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详解Python中的条件判断语句

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Release: 2016-06-06 11:16:14
Original
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 一个else语句可以使用if语句结合起来。如果在if语句中的条件表达式解析为0或false值,那么else语句包含代码执行。

else语句是可选的声明,并if语句下面最多只有一个else语句。
语法:

if ... else语句的语法是:

if expression:
  statement(s)
else:
  statement(s)
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2015514110654164.jpg (264×368)

例子:

#!/usr/bin/python

var1 = 100
if var1:
  print "1 - Got a true expression value"
  print var1
else:
  print "1 - Got a false expression value"
  print var1

var2 = 0
if var2:
  print "2 - Got a true expression value"
  print var2
else:
  print "2 - Got a false expression value"
  print var2

print "Good bye!"

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当执行上面的代码,产生以下结果:

1 - Got a true expression value
100
2 - Got a false expression value
0
Good bye!

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elif 语句

elif语句可以检查多个表达式的真值,并执行一个代码块的条件之一计算结果为true。

if...elif 语句是可选的。然而不像else,对此可以有最多一个语句,if语句下边可以有任意数量elif语句。

if...elif 语句的语法是:

if expression1:
  statement(s)
elif expression2:
  statement(s)
elif expression3:
  statement(s)
else:
  statement(s)

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Python核心不提供switch或case语句在其他语言,但我们可以用if..elif...语句来模拟switch case如下:
例子

#!/usr/bin/python

var = 100
if var == 200:
  print "1 - Got a true expression value"
  print var
elif var == 150:
  print "2 - Got a true expression value"
  print var
elif var == 100:
  print "3 - Got a true expression value"
  print var
else:
  print "4 - Got a false expression value"
  print var

print "Good bye!"

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当执行上面的代码,产生以下结果:

3 - Got a true expression value
100
Good bye!

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