一、作业内容（习题27-34）：主要学习了逻辑关系、布尔表达式、if语句、if、else、elif运行条件、while及for循环、读取列表
二、习题
习题27：记住逻辑关系
计算机所谓的逻辑就是判断true或false：
and 与
or 或
not 非
！= 不等于
== 等于
= 大于等于
<=小于等于
True 真
Fasle假
程序运行时检查字符或者变量组合在一起表达的结果是真或假，逻辑关系的优先顺序中的and、or、not是同级，谁在前先运行谁，但有（）时，（）内容优先。通过简单的口诀可以明确逻辑关系的判断：非真即假，非假即真，非0即真。
习题28：布尔表达式
print(True and True)
print(False and False)
print(1==1 and 2==1)
print("test" == "test")
print(1==1 or 2!=1)
print(True and 1 == 1)
print(False and 0 != 0)
print(True or 1 == 1)
print("test" == "testing")
print(1 != 0 and 2 == 1)
print("test" != "testing")
print("test" == 1)
print(not (True and False))
print(not (1 == 1 and 0 != 1))
print(not (10 == 1 or 1000 == 1000))
print(not (1 != 10 or 3 == 4))
print(not ("testing" == "testing" and "Zed" == "Cool Guy"))
print(1 == 1 and not ("testing" == 1 or 1 == 0))
print("chunky" == "bacon" and not (3 == 4 or 3 == 3))
print(3 == 3 and not ("testing" == "testing" or "Python" == "Fun"))
print(3 != 4 and not ("testing" != "test" or "python" == "python"))
结果：
True
False
False
True
True
True
False
True
False
False
True
False
True
False
False
False
True
True
False
False
False
根据Python的逻辑表达式，在练习中可以思考并准确判断布尔表达式的运行结果是“真”还是“假”。
习题29：if语句
people = 34
cats = 20
dogs = 40
if 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 += 5
# if 表示如果满足if后的条件，则在运行后打印下一行代码
if 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.")
结果
C:\ProgramData\Anaconda3\python.exe E:/pyproject/ex29.py
Not many cats! The world is saved!
The world is drooled on!
People are less than or equal to dogs.
Process finished with exit code 0
如果满足if 语句，则执行if 语句条件下的命令行；如果不满足if语句，则执行else后的命令行。if语句执行自上往下，某个判断为“真”，则执行该判断下对应的语句；当不满足是，就继续else或elif后对应的语句。
习题30：Else和if
people = 30
cars = 40
buses = 15
if 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 buses > cars:
print("That's too many buses.")
elif buses < cars:
print("Maybe we could take the buses.")
else:
print("We still can't decide.")
if people > buses:
print("Alright, let's just take the buses.")
else:
print("Fine, let's stay home then.")
结果
We should take the cars.
Maybe we could take the buses.
Alright, let's just take the buses.
习题31. 作出决定
if， else和elif 可以用与创建程序的分支，当if语句内部再放一个if语句，可以用来创建嵌套的决定。
print("You enter a dark room with two doors. Do you go through door #1 or door #2?")
door = input(2)
if door == "1":
print("there's a giant bear here eating a cheese cake. What do you do?")
print("1. Take the cake.")
print("2. Scream at the bear.")
bear = input(3)
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("Well, doing %s is probably better. Bear runs away." % bear)
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(2)
if insanity == "1" or insanity == "2":
print("Your body survives powered by a mind of jello. Good job!")
else:
print("The insanity rots your eyes into a pool of muck. Good job!")
else:
print("You stumble around and fall on a knife and die. Good job!")
结果：
You enter a dark room with two doors. Do you go through door #1 or door #2?
21
there's a giant bear here eating a cheese cake. What do you do?
1. Take the cake.
2. Scream at the bear.
23
Well, doing 3 is probably better. Bear runs away.
这里采用了input(), 在Python3 中赋值需要在键盘操作，在run区域用键盘输入数值。
习题32 循环和列表
for也可用于表示循环，并且循环的结果最好使用列表（list），列表相当于用按循环顺序存放东西的容器，列表需要与一些新的语法结合使用。
the_count = [1, 2, 3, 4,5]
fruits = ["apple", 'oranges', 'pears', 'apricots']
change = [1, 'pennies', 2, 'dimes', 3, 'quarters']
# this first kind of for-loop goes through a list
for number in the_count:
print("This is count %d" % number)
# same as above
for fruit in fruits:
print("A fruit of types: %s" % fruit)
# also we can go through mixed lists too
# notice we have to use %r since we don't know what's in it
for i in change:
print("I got %r" % i)
# we can also build lists, first start with an empty one
elements = []
# then use the range function to do 0 to 5 counts
for i in range (0, 6):
print("Adding %d to the list." %i)
# append is a function that lists understand
elements.append(i)
# now we can print them out too
for i in elements:
print("Element was: %d" % i)
结果：
This is count 1
This is count 2
This is count 3
This is count 4
This is count 5
A fruit of types: apple
A fruit of types: oranges
A fruit of types: pears
A fruit of types: 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.
Element was: 0
Element was: 1
Element was: 2
Element was: 3
Element was: 4
Element was: 5
注意：range()函数会从第一个数到最后一个，但不包含最后一个数字，即为含首不含尾的方式来循环for语句。
习题33：While循环
注意：while会一直执行它下面的代码片段，直到它对应的布尔表达式为False时才会停下来。while循环也是去检查布尔表达式的真假，执行完后再调回while所在位置，直到while表达式为False为止，因此，在Python代码中，大部分时候用for循环更好一些。
i = 1
numbers = []
while i < 9:
print("At the top i is %d" % i)
numbers.append(i)
i = i + 3
print("Numbers now:", numbers)
print("At the bottom i is %d" % i)
结果：
At the top i is 1
Numbers now: [1]
At the bottom i is 4
At the top i is 4
i = 1
numbers = []
while i < 6:
print("At the top i is %d" % i)
numbers.append(i)
i = i + 1
print("Numbers now:", numbers)
print("At the bottom i is %d" % i)
print("The numbers:")
for num in numbers:
print(num)
while i < 4:
print("At the top i is %d" % i)
numbers.append(i)
i = i + 1
print("Numbers now:", numbers)
print("At the bottom i is %d" % i)
print("The numbers:")
for num in numbers:
print(num)
while i < 9:
print("At the top i is %d" % i)
numbers.append(i)
i = i + 3
print("Numbers now:", numbers)
print("At the bottom i is %d" % i)
结果：
C:\ProgramData\Anaconda3\python.exe E:/pyproject/ex33.py
At the top i is 1
Numbers now: [1]
At the bottom i is 2
At the top i is 2
Numbers now: [1, 2]
At the bottom i is 3
At the top i is 3
Numbers now: [1, 2, 3]
At the bottom i is 4
At the top i is 4
Numbers now: [1, 2, 3, 4]
At the bottom i is 5
At the top i is 5
Numbers now: [1, 2, 3, 4, 5]
At the bottom i is 6
The numbers:
1
2
3
4
5
The numbers:
1
2
3
4
5
At the top i is 6
Numbers now: [1, 2, 3, 4, 5, 6]
At the bottom i is 9
Process finished with exit code 0
习题34：访问列表的元素
在列表中有序数和基数之分，Python列表是从0开始，因此列表中的元素以0开头索引。序数=基数+1，在Python中需要用基数表示，它意味着抓取任意元素，所以需要0元素。
animals = ['bear', 'python', 'peacock', 'kangaroo', 'whale', 'platypus']
print("The animal at 0 is the 1st and is", animals[0])
print("The animal at 3 is the 4th and is", animals[3])
print("The animal at 5 is the 6th and is", animals[5])
结果：
The animal at 0 is the 1st and is bear
The animal at 3 is the 4th and is kangaroo
The animal at 5 is the 6th and is platypus
