一、遍历整个列表
1-1.假设有一个魔术师名单,需要将其中每个魔术师的名字都打印出来。
# 用for循环来打印魔术师名单中的名字 magicians=[‘alice‘,‘david‘,‘carolina‘] for magician in magicians: #这里面的magician和magicians是便于让读者得知for循环在哪个列表中进行遍历 print(magician) #输出结果: alice david carolina
解析以上代码:
a. 先定义一个列表
b. 定义一个for循环:从列表magicians中取出一个名字,并将其存储在变量magician中
c. 让python打印前面存储到变量magician中的名字
d. 重复执行for循环并打印出来
1-2. 假设有一个魔术师名单,需要将所有魔术师打印出来,并且每个名字后面打印一条消息:指出他的表演太精彩。
magicians=[‘alice‘,‘david‘,‘carolina‘] for magician in magicians: print(magician.title() + ",that was a great trick!") #输出结果: Alice,that was a great trick! David,that was a great trick! Carolina,that was a great trick!
解析以上代码:
a. 先定义一个列表
b. 定义一个for循环:从列表magicians中取出一个名字,并将其存储在变量magician中
c. 让python打印前面存储到变量magician中的名字,用title方法让每个名字第一个字母大写,并用(+)拼接后面的语句
d. 重复执行for循环并打印出来
1-3.假设有一个魔术师名单,需要将所有魔术师打印出来,并且每个名字后面打印一条消息:指出他的表演太精彩。在最后一行代码下面加一行代码,告诉每位魔术师,我们期待他的下一次表演
magicians=[‘alice‘,‘david‘,‘carolina‘] for magician in magicians: print(magician.title() + ",that was a great trick!") print("I can‘t wait to see your next trick" + magician.title() + ".\n") #输出结果: Alice,that was a great trick! I can‘t wait to see your next trickAlice. David,that was a great trick! I can‘t wait to see your next trickDavid. Carolina,that was a great trick! I can‘t wait to see your next trickCarolina.
解析以上代码:
a. 由于两条print语句都缩进了,因此它们都将针对列表中的每位魔术师执行一次
b. 第二条print语句中的换行符“\n”:在每次迭代结束后插入一个空行
1-4. for循环的结束:在第3个实例中加入一条向全体魔术师致谢的消息,感谢他们的精彩表演。
magicians=[‘alice‘,‘david‘,‘carolina‘] for magician in magicians: print(magician.title() + ",that was a great trick!") print("I can‘t wait to see your next trick" + magician.title() + ".\n") print("Thank you,everyone.That was a great magic show!") #输出结果: Alice,that was a great trick! I can‘t wait to see your next trickAlice. David,that was a great trick! I can‘t wait to see your next trickDavid. Carolina,that was a great trick! I can‘t wait to see your next trickCarolina. Thank you,everyone.That was a great magic show!
解析以上代码:
a.for循环后面,没有缩进的代码都只执行一次,而不会重复执行。第三条print语句没有缩进,因此只执行一次
二、避免缩进错误
1.忘记缩进:对于for语句后面且属于循环组成部分的代码行,一定要缩进。如果没有缩进报错:IndentationError: expected an indented block
magicians=[‘alice‘,‘david‘,‘carolina‘] for magician in magicians: print(magician.title() + ",that was a great trick!") #输出结果: IndentationError: expected an indented block
2.忘记缩进额外的代码行。如下面的代码,并没有按照1-3的需求来执行
magicians=[‘alice‘,‘david‘,‘carolina‘] for magician in magicians: print(magician.title() + ",that was a great trick!") print("I can‘t wait to see your next trick" + magician.title() + ".\n") #输出结果: Alice,that was a great trick! David,that was a great trick! Carolina,that was a great trick! I can‘t wait to see your next trickCarolina.
3. 不必要的缩进:以下代码的print不属于前一行代码,所以不需要缩进
message ="Hello Python World!" print(message) #输出结果:IndentationError: unexpected indent
4. 循环后不必要的缩进:本该执行for循环结束的语句,进行了缩进,代码就重复执行三次
magicians=[‘alice‘,‘david‘,‘carolina‘] for magician in magicians: print(magician.title() + ",that was a great trick!") print("I can‘t wait to see your next trick" + magician.title() + ".\n") print("Thank you,everyone.That was a great magic show!") #输出结果: Alice,that was a great trick! I can‘t wait to see your next trickAlice. Thank you,everyone.That was a great magic show! David,that was a great trick! I can‘t wait to see your next trickDavid. Thank you,everyone.That was a great magic show! Carolina,that was a great trick! I can‘t wait to see your next trickCarolina. Thank you,everyone.That was a great magic show!
5. 遗漏冒号:for语句末尾的冒号是告知python,下一行是循环的第一行
magicians=[‘alice‘,‘david‘,‘carolina‘] for magician in magicians print(magician.title() + ",that was a great trick!") #输出结果:SyntaxError: invalid syntax
三、创建数值列表
1.使用函数range( ):轻松生成一系列的数字。函数range指定第一个值开始数,并到达指定第二个值后停止,因此输出不包含第二个值。
如果range输出不符合预期,可以尝试指定的值加1或减1.
for value in range(1,5): print(value) #输出结果: 1 2 3 4
2. 使用range( )创建数字列表
a.要创建数字列表,可使用函数list( )将range( )的结果直接转换为列表。如果将range( )作为list( )的参数,输出将为一个数字列表。
even_numbers = list(range(2,11,2)) #第一个2指的是从2开始数;最后一个2指的是步长,也就是每次叠加需要增加2,知道达到或超过终值11 print(even_numbers) #输出结果:[2, 4, 6, 8, 10]
b. 用range创建:前10个整数(1~10)的平方
squares = [] for value in range(1,11): square = value**2 squares.append(square) print(squares) #输出结果:[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
解析以上代码:
a. 创建一个空列表
b. 使用函数range( )让python遍历1~10的值
c. 在循环中,计算当前值得平方,并将结果存储到变量square中
d. 将新计算得到的平方值附加到列表squares末尾
e. 循环结束,打印列表squares
3. 对数字列表执行简单的统计计算
digits=[1,2,3,4,5,6,7,8,9,0] print(min(digits)) print(max(digits)) print(sum(digits)) #输出结果: 0 9 45
4. 列表解析
squares = [value**2 for value in range(1,11)] print(squares) #输出结果: [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
解析以上代码:
a. 指定一个列表名
b.定义一个表达式,用来生成要存储的列表的值,如上面的value**2
c. 编写一个for循环,用于给表达式提供值。如上面的for value in range(1,11),它将值1~10提供给表达式value**2。这里的for循环末尾不需要冒号
备注:如果需要些三四行代码来生成繁复的列表时,可以考虑创建列表解析
四、使用列表的一部分
1. 切片:指定要使用的第一个元素和最后一个元素的索引,到达指定的第二个索引前面的元素后停止。
players = [‘charles‘,‘martina‘,‘michael‘,‘florence‘,‘eli‘] print(players[1:4]) #打印前三名 print(players[:4]) #打印前四名,没有指定第一个元素就从头开始取值 print(players[2:]) #打印后三名,没有指定第二个元素就打印到结尾 #输出结果: [‘martina‘, ‘michael‘, ‘florence‘] [‘charles‘, ‘martina‘, ‘michael‘, ‘florence‘] [‘michael‘, ‘florence‘, ‘eli‘]
2. 遍历切片:遍历前三名队员,并打印他们的名字
players = [‘charles‘,‘martina‘,‘michael‘,‘florence‘,‘eli‘] print("Here are the first three players on my team:") for player in players[:3]: print(player.title()) #输出结果: Here are the first three players on my team: Charles Martina Michael
3. 复制列表:用同时省略起始索引和终止索引([ : ])的方法来实现,这个方法是创建一个适于第一个元素,终止于最后一个元素的切片,即复制整个列表。
my_foods=[‘pizza‘,‘falafel‘,‘carrot‘,‘cake‘] friend_foods = my_foods[:] my_foods.append(‘cannoli‘) friend_foods.append(‘ice cream‘) print("My favorite foods are:") print(my_foods) print("\nMy friend‘s favorite foods are:") print(friend_foods) #输出结果: My favorite foods are: [‘pizza‘, ‘falafel‘, ‘carrot‘, ‘cake‘, ‘cannoli‘] My friend‘s favorite foods are: [‘pizza‘, ‘falafel‘, ‘carrot‘, ‘cake‘, ‘ice cream‘]
解析以上代码:
a.my_foods的元素赋值到新列表friend_foods中,也就是将副本存储到friend_foods中
b. 在列表my_foods中添加“cannoli”
c. 在列表friend_foods中添加“ice cream”
d. 最后打印这两个列表
五、元组
1.定义元组:使用圆括号标识。定义元组后,就可以使用索引来访问元素,就想访问列表元素一样。元组里面的元素不能修改
dimensions = (200,50) print(dimensions[0]) dimensions[0] =250 print(dimensions[0]) #输出结果: 200 TypeError: ‘tuple‘ object does not support item assignment #由于元组中的元素不能修改,所以第二个打印报错
2. 遍历元组中的所有值:像列表一样使用for循环
dimensions = (200,50) for dimension in dimensions: print(dimension) #输出结果: 200 50
3. 修改元组变量:虽然不能修改元组的元素,但可以给存储元组的变量赋值。
dimensions = (200,50) print("Original dimensions:") for dimension in dimensions: print(dimension) dimensions= (400,100) print("\nModified dimensions:") for dimension in dimensions: print(dimension) #输出结果: Original dimensions: 200 50 Modified dimensions: 400 100
备注:此内容从《python编程:从入门到实践》中摘录
原文地址:https://www.cnblogs.com/heiqiuxixi/p/12322618.html