python快速入门【三】-----For 循环、While 循环

python入门合集：

python快速入门【一】-----基础语法

python快速入门【二】----常见的数据结构

python快速入门【三】-----For 循环、While 循环

python快速入门【四】-----各类函数创建

python快速入门【五】---- 面向对象编程

python快速入门【六】----真题测试

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For 循环

For循环是迭代对象元素的常用方法（在第一个示例中，列表）

具有可迭代方法的任何对象都可以在for循环中使用。

python的一个独特功能是代码块不被{} 或begin，end包围。相反，python使用缩进，块内的行必须通过制表符缩进，或相对于周围的命令缩进4个空格。

虽然这一开始可能看起来不直观，但它鼓励编写更易读的代码，随着时间的推移，你会学会喜欢它

In [1]

#取第一个列表成员（可迭代），暂称它数字（打印它）
#取列表的第二个成员（可迭代），暂时将其称为数字，等等......

for number in [23, 41, 12, 16, 7]:
    print(number)
print('Hi')

23
41
12
16
7
Hi

枚举

返回一个元组，其中包含每次迭代的计数（从默认为0开始）和迭代序列获得的值：

In [2]

friends = ['steve', 'rachel', 'michael', 'adam', 'monica']
for index, friend in enumerate(friends):
    print(index,friend)

0 steve
1 rachel
2 michael
3 adam
4 monica

Task

从文本中删除标点符号并将最终产品转换为列表:

On a dark desert highway, cool wind in my hair Warm smell of colitas, rising up through the air Up ahead in the distance, I saw a shimmering light My head grew heavy and my sight grew dim I had to stop for the night There she stood in the doorway; I heard the mission bell And I was thinking to myself, "This could be Heaven or this could be Hell" Then she lit up a candle and she showed me the way

(加州旅馆)

In [3]

text = '''On a dark desert highway, cool wind in my hair Warm smell of colitas, rising up through the air Up ahead in the distance, I saw a shimmering light My head grew heavy and my sight grew dim I had to stop for the night There she stood in the doorway; I heard the mission bell And I was thinking to myself, "This could be Heaven or this could be Hell" Then she lit up a candle and she showed me the way'''

In [4]

print(text)

On a dark desert highway, cool wind in my hair Warm smell of colitas, rising up through the air Up ahead in the distance, I saw a shimmering light My head grew heavy and my sight grew dim I had to stop for the night There she stood in the doorway; I heard the mission bell And I was thinking to myself, "This could be Heaven or this could be Hell" Then she lit up a candle and she showed me the way

基本上，任何具有可迭代方法的对象都可以在for循环中使用。即使是字符串，尽管没有可迭代的方法 - 但我们不会在这里继续。具有可迭代方法基本上意味着数据可以以列表形式呈现，其中有序地存在多个值。

In [5]

for char in '-.,;\n"\'':
    text = text.replace(char,' ')
print(text)

On a dark desert highway  cool wind in my hair Warm smell of colitas  rising up through the air Up ahead in the distance  I saw a shimmering light My head grew heavy and my sight grew dim I had to stop for the night There she stood in the doorway  I heard the mission bell And I was thinking to myself   This could be Heaven or this could be Hell  Then she lit up a candle and she showed me the way

In [6]

# Split converts string to list.
# Each item in list is split on spaces
text.split(' ')[0:20]

['On',
 'a',
 'dark',
 'desert',
 'highway',
 '',
 'cool',
 'wind',
 'in',
 'my',
 'hair',
 'Warm',
 'smell',
 'of',
 'colitas',
 '',
 'rising',
 'up',
 'through',
 'the']

In [7]

# Dont want to have non words in my list for example ''
# which in this case are things of zero length
len('')

0

In [8]

# Making new list with no empty words in it
cleaned_list = []

In [9]

for word in text.split(' '):
    word_length = len(word)
    if word_length > 0:
        cleaned_list.append(word)

In [10]

cleaned_list[0:20]

['On',
 'a',
 'dark',
 'desert',
 'highway',
 'cool',
 'wind',
 'in',
 'my',
 'hair',
 'Warm',
 'smell',
 'of',
 'colitas',
 'rising',
 'up',
 'through',
 'the',
 'air',
 'Up']

Continue

continue语句将转到循环的下一次迭代

continue语句用于忽略某些值，但不会中断循环

In [11]

cleaned_list = []

for word in text.split(' '):
    if word == '':
        continue
    cleaned_list.append(word)
cleaned_list[1:20]

['a',
 'dark',
 'desert',
 'highway',
 'cool',
 'wind',
 'in',
 'my',
 'hair',
 'Warm',
 'smell',
 'of',
 'colitas',
 'rising',
 'up',
 'through',
 'the',
 'air',
 'Up']

Break

break语句将完全打断循环

In [12]

cleaned_list = []

In [13]

for word in text.split(' '):
    if word == 'desert':
        print('I found the word I was looking for')
        break
    cleaned_list.append(word)
cleaned_list

I found the word I was looking for

['On', 'a', 'dark']

Task (顺道介绍一下Range函数)

1. 编写一个Python程序，它迭代整数从1到50（使用for循环）。对于偶数的整数，将其附加到列表even_numbers。对于奇数的整数，将其附加到奇数奇数列表中

In [14]

# Making empty lists to append even and odd numbers to.
even_numbers = []
odd_numbers = []

for number in range(1,51):
    if number % 2 == 0:
        even_numbers.append(number)
    else:
        odd_numbers.append(number)    

In [15]

print("Even Numbers: ", even_numbers)

Even Numbers:  [2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50]

In [16]

print("Odd Numbers: ", odd_numbers)

Odd Numbers:  [1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, 49]

Python 2 vs Python 3 (Range函数的不同点)

python 2 xrange and python 3 range are same (resembles a generator) python 2 range生成一个list

注意: 较长的列表会很慢

更多参考: http://pythoncentral.io/pythons-range-function-explained/

While 循环

For 循环	While 循环
遍历一组对象	条件为false时自动终止
没有break也可以结束	使用break语句才能退出循环

如果我们希望循环在某个时刻结束，我们最终必须使条件为False

In [1]

# Everytime through the loop, it checks condition everytime until count is 6
# can also use a break to break out of while loop.
count = 0
while count <= 5:
    print(count)
    count = count + 1

0
1
2
3
4
5

break语句

使用break可以完全退出循环

In [2]

count = 0
while count <= 5:
    if count == 2:
        break
    count += 1
    print (count)

1
2

while True条件使得除非遇到break语句，否则不可能退出循环

如果您陷入无限循环，请使用计算机上的ctrl + c来强制终止

In [3]

num = 0
while True:
    if num == 2:
        print('Found 2')
        break
    num += 1
    print (num)

1
2
Found 2

提醒：使用模运算符（％），它将数字左边的余数除以右边的数字

In [4]

# 1 divided by 5 is 0 remainder 1
1 % 5

1

In [5]

# 5 divided by 5 is 0 remainder 0
5 % 5

0

比较操作符	功能
<	小于
<=	小于或等于

| 大于 = | 大于或等于 == | 等于 != | 不等于

In [6]

x = 1
while x % 5 != 0:
    x += 1
    print(x)

2
3
4
5

当我们知道要循环多少次时，Range很有用

下面例子是: 从0开始，但不包括5

In [7]

candidates = list(range(0, 5))
candidates

[0, 1, 2, 3, 4]

In [8]

while len(candidates) > 0:
    first = candidates[0]
    candidates.remove(first)
    print(candidates)

[1, 2, 3, 4]
[2, 3, 4]
[3, 4]
[4]
[]