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。

# coding=utf8
# 例1：if 基本用法

flag = False
name = 'luren'
if name == 'python':         # 判断变量否为'python'
flag = True          # 条件成立时设置标志为真
print 'welcome boss'    # 并输出欢迎信息
else:
print name              # 条件不成立时输出变量名称

多重判断：

# coding=utf8
# 例2：elif用法

num = 5
if num == 3:            # 判断num的值
print 'boss'
elif num == 2:
print 'user'
elif num == 1:
print 'worker'
elif num < 0:           # 值小于零时输出
print 'error'
else:
print 'roadman'     # 条件均不成立时输出

python 并不支持 switch 语句

# coding=utf8
# 例3：if语句多个条件

num = 9
if num >= 0 and num <= 10:    # 判断值是否在0~10之间
print 'hello'
>>> hello        # 输出结果

num = 10
if num < 0 or num > 10:    # 判断值是否在小于0或大于10
print 'hello'
else:
print 'undefine'
>>> undefine        # 输出结果

num = 8
# 判断值是否在0~5或者10~15之间
if (num >= 0 and num <= 5) or (num >= 10 and num <= 15):
print 'hello'
else:
print 'undefine'
>>> undefine        # 输出结果

同一行，与IDL类似

#!/usr/bin/python

var = 100

if ( var  == 100 ) : print "Value of expression is 100"

print "Good bye!"

循环语句：

while

#!/usr/bin/python

count = 0
while (count < 9):
print 'The count is:', count
count = count + 1

print "Good bye!"

while... else

#!/usr/bin/python

count = 0
while (count < 9):
print 'The count is:', count
count = count + 1

print "Good bye!"

#!/usr/bin/python

flag = 1

while (flag): print 'Given flag is really true!'

print "Good bye!"

while循环嵌套

#coding=utf-8
#!/usr/bin/python

i = 2
while(i < 100):
j = 2
while(j <= (i/j)):
if not(i%j): break
j = j + 1
if (j > i/j) : print i, " 是素数"
i = i + 1

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)

#!/usr/bin/python

for letter in 'Python':     # First Example
print 'Current Letter :', letter

fruits = ['banana', 'apple',  'mango']
for fruit in fruits:        # Second Example
print 'Current fruit :', fruit

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

#!/usr/bin/python

fruits = ['banana', 'apple',  'mango']
for index in range(len(fruits)):
print 'Current fruit :', fruits[index]

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）

#!/usr/bin/python

for num in range(10,20):  #to iterate between 10 to 20
for i in range(2,num): #to iterate on the factors of the number
if num%i == 0:      #to determine the first factor
j=num/i          #to calculate the second factor
print '%d equals %d * %d' % (num,i,j)
break #to move to the next number, the #first FOR
else:                  # else part of the loop
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。

#!/usr/bin/python

for letter in 'Python':
if letter == 'h':
pass
print 'This is pass block'
print 'Current Letter :', letter

print "Good bye!"