At the top i is {i}")numbers.append(i)i = i + 1print("Numbers now: ",numbers)print(f"At the bottom i is {i}")print("The numbers: ")for num in numbers:print(num)print("Please type in a number:")
number = int(input(">>>")) #让使用者输入不同的数字
while_loop(number)

3、再增加一个变量给这个函数的参数，然后改变第 8 行的 +1，让它增加的值与之前不同。
如果去掉了第八行的+1，就会一直循环，按下 CTRL-C ，程序就会终止。
4、用这个函数重新写这个脚本，看看会产生什么样的效果。
5、用 for-loop 和 range 写这个脚本。你还需要中间的增加值吗？如果不去掉这个增加值会发生什么？
不需要中间的增加值，如果不去掉，程序会一直循环。

numbers = []for i in range(0,6):print(f"At the top i is {i}")numbers.append(i)print("Numbers now: ",numbers)print(f"At the bottom i is {i}")print("The numbers: ")for num in numbers:print(num)

At the top i is 0
Numbers now:  [0]
At the bottom i is 0
At the top i is 1
Numbers now:  [0, 1]
At the bottom i is 1
At the top i is 2
Numbers now:  [0, 1, 2]
At the bottom i is 2
At the top i is 3
Numbers now:  [0, 1, 2, 3]
At the bottom i is 3
At the top i is 4
Numbers now:  [0, 1, 2, 3, 4]
At the bottom i is 4
At the top i is 5
Numbers now:  [0, 1, 2, 3, 4, 5]
At the bottom i is 5
The numbers:
0
1
2
3
4
5

任何时候你在运行程序的时候它失控了，只用按下 CTRL-C ，程序就会终止。

3.3 分支和函数

3.3.1 代码实例

目前为止已经了解了 if 语句，函数以及列表。现在是时候深入学习一下了。照例输入如下代码，看看你能否明白程序在做什么。

from sys import exitdef gold_room():print("This room is full of gold. How much do you take?")choice = input("> ")if "0" in choice or "1" in choice:  # ?how_much = int(choice)else:dead("Man, learn to type a number.")if how_much < 50:print("Nice, you're not greedy, you win!")exit(0)else:dead("You greedy bastard!")def bear_room():print("There is a bear here.")print("The bear has a bunch of honey.")print("The fat bear is in front of another door.")print("How are you going to move the bear?")bear_moved = Falsewhile True:choice = input("> ")if choice == "take honey":dead("The bear looks at you then slaps your face")elif choice == "taunt bear" and not bear_moved:print("The bear has moved from the door.")print("You can go through it now.")bear_moved = Trueelif choice == "taunt bear" and bear_moved:dead("The bear gets pissed off and chews your leg.")elif choice == "open door" and bear_moved:gold_room()else:print("I got no idea what that means.")def cthulhu_room():print("Here you see the great evil Cthulhu.")print("He, it, whatever stares at you and you go insane.")print("Do you flee for your life or eat your head?")choice = input("> ")if "flee" in choice:start()elif "head" in choice:dead("Well that was tasty!")else:cthulhu_room()def dead(why):print(why, "Good job!")exit(0)def start():print("You are in a dark room.")print("There is a door to your right and left.")print("Which one do you take?")choice = input("> ")if choice == "left":bear_room()elif choice == "right":cthulhu_room()else:dead("You stumble around the room until you starve.")
start()

You are in a dark room.
There is a door to your right and left.
Which one do you take?
> left
There is a bear here.
The bear has a bunch of honey.
The fat bear is in front of another door.
How are you going to move the bear?
> taunt bear
The bear has moved from the door.
You can go through it now.
> open door
This room is full of gold. How much do you take?
> 100
You greedy bastard! Good job!
An exception has occurred, use %tb to see the full traceback.SystemExit: 0

3.3.2 附加练习

1、画一个这个游戏的流程图，并指出它是如何运转的。
详见2，在每个语句后面写上注释就不难理解他的工作流程了。
2、为你不理解的函数写上注释。

from sys import exit # 引入 sys 部分模块 exitdef gold_room (): # 定义函数 gold_room print("This room is full of gold. How much do you take?") # 打印字符串choice = input (" > ") # 输出提示信息">"，接收输入内容，并将输入的内容赋值给变量 choiceif "0" in choice or "1" in choice: # 如果变量 choice 的值为 0 或者 1how_much = int(choice) # 调用 int()函数，将变量 choice 的值转换为整型，并赋值给变量 how_muchelse: # 如果变量 choice 的值不是 0 或 1dead("Man, learn to type a number.") # 调用 dead() 函数,打印字符串 if how_much < 50: # 如果变量 how_much 的值小于 50 print("Nice, you're not greedy, you win!") #打印字符串exit(0) # 程序正常退出else: # 如果 if 后面的条件语句为 False ，则判断 elif 后面的条件语句,若为 True，则调用 dead() 函数dead("You greedy bastard!") # 调用 dead() 函数,打印字符串 def bear_room(): # 定义函数 bear_roomprint("There is a bear here.") #打印字符串print("The bear has a bunch of honey.") # 打印字符串print("The fat bear is in front of another door.") # 打印字符串print("How are you going to move the bear?") # 打印字符串bear_moved = False # 将布尔值 False 赋值给变量 bear_moved  while True: # 判断布尔表达式的真假,如果为 True,则一直循环直到表达式为False为止choice = input(" > ") # 输出提示信息">"，接收输入内容，并将输入的内容赋值给变量 choiceif choice == "Take honey": # 如果变量 choice 等于 "Take money"dead("The bear looks at you then slaps your face off.") # 调用 dead() 函数，打印字符串elif choice == "taunt bear" and not bear_moved: # 如果 if 后面的条件语句为 False ，则判断 elif 后面的条件语句,若为 True，则打印以下字符串并将布尔值 True 赋值给变量 bear_movedprint("The bear has moved from the door.")print("You can go through it now.")bear_moved = True elif choice == "taunt bear" and bear_moved: # 如果 if 后面的条件语句为 False ，则判断 elif 后面的条件语句,若为 True，则调用 dead() 函数并打印字符串dead("The bear gets pissed off and chews your leg off.")elif choice == "open door" and bear_moved: # # 如果 if 后面的条件语句为 False ，则判断 elif 后面的条件语句,若为 True，则调用 gold_room() 函数gold_room()else: # 若if和elif后面的条件语句都是为False, 则打印else后面的字符串print("I got no idea what that means.")def cthulhu_room(): # 定义函数 cthulhu_roomprint("Here you see the great evil Cthulhu.") # 打印字符串print("He, it, whatever stares at you and you go insane.") # 打印字符串print("Do you flee for your life or eat your head?") # 打印字符串choice == input(" > ") # 输出提示信息">"，接收输入内容，并将输入的内容赋值给变量 choiceif "flee" in choice: # 如果变量 choice 的值为 fleestart() # 调用 start() 函数elif "head" in choice: # 如果变量 choice 的值为 headdead("Well that was tasty!") # 调用 dead() 函数并打印字符串else: # 如果两者都不是则调用 cthulhu_room 函数cthulhu_room()def dead(why): # 定义函数 dead() ，位置参数为 whyprint(why, "Good job!") # 打印字符串exit(0) # 程序正常退出def start(): # 定义函数 start()print("You are in a dark room.") # 打印字符串print("There is a door to your right and left.") # 打印字符串print("Which one do you take?" ) # 打印字符串choice = input(" > ") # 输出提示信息">"，接收输入内容，并将输入的内容赋值给变量 choiceif choice == "left": # 如果变量 choice 的值为 leftbear_room() # 调用 bear_room 函数elif choice == "right": # 如果变量 choice 的值为 rightcthulhu_room() # 调用 cthulhu_room 函数else: # 如果两者都不是则调用 dead() 函数并打印字符串dead("You stumble around the room until you starve.")start() # 调用函数 start() 游戏开始

3、这个 gold_room 让你输入数字的方式有点奇怪。这样做有哪些 bug ？你能改善我的代码吗？可以查查看 int() 的相关知识。
这里用了一个判断输入的数字是否含1或者0来检查是否输入的是数字，很明显的bug就是避开0和1的数字会被当作非数字来执行。
int（）相关知识
描述
int() 函数用于将一个字符串或数字转换为整型。
语法
以下是 int() 方法的语法:
class int(x, base=10)
参数
x – 字符串或数字。
base – 进制数，默认十进制。
返回值
返回整型数据。
参考链接：https://www.runoob.com/python/python-func-int.html

3.3.3 常见问题

1、exit(0) 是干什么用的？
在很多操作系统中，一个程序可以用 exit(0) 来结束，其中传入的数字代表是否有错误。如果你用 exit(1) 代表有 1 个错误， exit(0) 则代表程序正常退出。它不同于通常的布尔逻辑（0==False），因为你可以用不同的数字来表示不同的错误结果。你可以用 exit(100) 来表示与 exit(2) 或者 exit(1) 不同的错误结果。
2、为什么 input() 有时会被写成 input(’> ')？
input 的参数是一个字符串，所以要在获取用户输入的内容前面加一个提示符。这里 > 也可以换成想要提示用户的文字。

四、总结

Task06 主要练习了For、If以及While语句，重点需要掌握的有：
1、elif 和 else 的作用与区别.
2、If的嵌套使用，以及尝试自己编写脚本或修复其他人的代码（这也用到了task5中防御性编程的思想）。
3、while-loop和for-loop用法
4、当一个循环运行的时候，它会过一遍代码块，到结尾之后再跳到顶部。为了直观表现这个过程，可以用 print 打印出循环的整个过程，把 print 行写在循环的前面、顶部、中间、结尾。研究一下输出的内容，试着理解它是如何运行的。
5、在每行代码下面写下注释，弄清楚这一行是做什么的，就很容易明白。确保注释和代码一样简洁。