Python学习入门基础教程(learning Python)--4.2 Python的计数循环体for语句
Python的另外一种循环体结构是计数循环体for,通过for循环实现某些语句块循环执行一定次数,其使用语法结构如下。
Python的for循环应用思想和C等其他高级语言一样,当for的条件满足时,for下的语句块执行,不同的是for的条件的写法和其他高级语言有些不太一样。
[python] view plaincopy- for variable in [value1, value2,.....]:
- (TAB)statement
- (TAB)statement
- (TAB)etc.
这里第一次出现了左方括号 ‘[’ 和右方括号 ‘]’, 用左右方括号括起来的数据序列我们称之为list列表,有关list列表的知识我们以后再详细讲解。
这里有一点需要注意就是[value1,value2,.....]后边要有冒号:,否则会有语法出错错误。
for的循环原理是这样的每次从in后边的list取出一个值valuex并赋值for后边variable变量,那循环多少次呢?list里有几个数据就循环几次,当list里的所有数据都取完了,for就结束了,故for执行的次数取决于list里的数据个数,下面我举个例子来说明一下。
[python] view plaincopy
- def main():
- print('I will display the numbers 1 through 5.')
- for num in [1, 2, 3, 4, 5]:
- print(num)
- # Call the main function.
- main()
代码第3行是个for循环每次从[1,2,3,4,5]这个list里取出一个值赋值给num,in后边的list([1, 2, 3, 4, 5])共有5个数据,可知for可以循环5次之后就能结束。
运行结果如下
结下来我们分析一下结果为何是这样的?
每次循环for都会从[ 1, 2, 3, 4, 5]里提取一个数据给num。

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