本文共 3726 字，大约阅读时间需要 12 分钟。

笔记29 笨办法学python练习38 列表前的for与while简略比较

这个习题38，在讲述列表操作之前，特地提到了两个条件操作符号，一个是while，另一个是for。这两个仍然是让我感到困惑的，先温习温习，再来看列表吧。
While是一个有点难以理解的概念，它和for一样，也被称为循环指令。但这个循环的条件是，循环语句中的布尔值为true。它和for的区别何在呢？
相同之处大概在于循环，循环的对象常常就是一个列表中陈列的对象，所以这两个指令都和列表关系密切。还是把书中的例题稍加改造，在本文中先对比一下它们，然后概括出两者间的简单区别吧。
使用for-loop的习题ex38.0.py

在这里插入代码片cities = ['guangzhou', 'wuhan', 'shenzhen', 'nanjing', 'shenyang']persons = ['zhang', 'wang', 'liu', 'li', 'chen']pairs = ['guangzhou', 'one', 'wuhan', 'second', 'shenzhen', 'third', 'nanjing', 'forth']# this first kind for-loop goes through a city listfor city in cities:    print(f"This is a large city {cities}:")# same as above for loop personsfor name in persons:    print(f"A name of somebody: {persons}")# also we can go through mixed lists too, one to one# notice we have to use {} since we don't know what's in itfor i in pairs:    print(f"I get {i}")# we can also build lists, first start with an empty oneempty_elements = []# then use the range function to do 5 to 10 countsfor i in range(5, 10):    print(f"Adding {i} to the list.")    # append is a function that lists Understand    empty_elements.append(i)# now we can print them out toofor i in empty_elements:    print(f"Element was: {i}")

运行结果在这里插入代码片PS C:\Users\lenovo> cd 1pythonw
PS C:\Users\lenovo\1pythonw> python ex38.0.py
This is a large city [‘guangzhou’, ‘wuhan’, ‘shenzhen’,
This is a large city [‘guangzhou’, ‘wuhan’, ‘shenzhen’,
This is a large city [‘guangzhou’, ‘wuhan’, ‘shenzhen’,
This is a large city [‘guangzhou’, ‘wuhan’, ‘shenzhen’,
This is a large city [‘guangzhou’, ‘wuhan’, ‘shenzhen’,
A name of somebody: [‘zhang’, ‘wang’, ‘liu’, ‘li’, 'chen
A name of somebody: [‘zhang’, ‘wang’, ‘liu’, ‘li’, 'chen
A name of somebody: [‘zhang’, ‘wang’, ‘liu’, ‘li’, 'chen
A name of somebody: [‘zhang’, ‘wang’, ‘liu’, ‘li’, 'chen
A name of somebody: [‘zhang’, ‘wang’, ‘liu’, ‘li’, 'chen
I get guangzhou
I get one
I get wuhan
I get second
I get shenzhen
I get third
I get nanjing
I get forth
Adding 5 to the list.
Adding 6 to the list.
Adding 7 to the list.
Adding 8 to the list.
Adding 9 to the list.
Element was: 5
Element was: 6
Element was: 7
Element was: 8
Element was: 9

使用while的习题ex38.1.py

在这里插入代码片zhang = input("> ")if zhang == "0":    print("begin to while")i = 1numbers = []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)

运行结果

在这里插入代码片PS C:\Users\lenovo\1pythonw> python ex38.1.py> 0begin to whileAt the top i is 1Numbers now:  [1]At the bottom i is 2At the top i is 2Numbers now:  [1, 2]At the bottom i is 3At the top i is 3Numbers now:  [1, 2, 3]At the bottom i is 4At the top i is 4Numbers now:  [1, 2, 3, 4]At the bottom i is 5At the top i is 5Numbers now:  [1, 2, 3, 4, 5]At the bottom i is 6The numbers:12345

单纯留下while指令形成ex38.2.py，也运行顺利。
练习ex38.2.py

在这里插入代码片zhang = input("> ")if zhang == "0":    print("begin to while")i = 1numbers = []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: ")

运行结果

在这里插入代码片PS C:\Users\lenovo\1pythonw> python ex38.2.py> 0begin to whileAt the top i is 1Numbers now:  [1]At the bottom i is 2At the top i is 2Numbers now:  [1, 2]At the bottom i is 3At the top i is 3Numbers now:  [1, 2, 3]At the bottom i is 4At the top i is 4Numbers now:  [1, 2, 3, 4]At the bottom i is 5At the top i is 5Numbers now:  [1, 2, 3, 4, 5]At the bottom i is 6The numbers:PS C:\Users\lenovo\1pythonw>

两者的简单比较：
这两种循环都是把列表中的元素用同样的操作进行一次，但如同在练习33中所看到的，while循环有可能无法终止，一直进行，for好像没有这个问题。而且就我的理解，你把for循环可以并列列出多个也容易操作运行，但如果while的指令似乎就不能够做到这样。
继续看这个ex38练习。