Python 編程中 while 語句用于循環(huán)執(zhí)行程序,即在某條件下,循環(huán)執(zhí)行某段程序,以處理需要重復處理的相同任務。其基本形式為:
while 判斷條件:
執(zhí)行語句……
執(zhí)行語句……
執(zhí)行語句可以是單個語句或語句塊。判斷條件可以是任何表達式,任何非零、或非空(null)的值均為true。
當判斷條件假false時,循環(huán)結束。
執(zhí)行流程圖如下:

實例:
#!/usr/bin/python
count = 0
while (count < 9):
print 'The count is:', count
count = count + 1
count = 0
while (count < 9):
print 'The count is:', count
count = count + 1
print "Good bye!"
以上代碼執(zhí)行輸出結果:
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!
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!
while 語句時還有另外兩個重要的命令 continue,break 來跳過循環(huán),continue 用于跳過該次循環(huán),break 則是用于退出循環(huán),此外"判斷條件"還可以是個常值,表示循環(huán)必定成立,具體用法如下:
# continue 和 break 用法
i = 1
while i < 10:
i += 1
if i%2 > 0: # 非雙數(shù)時跳過輸出
continue
print i # 輸出雙數(shù)2、4、6、8、10
i = 1
while 1: # 循環(huán)條件為1必定成立
print i # 輸出1~10
i += 1
if i > 10: # 當i大于10時跳出循環(huán)
break
i = 1
while i < 10:
i += 1
if i%2 > 0: # 非雙數(shù)時跳過輸出
continue
print i # 輸出雙數(shù)2、4、6、8、10
i = 1
while 1: # 循環(huán)條件為1必定成立
print i # 輸出1~10
i += 1
if i > 10: # 當i大于10時跳出循環(huán)
break
無限循環(huán)
如果條件判斷語句永遠為 true,循環(huán)將會無限的執(zhí)行下去,如下實例:
#!/usr/bin/python
# -*- coding: UTF-8 -*-
var = 1
while var == 1 : # 該條件永遠為true,循環(huán)將無限執(zhí)行下去
num = raw_input("Enter a number :")
print "You entered: ", num
print "Good bye!"
以上實例輸出結果:
Enter a number :20
You entered: 20
Enter a number :29
You entered: 29
Enter a number :3
You entered: 3
Enter a number between :Traceback (most recent call last):
File "test.py", line 5, in <module>
num = raw_input("Enter a number :")
KeyboardInterrupt
# -*- coding: UTF-8 -*-
var = 1
while var == 1 : # 該條件永遠為true,循環(huán)將無限執(zhí)行下去
num = raw_input("Enter a number :")
print "You entered: ", num
print "Good bye!"
以上實例輸出結果:
Enter a number :20
You entered: 20
Enter a number :29
You entered: 29
Enter a number :3
You entered: 3
Enter a number between :Traceback (most recent call last):
File "test.py", line 5, in <module>
num = raw_input("Enter a number :")
KeyboardInterrupt
注意:以上的無限循環(huán)你可以使用 CTRL+C 來中斷循環(huán)。
循環(huán)使用 else 語句
在 python 中,for … else 表示這樣的意思,for 中的語句和普通的沒有區(qū)別,else 中的語句會在循環(huán)正常執(zhí)行完(即 for 不是通過 break 跳出而中斷的)的情況下執(zhí)行,while … else 也是一樣。
#!/usr/bin/python
count = 0
while count < 5:
print count, " is less than 5"
count = count + 1
else:
print count, " is not less than 5"
count = 0
while count < 5:
print count, " is less than 5"
count = count + 1
else:
print count, " is not less than 5"
以上實例輸出結果為:
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
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
簡單語句組
類似if語句的語法,如果你的while循環(huán)體中只有一條語句,你可以將該語句與while寫在同一行中, 如下所示:
#!/usr/bin/python
flag = 1
while (flag): print 'Given flag is really true!'
print "Good bye!"
flag = 1
while (flag): print 'Given flag is really true!'
print "Good bye!"
注意:以上的無限循環(huán)你可以使用 CTRL+C 來中斷循環(huán)。