樱花浪子'S BLOG

4.Python:控制流

当你的选择不同时，程序的流程也就不同，这是靠控制流来实现的。Python有三种控制流语句：if，for 和while
if

如果条件为真，则执行一块代码，否则执行另一块代码。else块是可选的。

number=23

guess=int(input('Enter an integer:'))

if guess == number:

    print('Congratulations, you guessed it.')

    print('(but you do not win any prize!)')

elif guess<number:

    print('No, it is a little higher than that')

else:

    print('No, it is a little lower than that')

print('Done')

运行输出

>>>

Enter an integer:10

No, it is a little higher than that

Done

>>>
While

只要条件为真就重复执行某块代码，也可以带有else块。

number=23

running =True

while running:

    guess=int(input('Enter an integer:'))

    if guess == number:

        print('Congratulations, you guessed it.')

        print('(but you do not win any prize!)')

        running = False

    elif guess<number:

        print('No, it is a little higher than that')

    else:

        print('No, it is a little lower than that')

else:

    print('The while loop is over!')

print('Done')

运行输出：

>>>

Enter an integer:10

No, it is a little higher than that

Enter an integer:20

No, it is a little higher than that

Enter an integer:30

No, it is a little lower than that

Enter an integer:25

No, it is a little lower than that

Enter an integer:24

No, it is a little lower than that

Enter an integer:23

Congratulations, you guessed it.

(but you do not win any prize!)
for

for…in语句表示迭代序列中的每一个元素，该序列必须是有序的。

for i in range(1,5):

    print(i)

else:

　　print('The loop is over')

运行输出：

>>>

1

2

3

4

The loop is over
break

break语句用来跳出循环，如果跳出的是for或者while循环，那么相关的else块也不会被执行。

while True:

    s=(input('Enter something:'))

    if (s=='quit') or (s=='exit'):

        break

    print('Length of the string is ', len(s))

    print('缩进测试')

print('Done')

运行输出：

>>>    

Enter something:hello

Length of the string is 5

缩进测试

Enter something:exit

Done

while True:

    s=(input('Enter something:'))

    if (s=='quit') or (s=='exit'):

        break

    print('Length of the string is ', len(s))

print('缩进测试')

print('Done')

运行输出：

>>>

Enter something:hello

Length of the string is 5

Enter something:quit

缩进测试

Done

说明while循环仅靠缩进改变来结束，以后要小心。
continue

continue语句用来跳出本次循环，进入下次循环迭代。

while True:

    s=(input('Enter something:'))

    if (s=='quit') or (s=='exit'):

        break

   if len(s) < 3:

        print('Too small')

        continue

    print('Input is of sufficient length')

print('Done')

运行输出：

>>>

Enter something:he

Too small

Enter something:hello

Input is of sufficient length

Enter something:exit

Done