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新手学python-笨方法学python-ex33-While 循环

跟着笨方法学python个教程学习，边学边做习题边记录。

新手学python-笨方法学python-ex33-While 循环 - 学习记录

课程内容：

接下来是一个更在你意料之外的概念： while-loop``（while 循环）。``while-loop 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。

等等，你还能跟得上这些术语吧？如果你的某一行是以 : （冒号, colon）结尾，那就意味着接下来的内容是一个新的代码片段，新的代码片段是需要被缩进的。只有将代码用这样的方式格式化，Python 才能知道你的目的。如果你不太明白这一点，就回去看看“if 语句”和“函数”的章节，直到你明白为止。

接下来的练习将训练你的大脑去阅读这些结构化的代码。这和我们将布尔表达式烧录到你的大脑中的过程有点类似。

回到 while 循环，它所作的和 if 语句类似，也是去检查一个布尔表达式的真假，不一样的是它下面的代码片段不是只被执行一次，而是执行完后再调回到 while 所在的位置，如此重复进行，直到 while 表达式为 False 为止。

While 循环有一个问题，那就是有时它会永不结束。如果你的目的是循环到宇宙毁灭为止，那这样也挺好的，不过其他的情况下你的循环总需要有一个结束点。

为了避免这样的问题，你需要遵循下面的规定：

1.尽量少用while-loop，大部分时候for-loop是更好的选择。
2.重复检查你的while语句，确定你测试的布尔表达式最终会变成False。
3.如果不确定，就在while-loog的结尾打印出你要测试的值。看看它的变化。

在这节练习中，你将通过上面的三样事情学会while-loop：


i = 0
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

 

 

运行该程序：

$python ex33.py
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

$

 

加分习题:
1.将这个 while 循环改成一个函数，将测试条件(i < 6)中的 6 换成一个变量。

ex33_1.py

def while_list(number):
    i = 0
    numbers = []
    
    while i < number:
        print "At the top i is %d" %i
        numbers.append(i)
    
        i += 1
        print "Numbers now: ", numbers
        print "At the bottom i is %d" % i

调用该函数试试看：

 

>>> import ex33_1
>>> ex33_1.while_list(1)
At the top i is 0
Numbers now:  [0]
At the bottom i is 1
>>> ex33_1.while_list(2)
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
>>> ex33_1.while_list(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
>>>

2.使用这个 函数重写你的脚本，并用不同的数字进行测试。

ex33_2.py


def while_list(number):
    i = 0
    numbers = []
    
    while i < number:
        print "At the top i is %d" % i
        numbers.append(i)
        
        i += 1
        print "Numbers now: ", numbers
        print "At the bottom i is %d" % i
    
    print "The numbers: "
    for num in numbers:
        print num

nums = int(raw_input("Pls input an int> "))
while_list(nums)

 


3.为函数添加另外一个参数，这个参数用来定义第 8 行的加值 + 1 ，这样你就可以让它任意加值了。

ex33_3.py

def while_lists(number, adds):
    i = 0
    numbers = []
    
    while i < number:
        print "At the top i is %d" % i
        numbers.append(i)
        
        i = i + adds
        print "Numbers now: ", numbers
        print "At the bottom i is %d" % i


调用该函数试试看：

 

>>> import ex33_3
>>> ex33_3.while_lists(10, 2)
At the top i is 0
Numbers now:  [0]
At the bottom i is 2
At the top i is 2
Numbers now:  [0, 2]
At the bottom i is 4
At the top i is 4
Numbers now:  [0, 2, 4]
At the bottom i is 6
At the top i is 6
Numbers now:  [0, 2, 4, 6]
At the bottom i is 8
At the top i is 8
Numbers now:  [0, 2, 4, 6, 8]
At the bottom i is 10
>>>

4.再使用该函数重写一遍这个脚本。看看效果如何。
ex33_4.py

def while_lists(number, adds):
    i = 0
    numbers = []
    
    while i < number:
        print "At the top i is %d" % i
        numbers.append(i)
        
        i = i + adds
        print "Numbers now: ", numbers
        print "At the bottom i is %d" % i
        
    print "The numbers: "
    
    for num in numbers:
        print num

while_lists(20, 3)


运行该程序：

$python ex33_4.py
At the top i is 0
Numbers now:  [0]
At the bottom i is 3
At the top i is 3
Numbers now:  [0, 3]
At the bottom i is 6
At the top i is 6
Numbers now:  [0, 3, 6]
At the bottom i is 9
At the top i is 9
Numbers now:  [0, 3, 6, 9]
At the bottom i is 12
At the top i is 12
Numbers now:  [0, 3, 6, 9, 12]
At the bottom i is 15
At the top i is 15
Numbers now:  [0, 3, 6, 9, 12, 15]
At the bottom i is 18
At the top i is 18
Numbers now:  [0, 3, 6, 9, 12, 15, 18]
At the bottom i is 21
The numbers:
0
3
6
9
12
15
18

$

 

5.接下来使用 for-loop 和 range 把这个脚本再写一遍。你还需要中间的加值操作吗？如果你不去掉它，会有什么样的结果？

ex33_5.py

numbers = []
for i in range(6):
    print "At the top i is %d" % i
    numbers.append(i)
    
    print "Numbers now: ", numbers
    print "At the bottom i is %d" % i

print "The numbers: "

for num in numbers:
    print num

   

运行该 程序：


$python ex33_5.py
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

$

 

 


6.很有可能你会碰到程序跑着停不下来了，这时你只要按着 CTRL 再敲 c (CTRL-c)，这样程序就会中断下来了。


至此，-ex33-While 循环这一小节的学习结束了。
Tags:
Saturday, November 14, 2015 | Python

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