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第四章操作列表（for循环）

2021-07-05 13:32:30 作者：互联网

第四章操作列表

4.1遍历整个列表

使用for循环打印

将列表heros的元素与he（随便取一个变量）相关联，打印赋给变量he的内容

heros = ['zhangsan','yaso','mangzai']
for he in heros:
    print(he)

zhangsan
yaso
mangzai

4.1.1 深入循环研究

for he in heros:

这行代码让python获取列表heros中的第一个值‘zhangsan’，并将其与变量’he’相关联，接下来读取下一行代码

    print(he)

让打印’he’的值，依然是’zhangsan’,该列表还包含其他值，python返回到循环的第一行

for he in heros:
    print(he)  # 然后获取第二个元素关联'he'，再打印，以此类推

循环中有多少个元素就执行多少次循环，直到最后一个元素
至于变量也是可以随便定名称，但是用单数和双数区分可以更有可读性：
for cat in cats: for message in messages

4.1.2 在for循环中执行更多操作

附加消息

heros = ['zhangsan','yaso','mangzai']
for hero in heros:
    print(f"{hero.title()},who is a good hero")

for hero in heros:
    print(f"{hero.title()},who is a good hero")
    print(f"I can't wait to see your next performance,{hero.title()}.\n")

4.1.3在for循环结束后执行一些操作

for hero in heros:
    print(f"{hero.title()},who is a good hero")
    print(f"I can't wait to see your next performance,{hero.title()}.\n")
print("Thank you,everyone, that was a good game show!")   # 主要看缩进

看缩进辨别是否在循环里，有缩进的在循环里，没有的是全局的语句

4.2避免缩进错误

python中根据缩进来判断代码行与前一个代码行的关系

4.2.1忘记缩进

对于for循环的组成代码行必须缩进

heros = ['zhangsan','yaso','mangzai']
for hero in heros:
print(hero)   # expected an indented block(应为缩进块)

4.2.2忘记缩进额外的代码行

忘了某一行

for hero in heros:
    print(f"{hero.title()},who is a good hero")
print(f"I can't wait to see your next performance,{hero.title()}.\n")

这是一个逻辑错误，语法合理，但结果不符合预期

4.2.3不必要的缩进

message = 'Hello world'
    print(message)   # unexpected indent(不被期望的缩进)

4.2.4循环后不必要的缩进

for hero in heros:
    print(f"{hero.title()},who is a good hero")
    print(f"I can't wait to see your next performance,{hero.title()}.\n")
    print("Thank you,everyone, that was a good game show!")

逻辑错误

4.2.5 遗漏了冒号（重要）

for那一行末尾要跟冒号：

for hero in heros     # invalid syntax（无效语法）
    print(f"{hero.title()},who is a good hero")

4.2.6练习

4-1 披萨

pizzas = ['Sausage pizza','Cheese pizza','Corn pizza']
for pizza in pizzas:
    print(pizza)
    print(f"i love {pizza}.\n")
print("I really like pizza!")

4-2 动物

dogs = ['柯基','二哈','拉布拉多','牧羊犬']
for dog in dogs:
    print(f"{dog}喜欢吃骨头")
print("他们都是很好的宠物狗")

4.3创建数值列表

4.3.1 使用函数`range()`

生成数据

for num in range(1,5):  # 1开始，5截至（不包括5）
    print(num)

range只打印了1-4，由于编程语言中常见的差一行为的结果，函数rangr（）指定第一个数开始，第二个数处停止

所以要输出1~5得用range(1,6)

for num in range(1,6):
    print(num)

调用`range()`时，也可指定一个参数，这样它将从0开始。

for num in range(5):  # 从0开始五个数
    print(num)

4.3.2 使用range()创建数字列表

可以用list()函数直接将range（）的结果转换为列表

number = list(range(5))
print(number)

[0, 1, 2, 3, 4]

range()函数还可以指定步长range(start,stop,step)

num = list(range(5,18,3))  # 从5开始，每次+3，到18结束
print(num)

[5, 8, 11, 14, 17]

用range()可以创建任何需要的数集，活用for，列表操作的函数，增加删除等

如：创建由前十个数平方组成的列表

squares = []   # 创个空列表，到时候用append添加
for value in range(1,11):
    square = value ** 2
    squares.append(square)   # 每次的结果都添加到列表中，用向末尾加的方法
    
print(squares)

[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

    squares.append(value ** 2)   #简洁写法

4.3.3 对数字列表进行简单的统计计算

函数：min(),sum(),max()

digits = []
for num in range(10):
    digits.append(num)
print(digits)
max(digits)

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]





9

min(digits)

sum(digits)

4.3.4 列表解析

将for循环和创建新元素的代码合成一行，并自动附加新元素

格式：list = [要存数据的表达式 for value in range()] 注意这里的for循环没有冒号

squares = [value ** 2 for value in range(1,10)]  # 没有冒号
print(squares)

[1, 4, 9, 16, 25, 36, 49, 64, 81]

4.3.5 练习

4-3 数到20

for num in range(1,21):
    print(num)

练习4-4：一百万

million = [value for value in range(1,10001)] # 100万太大输不出来
print(million)

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, ......]

4-5 100万求和

million = [value for value in range(1,1000001)]
print(min(million))
print(max(million))
print(sum(million)) # 秒出

1
1000000
500000500000

4-6 奇数

odd_num = list(range(1,20,2))
for o in odd_num:
    print(o)

4-7 3的倍数

three = list(range(3,31,3))
for num in three:
    print(num)

4-8 立方

cube = [num ** 3 for num in range(1,11)]
for value in cube:
    print(value)

4.4 使用列表的一部分(切片)

4.4.1 切片

要创建切片，可指定要使用的起始元素和终止元素

一般就是[a,b]，a是访问起始元素-1，b就是终止元素，如访问3.4个【2：4】

players = ['A','B','C','D','E']
print(players[0:3]) # 输出第1~3个

['A', 'B', 'C']

# 指定输出第3，4个元素
print(players[2:4])

['C', 'D']

不指定起始默认从表头开始

# 不指定起始默认从表头开始
print(players[:4])

['A', 'B', 'C', 'D']

# 要让切片终止于末尾，同样操作
print(players[2:])

['C', 'D', 'E']

# 上章的负索引也是同样，如输出后三个元素
print(players[-3:])

['C', 'D', 'E']

# 还可以指定步长，如访问第2到5个元素，间隔为2
print(players[1:5:2])

['B', 'D']

4.4.2 遍历切片

players = ['A','B','C','D','E']
print("Here are the first three players on my team")

for memb in players[:3]:
    print(memb)

Here are the first three players on my team
A
B
C

4.4.3 复制列表

创建一个包含整个列表的切片[:]通过不加起始和终止索引

my_foods = ['noodles','cake','pizza','bread']
friend_foods = my_foods[:]

print("My favorite foods are:")
print(my_foods)

print("\nMy friend's favorite foods are:")
print(friend_foods)

My favorite foods are:
['noodles', 'cake', 'pizza', 'bread']

My friend's favorite foods are:
['noodles', 'cake', 'pizza', 'bread']

# 核实是否是两个列表，分别添加东西
my_foods.append('water')
friend_foods.append("cola")

print(my_foods)
print(friend_foods)

['noodles', 'cake', 'pizza', 'bread', 'water']
['noodles', 'cake', 'pizza', 'bread', 'cola']

复制必须切片，不切片直接=，会导致两个代表同一个列表，对两个表的操作其实是在操作用一个表

my_foods = ['noodles','cake','pizza','bread']
my_foods = friend_foods
my_foods.append('water')
friend_foods.append("cola")

print(my_foods)
print(friend_foods)

['noodles', 'cake', 'pizza', 'bread', 'cola', 'water', 'cola']
['noodles', 'cake', 'pizza', 'bread', 'cola', 'water', 'cola']

4.4.4 练习

4-10切片

players = ['noodles', 'cake', 'pizza', 'bread', 'cola', 'water', 'cola']

print("The first three items in the list are:")
print(players[:3])

print("\nThree items from the middle of the list are:")
print(players[2:5])

print("\nThe last three items in the list are:")
print(players[-3:])

The first three items in the list are:
['noodles', 'cake', 'pizza']

Three items from the middle of the list are:
['pizza', 'bread', 'cola']

The last three items in the list are:
['cola', 'water', 'cola']

4-11 你的皮萨，我的披萨

my_pizzas = ['Sausage pizza','Cheese pizza','Corn pizza']
friend_pizzas = my_pizzas[:]

my_pizzas.append("beef pizza")
friend_pizzas.append("cola pizza")

print("my favorite pizzas are:")
for pizza in my_pizzas:
    print(f"- {pizza}")

print("\nMy friend's favorite pizzas are:")
for pizza in friend_pizzas:
    print(f"- {pizza}")

my favorite pizzas are:
- Sausage pizza
- Cheese pizza
- Corn pizza
- beef pizza

My friend's favorite pizzas are:
- Sausage pizza
- Cheese pizza
- Corn pizza
- cola pizza

4.5 元组

⭐列表非常适合用于存储在程序运行期间可能变化的数据集。列表是可修改的

⭐元组则是不可变的列表。Python将不能修改的值称为不可变的

4.5.1 定义元组

元组看起来像列表，但用（）圆括号而不是[]方括号，其他的索引，遍历和列表一样

dimensions = (200,50) # 两个元素的元组
print(dimensions[0])
print(dimensions[1])

200
50

# 尝试修改
dimensions = (200,50)
dimensions[0] = 255 #'tuple'(元组) object does not support item assignment不支持赋值

---------------------------------------------------------------------------

TypeError                                 Traceback (most recent call last)

<ipython-input-30-99dbc19ec1e2> in <module>
      1 # 尝试修改
      2 dimensions = (200,50)
----> 3 dimensions[0] = 255 #'tuple'(元组) object does not support item assignment不支持赋值


TypeError: 'tuple' object does not support item assignment

注意：严格的说，元组是由逗号标识的，（）只是为了美观。如果只要定义一个元素的元组，必须在这个元素后面加上逗号：
my_t = (3,)

# 试验
neirong = (1,2)
hao = (3,)
women = 4,5,7  # 不加括号也没有问题

print(neirong)
print(hao)
print(women[1])
print(women)

(1, 2)
(3,)
5
(4, 5, 7)

4.5.2遍历元组中的所有值

num = (4,7,8,9)
for v in num:
    print(v)

4.5.3修改元组中的变量

虽然不能改变元组的元素，但能给这些元素重新赋值来改变

num = (4,7,8,9)
print("original num:")
for v in num:
    print(v)
    
num = (2,5,3,3)
print("modified num:")
for v in num:
    print(v)

original num:
4
7
8
9
modified num:
2
5
3
3

4.5.4 练习

4-13 自助餐

foods = ('noodles', 'cake', 'pizza', 'bread', 'cola')
for con in foods:
    print(con)

noodles
cake
pizza
bread
cola

foods[1] = 'xuebi'

---------------------------------------------------------------------------

TypeError                                 Traceback (most recent call last)

<ipython-input-35-378c38c533e1> in <module>
----> 1 foods[1] = 'xuebi'


TypeError: 'tuple' object does not support item assignment

foods = ('noodles', 'zhou', 'milk', 'bread', 'cola')
for new in foods:
    print(new)

4.6 设置代码格式

⭐缩进：每级缩进都用四个空格,一般文字文档用制表符，要防止制表符和空格混乱

⭐每行最好不要超过80字符

⭐空行在不同操作间用，空行不会影响运行，一般只用一行，不用多行

标签：num,range,列表,foods,循环,print,hero,第四章,pizza
来源： https://blog.csdn.net/xuezhale/article/details/118487761

第四章 操作列表（for循环）

第四章 操作列表