Python基础课程-for循环

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')

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('')

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 == '':continuecleaned_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')breakcleaned_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)

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