【课程5.4】 循环语句：while循环
执行语句可以是单个语句或语句块
判断条件可以是任何表达式，任何非零、或非空（null）的值均为true。
当判断条件假false时，循环结束。
1.基本运行逻辑
count = 0
while count < 9:
print( 'The count is:', count)
count = count + 1
print( "Good bye!")
# 这里count<9是一个判断语句，当判断为True时，则继续运行
----------------------------------------------------------------------
The count is: 0
The count is: 1
The count is: 2
The count is: 3
The count is: 4
The count is: 5
The count is: 6
The count is: 7
The count is: 8
Good bye!
2.关于无限循环：如果条件判断语句永远为 true，循环将会无限的执行下去
var = 1
while var == 1 :
num = input("Enter a number :")
print( "You entered: ", num)
print( "Good bye!")
# 该条件永远为true，循环将无限执行下去
# 一定要避免无限循环！！
----------------------------------------------------------------------
3.while-else语句
count = 0
while count < 5:
print(count, " is less than 5")
count = count + 1
else:
print(count, " is not less than 5")
# 逻辑和if-else一样
----------------------------------------------------------------------
0 is less than 5
1 is less than 5
2 is less than 5
3 is less than 5
4 is less than 5
5 is not less than 5
