流程控制if

if 语句

if expression:

statement(s)

else

else语句：

if 语句,else语句

if expression:

statement(s)

else:

statement(s)

elif语句：

if expression1:

statement1(s)

elif expression2(s):

statements2(s)

else:

statement2(s)

注：Python使用缩进作为其语法分组的方法，建议使用4个空格

逻辑值（bool）包含了两个值：

True:表示非空的量（比如：string，tuple，list，set，dictonary）所有非零数。

false：表示0，None,空的量等，包括空的元组、空的列表、空的字典。

[root@localhost ~]# vim 1.py
#!/usr/bin/python
score = int(raw_input("Please input you score"))
if  score >= 90:print 'A'print 'very good'
elif score >= 80:print 'B'print 'good'
elif score >= 70:print 'C'print 'pass'
else : print 'D fail'
print 'End'
[root@localhost ~]# python 1.py
Please input you score90
A
very good
End
[root@localhost ~]# python 1.py
Please input you score80
B
good
End
[root@localhost ~]# python 1.py
Please input you score70
C
pass
End
[root@localhost ~]# python 1.py
Please input you score10
D fail
End

[root@localhost ~]# vim 2.py
#!/usr/bin/python
yn = raw_input("Please input [Yes/No]")
yn = yn.lower()
if yn == 'y' or yn == 'yes':print "program is running"
elif yn == 'n' or yn == 'no':print "program is exit"
else:print "please input [Yes/No]"[root@localhost ~]# python 2.py
Please input [Yes/No]yes
program is running
[root@localhost ~]# python 2.py
Please input [Yes/No]no
program is exit

流程控制for循环

for循环：

在序列里面，使用for循环遍历

语法：

for interating_var in sequence:

statement(s)

#对序列做一个遍历
[root@localhost ~]# vim 3.py
#!/usr/bin/python
for i in range(1,11):print (i)
[root@localhost ~]# python 3.py
1
2
3
4
5
6
7
8
9
10#打印range(1,11)，也就是打印1-10
[root@localhost ~]# vim 3.py
#!/usr/bin/python
print [i for i in range(1,11)]
[root@localhost ~]# python 3.py
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]#打印range(1,11)*2，也就是打印（1-10）*2
[root@localhost ~]# vim 3.py
#!/usr/bin/python
print [i*2 for i in range(1,11)]
[root@localhost ~]# python 3.py
[2, 4, 6, 8, 10, 12, 14, 16, 18, 20]#判断是不是偶数，是的话放在列表中，并打印出来
[root@localhost ~]# vim 3.py
#!/usr/bin/python
print [i for i in range(1,11) if i%2 == 0 ]
[root@localhost ~]# python 3.py
[2, 4, 6, 8, 10]#打印range(1,11)的三次方，也就是打印（1-10）的三次方
[root@localhost ~]# vim 3.py
#!/usr/bin/python
print [i**3 for i in range(1,11)]
[root@localhost ~]# python 3.py
[1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]#列表重写，打印range(1,11)的三次方，也就是打印（1-10）的三次方
[root@localhost ~]# vim 3.py
#!/usr/bin/python
for j in [i**3 for i in range(1,11)]:print j,
[root@localhost ~]# python 3.py
1 8 27 64 125 216 343 512 729 1000#列表重写，只是奇数的range(1,11)的三次方，打印1,3,5,7,9的三次方
[root@localhost ~]# vim 3.py
#!/usr/bin/pytho
for j in [i**3 for i in range(1,11) if i % 2 !=0 ]:print j,
[root@localhost ~]# python 3.py
1 27 125 343 729xrange object对象，不占用内存，遍历才能取到值

#用shell实现乘法口诀，", print"  ，表示不换行，print表示换行

[root@localhost ~]# vim 6.py
#!/usr/bin/python
for i in xrange(1,10):for j in xrange(1,i+1):print "%s * %s = %s" %(j, i, j*i),print
[root@localhost ~]# python 6.py
1 * 1 = 1
1 * 2 = 2 2 * 2 = 4
1 * 3 = 3 2 * 3 = 6 3 * 3 = 9
1 * 4 = 4 2 * 4 = 8 3 * 4 = 12 4 * 4 = 16
1 * 5 = 5 2 * 5 = 10 3 * 5 = 15 4 * 5 = 20 5 * 5 = 25
1 * 6 = 6 2 * 6 = 12 3 * 6 = 18 4 * 6 = 24 5 * 6 = 30 6 * 6 = 36
1 * 7 = 7 2 * 7 = 14 3 * 7 = 21 4 * 7 = 28 5 * 7 = 35 6 * 7 = 42 7 * 7 = 49
1 * 8 = 8 2 * 8 = 16 3 * 8 = 24 4 * 8 = 32 5 * 8 = 40 6 * 8 = 48 7 * 8 = 56 8 * 8 = 64
1 * 9 = 9 2 * 9 = 18 3 * 9 = 27 4 * 9 = 36 5 * 9 = 45 6 * 9 = 54 7 * 9 = 63 8 * 9 = 72 9 * 9 = 81

for循环

for

else

for循环如果正常结束，才会执行else语句，

break退出整个循环，break异常退出不执行else内容，

continue退出当前的循环，不执行循环语句内容

pass是占位

import sys    sys.exit()退出整个脚本

import time   [root@localhost ~]# vim 7.py

range(10)直接分配，占用内存资源

b=xrange（10）产生对象，只有对对象做遍历，才能产生值，才会占用内存资源

#!/usr/bin/python
for i in xrange(10):print i
else:print 'main end
[root@localhost ~]# python 7.py
0
1
2
3
4
5
6
7
8
9
main end
#ctrl+C或者break异常退出不执行else内容
[root@localhost ~]# vim 7.py
#!/usr/bin/python
import time
for i in xrange(10):print itime.sleep (1)if i==5:break
else:print 'main end
[root@localhost ~]# python 7.py
0
1
2
3
4
5

#continue跳出本次i=3的循环，继续i=4的循环

[root@localhost ~]# vim 7.py
#!/usr/bin/python
import time
for i in xrange(10):if i==3:continueelif i==5:breakelif i==6:pass            print i
else:print 'main end'
[root@localhost ~]# python 7.py
0
1
2
4[root@133 filevalue]# vim for_else.py #!/usr/bin/pythonimport time
for i in xrange(10):if i == 3:  #i=3的时候跳出本次循环，继续下一次循环，执行i=4continueelif i == 5:break    #i=5的时候break直接跳出循环elif i == 6:passtime.sleep(1)print i
else:print "main end"[root@133 filevalue]# python for_else.py
0
1
2
4

现在让我们用python写一个练习题：

系统随机生成一个20以内的随机整数，

玩家有6次机会进行猜猜看，每次猜测都有反馈（猜大了，猜小了，猜对了，结束）

6次中，猜对了，玩家赢了

否则，系统赢了

[root@133 filevalue]# vim random20.py
#!/usr/bin/python
import random
import sysprint 'The random have been created'
random_num = random.randint(1,20)for i in xrange(1,7) :enter_num = int(raw_input('Please input guess num'))if random_num < enter_num:print 'biger'elif random_num > enter_num:print 'lower'elif random_num == enter_num:print 'you are win'sys.exit()
print 'game over'
[root@133 filevalue]# python random20.py
The random have been created
Please input guess num10
lower
Please input guess num15
lower
Please input guess num18
you are win

while与for相比

for循环用在有次数的循环上

while循环用在有条件的控制上

while循环

while循环，直到表达式为假，才退出while循环，表达式是一个逻辑表达式，必须返回一个True或者False，注意是大写，不是小写

语法：

while expression：

statement(s)

[root@localhost ~]# vim 11.py
#!/usr/bin/python
n=0
while True:if n==10:breakprint n,'hello'n+=1
[root@localhost ~]# python 11.py
0 hello
1 hello
2 hello
3 hello
4 hello
5 hello
6 hello
7 hello
8 hello
9 hell

while循环可以使用break退出，例子是用户输入=q的时候，break退出

[root@localhost ~]# vim 11.py
#!/usr/bin/python
while True:print 'hello'input = raw_input("please input somethong,q for quit:")if input=="q":break
[root@localhost ~]# python 11.py
hello
please input somethong,q for quit:a
hello
please input somethong,q for quit:b
hello
please input somethong,q for quit:q[root@localhost ~]# vim 11.py
#!/usr/bin/python
x=''
while x!='q': #输入值不为q，x!='q'为真True，输出hello，输入为q，x!='q'为假False，执行else,输出hello worldprint 'hello'x = raw_input("please input something,q for quit:")if not x: #当输入为空时，not x为true，if true break，直接跳出break
else:print "hello world"
[root@localhost ~]# python 11.py
hello
please input somethong,q for quit:a
hello
please input somethong,q for quit:q
hello world
[root@localhost ~]# python 11.py
hello
please input somethong,q for quit:

#用continue，条件满足，不执行continue的循环内容，重新开始循环
[root@localhost ~]# vim 11.py
#!/usr/bin/python
x=''
while x!='q':print 'hello'x = raw_input("please input somethong,q for quit:")if not x:breakif x=='quit':continueprint 'continue'
else:print "hello world"
[root@localhost ~]# python 11.py
hello
please input somethong,q for quit:a
continue
hello
please input somethong,q for quit:quit
hello
please input somethong,q for quit:q
continue
hello world #x=q，所以不执行while循环，而是执行esle： 输出hello world