Python 入门教程/for循环
好吧，在第一节关于循环的课中，我说我会等到我们学到列表后才教你for循环。现在，它来了！
基本上，for 循环对列表中的每个值执行操作。它的写法有点令人困惑，但除此之外非常简单。以下是在代码中的示例
- 代码示例 1 - for 循环
# Example 'for' loop
# First, create a list to loop through:
newList = [45, 'eat me', 90210, "The day has come, the walrus said, to speak of many things", -67]
# create the loop:
# Goes through newList, and sequentially puts each bit of information
# into the variable value, and runs the loop
for value in newList:
print(value)
如您所见，当循环执行时，它会遍历 'in' 后面提到的列表中的所有值。然后它将它们放入 value 中，并执行循环，每次 value 的值都不同。让我们再看一个例子，一个我们都知道的经典啦啦队口号
- 代码示例 2 - for 循环示例
# cheerleading program
word = input("Who do you go for? ")
for letter in word:
call = "Gimme a", letter + "!"
print(call)
print(letter + "!")
print("What does that spell?")
print(word + "!")
您刚刚学习了几件事
如您所见，字符串（记住 - 字符串是文本行）只是包含大量字符的列表。程序遍历了 word 中的每个字母（或值），并在屏幕上打印了它们。
- 代码示例 3 - 循环遍历范围
有时，您可能只需要循环特定次数（例如 10 次）。如果您来自其他编程语言，您可能正在寻找类似于此格式的普通 for 循环
for (var i = 1 ; i < 10 ; i++ ) {
}
Python 没有这种类型的循环。如果您想循环特定次数，只需使用 range()。
# The below code will print numbers from 0 through 9
for integer in range(0,10):
print(integer)
# The above code can further be simplified to:
# for integer in range(10):
# print(integer)
您必须注意几件事
- 范围从起始数字开始，一直到（但不包括）结束数字。如果您想从 0 循环到 100，您的范围函数将是 range(0,101)，因为该函数不包含结束数字。
- range 函数的参数是起始值（默认值为 0）、结束值和步长（默认值为 1）。步长只是 range() 的递增量。如果您想从 100 循环到 0，只需使用 -1 的步长。换句话说：range(100,0,-1)。
现在进入这节课的重点。让我们开始编写程序。到目前为止，我们已经学习了变量、列表、循环和函数。这几乎是我们进行大量编程所需的一切。因此，让我们给自己设定一个任务。
- 代码示例 4 - 菜单函数
# THE MENU FUNCTION
# The program asks for a string with all the menu options in it,
# and a text string asking a question.
# make sure every menu entry is unique.
def menu(list, question):
for entry in list:
print(1 + list.index(entry),)
print(") " + entry)
return input(question) - 1
# def menu(list, question): is telling the function to
# ask for two bits of information:
# A list of all the menu entries,
# and the question it will ask when all the options have been printed
# for entry in list: is pretty much saying;
#'for every entry in the list, do the following:'
# print list.index(entry) + 1 uses the .index() function to find
# where in the list the entry is in. print function then prints it
# it adds 1 to make the numbers more intelligible.
# print ") " + entry prints a bracket, and then the entry name
# after the for loop is finished, input(question) - 1 asks the question,
# and returns the value to the main program (minus 1, to turn it back to
# the number the computer will understand).
这并不难，是吗？实际的程序只占用了五行——这就是我们到目前为止所学内容的奇妙之处！我所有的注释占用了十六行——超过了程序长度的三倍。对程序进行大量注释是一个好主意。请记住，如果您要公开发布您的代码，将会有很多人查看您编写的代码。我们将在第一个示例程序中看到我们刚刚编写的函数。
我们的第一个示例程序是什么？如何做一个（非常）简单的文字冒险游戏？听起来很有趣！它只包含一个房间，并且非常简单。将有五样东西和一扇门。在五样东西中，有一把通往门的钥匙。您需要找到钥匙，然后打开门。我将先给出普通英语的版本，然后再用 Python 编写
- 代码示例 5 - 代码的普通英语版本
#Plain-english version of our 'game'
Tell the computer about our menu function
Print a welcoming message, showing a description of the room.
We will give the player six things to look at: plant, painting,\
vase, lampshade, shoe, and the door
Tell the computer that the door is locked
Tell the computer where the key is
present a menu, telling you what things you can 'operate':
It will give you the six options
It will ask the question "what will you look at?"
if the user wanted to look at:
plant:
If the key is here, give the player the key
otherwise, tell them it isn't here
painting:
same as above
etc.
door:
If the player has the key, let them open the door
Otherwise, tell them to look harder
Give the player a well done message, for completing the game.
从这里，我们可以编写一个真正的程序。准备好了吗？它来了（跳过键入注释）
- 代码示例 6 - 文字冒险游戏
#TEXT ADVENTURE GAME
#the menu function:
def menu(list, question):
for entry in list:
print(1 + list.index(entry),)
print(") " + entry)
return int(input(question)) - 1
#Give the computer some basic information about the room:
items = ["plant","painting","vase","lampshade","shoe","door"]
#The key is in the vase (or entry number 2 in the list above):
keylocation = 2
#You haven't found the key:
keyfound = 0
loop = 1
#Give some introductory text:
print("Last night you went to sleep in the comfort of your own home.")
print("Now, you find yourself locked in a room. You don't know how")
print("you got there, or what time it is. In the room you can see")
print(len(items), "things:")
for x in items:
print(x)
print("")
print("The door is locked. Could there be a key somewhere?")
#Get your menu working, and the program running until you find the key:
while loop == 1:
choice = menu(items,"What do you want to inspect? ")
if choice == 0:
if choice == keylocation:
print("You found a small key in the plant.")
print("")
keyfound = 1
else:
print("You found nothing in the plant.")
print("")
elif choice == 1:
if choice == keylocation:
print("You found a small key behind the painting.")
print("")
keyfound = 1
else:
print("You found nothing behind the painting.")
print("")
elif choice == 2:
if choice == keylocation:
print("You found a small key in the vase.")
print("")
keyfound = 1
else:
print("You found nothing in the vase.")
print("")
elif choice == 3:
if choice == keylocation:
print("You found a small key in the lampshade.")
print("")
keyfound = 1
else:
print("You found nothing in the lampshade.")
print("")
elif choice == 4:
if choice == keylocation:
print("You found a small key in the shoe.")
print("")
keyfound = 1
else:
print("You found nothing in the shoe.")
print("")
elif choice == 5:
if keyfound == 1:
loop = 0
print("You put in the key, turn it, and hear a click")
print("")
else:
print("The door is locked, you need to find a key.")
print("")
print("Light floods into the room as you open the door to your freedom.")
嗯，一个非常简单但有趣的遊戲。不要被代码量吓到，53 行只是 'if' 语句，这是最容易阅读的部分。（一旦您理解了所有缩进，您就可以制作自己的游戏，并且可以根据自己的喜好制作它，既可以简单，也可以复杂。）
您应该问的第一个问题是“这个程序能运行吗？”答案是肯定的。然后您应该问“这个程序运行得很好吗？”——不完全是。menu() 函数很棒——它减少了很多键入工作。然而，我们使用的 'while' 循环有点乱——对于一个简单的程序来说，有四级缩进。我们可以做得更好！
现在，当我们介绍类时，这将变得非常非常简单。但那必须等待。在此之前，让我们创建一个函数来减少我们的混乱。我们将传递两件事：我们选择的菜单和钥匙的位置。它将返回一件事——是否找到了钥匙。让我们看看
- 代码示例 7 - 创建检查函数
def inspect(choice,location):
if choice == location:
print("")
print("You found a key!")
print("")
return 1
else:
print("")
print("Nothing of interest here.")
print("")
return 0
现在主程序可以简化一点。让我们从 while 循环中提取它，并更改一下
- 代码示例 8 - 新游戏
while loop == 1:
keyfound = inspect(menu(items,"What do you want to inspect? "), keylocation)
if keyfound == 1:
print("You put the key in the lock of the door, and turn it. It opens!")
loop = 0
print("Light floods into the room, as you open the door to your freedom.")
现在程序变得非常短——从繁琐的 83 行缩减到非常紧凑的 50 行！当然，您会失去很多通用性——房间中的所有物品都做同样的事情。当您找到钥匙时，您会自动打开门。游戏变得不那么有趣了。它也变得更难改变。