Python学习笔记四：过程控制

条件语句：

#以缩进来区分表示同一范围。

 1 # coding=utf8
 2 # 例1：if 基本用法
 3 
 4 flag = False
 5 name = 'luren'
 6 if name == 'python':         # 判断变量否为'python'
 7     flag = True          # 条件成立时设置标志为真
 8     print 'welcome boss'    # 并输出欢迎信息
 9 else:
10     print name              # 条件不成立时输出变量名称

多重判断：

 1 # coding=utf8
 2 # 例2：elif用法
 3 
 4 num = 5     
 5 if num == 3:            # 判断num的值
 6     print 'boss'        
 7 elif num == 2:
 8     print 'user'
 9 elif num == 1:
10     print 'worker'
11 elif num < 0:           # 值小于零时输出
12     print 'error'
13 else:
14     print 'roadman'     # 条件均不成立时输出

python 并不支持 switch 语句

 1 # coding=utf8
 2 # 例3：if语句多个条件
 3 
 4 num = 9
 5 if num >= 0 and num <= 10:    # 判断值是否在0~10之间
 6     print 'hello'
 7 >>> hello        # 输出结果
 8 
 9 num = 10
10 if num < 0 or num > 10:    # 判断值是否在小于0或大于10
11     print 'hello'
12 else:
13     print 'undefine'
14 >>> undefine        # 输出结果
15 
16 num = 8
17 # 判断值是否在0~5或者10~15之间
18 if (num >= 0 and num <= 5) or (num >= 10 and num <= 15):    
19     print 'hello'
20 else:
21     print 'undefine'
22 >>> undefine        # 输出结果

同一行，与IDL类似

1 #!/usr/bin/python 
2  
3 var = 100 
4  
5 if ( var  == 100 ) : print "Value of expression is 100" 
6  
7 print "Good bye!"

循环语句：

while

1 #!/usr/bin/python
2 
3 count = 0
4 while (count < 9):
5    print 'The count is:', count
6    count = count + 1
7 
8 print "Good bye!"

while... else

1 #!/usr/bin/python
2 
3 count = 0
4 while (count < 9):
5    print 'The count is:', count
6    count = count + 1
7 
8 print "Good bye!"

1 #!/usr/bin/python
2 
3 flag = 1
4 
5 while (flag): print 'Given flag is really true!'
6 
7 print "Good bye!"

while循环嵌套

 1 #coding=utf-8
 2 #!/usr/bin/python
 3 
 4 i = 2
 5 while(i < 100):
 6    j = 2
 7    while(j <= (i/j)):
 8       if not(i%j): break
 9       j = j + 1
10    if (j > i/j) : print i, " 是素数"
11    i = i + 1
12 
13 print "Good bye!"

for

遍历任何序列的项目，列表或字符串

for i in sequence

statements(s)

 1 #!/usr/bin/python
 2 
 3 for letter in 'Python':     # First Example
 4    print 'Current Letter :', letter
 5 
 6 fruits = ['banana', 'apple',  'mango']
 7 for fruit in fruits:        # Second Example
 8    print 'Current fruit :', fruit
 9 
10 print "Good bye!"

>

Current Letter : P
Current Letter : y
Current Letter : t
Current Letter : h
Current Letter : o
Current Letter : n
Current fruit : banana
Current fruit : apple
Current fruit : mango
Good bye!

索引

1 #!/usr/bin/python
2 
3 fruits = ['banana', 'apple',  'mango']
4 for index in range(len(fruits)):
5    print 'Current fruit :', fruits[index]
6 
7 print "Good bye!"

内置函数 len() 和 range(),函数 len() 返回列表的长度，即元素的个数。 range返回一个序列的数。

for.... else...（for循环嵌套）

 1 #!/usr/bin/python
 2 
 3 for num in range(10,20):  #to iterate between 10 to 20
 4    for i in range(2,num): #to iterate on the factors of the number
 5       if num%i == 0:      #to determine the first factor
 6          j=num/i          #to calculate the second factor
 7          print '%d equals %d * %d' % (num,i,j)
 8          break #to move to the next number, the #first FOR
 9    else:                  # else part of the loop
10       print num, 'is a prime number'

break跳出整个循环 continue跳出本次循环 pass空语句，是为了保持程序结构的完整性。

1 #!/usr/bin/python
2 
3 for letter in 'Python': 
4    if letter == 'h':
5       pass
6       print 'This is pass block'
7    print 'Current Letter :', letter
8 
9 print "Good bye!"