python 教程 第四章、 控制流

第四章、 控制流
控制语句后面要加冒号:
1)    if语句

if guess == number:

    print 'Congratulations, you guessed it.' # New block starts here

elif guess < number:

    print 'No, it is a little higher than that' # Another block

else:

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

if not False and True: #组合条件

    print "OK"

注：Python暂时没有switch语句

2)    while语句
注：while语句有一个可选的else从句

while running:

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

    if guess == number:

        print 'Congratulations, you guessed it.' 

        running = False # this causes the while loop to stop

    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.' 

3)    range语句

print range(10) #[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

print range(5,10) #[5, 6, 7, 8, 9]

print range(1,10,3) #[1, 4, 7]

print range(-10, -100, -30) #[-10, -40, -70]

用法参考help(range)

4)    for循环

a = ['apple', 'banana', 'carrot']

for i in range(len(a)): #range()和len()一起用于字符串索引

print a[i]

#apple

#banana

#carrot

带逗号的print语句输出的元素之间会自动添加空格

for i in range(len(a)):

print a[i],  #带,的print语句

# apple banana carrot

C/C++中的for (int i = 0; i < 5; i++)，等价于Python：for i in range(0,5)。

5)    break语句

while True:

    s = raw_input('Enter something : ')

    if s == 'quit':

        break

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

print 'Done' 

6)    continue语句

while True:

    s = raw_input('Enter something : ')

    if s == 'quit':

        break

    if len(s) < 3:

        continue

print 'Input is of sufficient length' 

7)    条件表达式

x, y = 3, 4

small = x if x < y else y

print small #3