[Python知识库]第六章——FOR、IF以及while

第六章——FOR、IF以及WHILE

1.if语句

1.1if

#if条件判断学习
people =20
cats = 30
dogs = 15
if people <cats:#条件判断人数数量是否小于猫，若小于，则执行if后面的语句（这个要输出）
    print("Too many cats! The world is doomed")
 
if people>cats:#判断人的数量是否小于猫
    print("Not many cats! The world is saved")
if people<dogs:#判断人的数量是否小于狗
    print("The world is drooled on")
if people >dogs:#判断人的数量是大于狗 这个要输出
    print("The world is dry")
    
    
dogs+=5#更新参数 目前dogs变成了20
if people>=dogs:#是等于的
    print("People are greater than or equal to dogs")
if people<=dogs:
    print("People are greater than or equal to dogs")
if people==dogs:
    print("People are dogs.")

if语句对下面的代码起一个“开关”作用，如果if语句表达式为真true,就执行下面的代码块。

if 下面的代码缩进 4 个空格是为了告诉python与当前if语句匹配的程序代码。

如果没有缩进，你很可能收到一个错误提示。Python 一般会让你在一个带 ： 的代码行下面缩进一些内容。

1.2 else和if

   people = 30
   cars = 40
   trucks = 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 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 homethen.")

1.当条件互斥时使用if-elif-else,

2.else、elif为子块，不能独立使用,一个if语句中可以包含多个elif语句，但结尾只能有一个else语句

1.3 if嵌套使用

#嵌套if语句
print("""You enyer a darker room with two doors.
Do you go through door #1 or door #2?""")#打印这个和语句
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 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 an die.Good job")

if 语句里面又放了一个 if 语句。这在创建“嵌套”（nested）

2.FOR语句

   the_count = [1, 2, 3, 4, 5]
   fruits = ['apples', '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(f"This is count {number}")

   # same as above
   for fruit in fruits:
       print(f"A fruit of type: {fruit}")
  # also we can go through mixed lists too
  # notice we have to use {} since we don't know what's in it
  for i in change:
      print(f"I got {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(f"Adding {i} to the list.")
  # append is a function that lists understand
      elements.append(i)

  # now we can print them out too
  for i in elements:
      print(f"Element was: {i}")

range(0, 6) 中的 i 只循环了6次而不是7次？ range() 函数只处理从第一个到最后一个数，但不包括最后一个数，所以它在 5就结束了。这是这类循环的通用做法

element.append() 的作用是把东西追加到列表的末尾

3.while语句

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。

    i = 0
    numbers = []

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

结果展示：

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

for-loop 和 while-loop 的区别是什么？ for-loop 只能迭代（循环）一些东西的集合，而 while-loop 能够迭代（循环）任何类型的东西。

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。

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。