Python programming 5

基础永远值得花费90%的精力去学习加强。认识实践的重要性。

运算符相对于语法里的动词，加减乘除。

Ax Python Basic Operators | 运算符

运算符可以操作数据，比如4+5=9，加号和等于号就相对于运算符。

类型

• Arithmetic Operators | 算术
• Comparison (Relational) Operators | 关系或比较
• Assignment Operators | 赋值
• Logical Operators | 逻辑
• Bitwise Operators | 按位
• Membership Operators | 成员
• Identity Operators | 身份

算术

假设变量a为10，变量b为20，则−

Operator	Description	Example
+ Addition	在操作符的任意一边添加值。	a + b = 30
- Subtraction	从左操作数减去右操作数。	a – b = -10
* Multiplication	将运算符两边的值相乘	a * b = 200
/ Division	将左操作数除以右操作数	b / a = 2
% Modulus	左操作数除以右操作数，返回余数	b % a = 0
** Exponent	对运算符执行指数(幂)计算	a**b =10 to the power 20
//	上下除法——对操作数进行除法，其结果是去掉小数点后数字的商。但如果其中一个操作数为负，则结果将向下取整，即舍入0(趋近负无穷)−	9//2 = 4 and 9.0//2.0 = 4.0, -11//3 = -4, -11.0//3 = -4.0

例

假设变量 a 持有 21，变量 b 持有 10

#!/usr/bin/python

a = 21
b = 10
c = 0

c = a + b
print "Line 1 - Value of c is ", c

c = a - b
print "Line 2 - Value of c is ", c 

c = a * b
print "Line 3 - Value of c is ", c 

c = a / b
print "Line 4 - Value of c is ", c 

c = a % b
print "Line 5 - Value of c is ", c

a = 2
b = 3
c = a**b 
print "Line 6 - Value of c is ", c

a = 10
b = 5
c = a//b 
print "Line 7 - Value of c is ", c

执行

Line 1 - Value of c is 31
Line 2 - Value of c is 11
Line 3 - Value of c is 210
Line 4 - Value of c is 2
Line 5 - Value of c is 1
Line 6 - Value of c is 8
Line 7 - Value of c is 2

比较

这些运算符比较它们两边的值，并决定它们之间的关系。它们也被称为关系操作符。

假设变量a为10，变量b为20，则−

Operator	Description	Example
==	如果两个操作数的值相等，则条件为真。	(a == b) is not true.
!=	如果两个操作数的值不相等，则条件为真	(a != b) is true.
<>	如果两个操作数的值不相等，则条件为真	(a <> b) is true. This is similar to != operator.
>	如果左操作数的值大于右操作数的值，则条件为真	(a > b) is not true.
<	如果左操作数的值小于右操作数的值，则条件为真。	(a < b) is true.
>=	如果左操作数的值大于或等于右操作数的值，则条件为真。	(a >= b) is not true.
<=	如果左操作数的值小于或等于右操作数的值，则条件为真。	(a <= b) is true.

例

假设变量 a 为 10，变量 b 为 20

#!/usr/bin/python

a = 21
b = 10
c = 0

if ( a == b ):
   print "Line 1 - a is equal to b"
else:
   print "Line 1 - a is not equal to b"

if ( a != b ):
   print "Line 2 - a is not equal to b"
else:
   print "Line 2 - a is equal to b"

if ( a <> b ):
   print "Line 3 - a is not equal to b"
else:
   print "Line 3 - a is equal to b"

if ( a < b ):
   print "Line 4 - a is less than b" 
else:
   print "Line 4 - a is not less than b"

if ( a > b ):
   print "Line 5 - a is greater than b"
else:
   print "Line 5 - a is not greater than b"

a = 5;
b = 20;
if ( a <= b ):
   print "Line 6 - a is either less than or equal to  b"
else:
   print "Line 6 - a is neither less than nor equal to  b"

if ( b >= a ):
   print "Line 7 - b is either greater than  or equal to b"
else:
   print "Line 7 - b is neither greater than  nor equal to b"

执行

Line 1 - a is not equal to b
Line 2 - a is not equal to b
Line 3 - a is not equal to b
Line 4 - a is not less than b
Line 5 - a is greater than b
Line 6 - a is either less than or equal to b
Line 7 - b is either greater than or equal to b

赋值

假设变量 a 为 10，变量 b 为 20

Operator	Description	Example
=	将右操作数赋值给左操作数	c = a + b assigns value of a + b into c
+= Add AND	它将右操作数添加到左操作数，并将结果赋给左操作数	c += a is equivalent to c = c + a
-= Subtract AND	它从左操作数减去右操作数，并将结果赋值给左操作数	c -= a is equivalent to c = c - a
*= Multiply AND	它将右操作数与左操作数相乘，并将结果赋给左操作数	c *= a is equivalent to c = c * a
/= Divide AND	它将左操作数与右操作数除法，并将结果赋值给左操作数	c /= a is equivalent to c = c / a
%= Modulus AND	它使用两个操作数取模，并将结果赋值给左操作数	c %= a is equivalent to c = c % a
**= Exponent AND	对运算符执行指数(幂)计算，并将值赋给左操作数	c **= a is equivalent to c = c ** a
//= Floor Division	它对操作符执行下除法，并将值赋给左操作数	c //= a is equivalent to c = c // a

例

假设变量 a 为 10，变量 b 为 20

#!/usr/bin/python

a = 21
b = 10
c = 0

c = a + b
print "Line 1 - Value of c is ", c

c += a
print "Line 2 - Value of c is ", c 

c *= a
print "Line 3 - Value of c is ", c 

c /= a 
print "Line 4 - Value of c is ", c 

c  = 2
c %= a
print "Line 5 - Value of c is ", c

c **= a
print "Line 6 - Value of c is ", c

c //= a
print "Line 7 - Value of c is ", c

执行

Line 1 - Value of c is 31
Line 2 - Value of c is 52
Line 3 - Value of c is 1092
Line 4 - Value of c is 52
Line 5 - Value of c is 2
Line 6 - Value of c is 2097152
Line 7 - Value of c is 99864

按位

Python 语言支持以下按位运算符

Operator	Description	Example
& Binary AND	如果两个操作数中都存在一个位，操作符就向结果复制一个位	(a & b) (means 0000 1100)
| Binary OR	如果它存在于任意一个操作数中，它就复制一个位	(a | b) = 61 (means 0011 1101)
^ Binary XOR	如果位是在一个操作数中设置的，而不是在两个操作数中设置的，则复制该位。	(a ^ b) = 49 (means 0011 0001)
~ Binary Ones Complement	它是一元的，具有“翻转”比特的效果。	(~a ) = -61 (means 1100 0011 in 2’s complement form due to a signed binary number.
<< Binary Left Shift	左操作数值向左移动由右操作数指定的位数。	a << 2 = 240 (means 1111 0000)
>> Binary Right Shift	左操作数的值会向右移动由右操作数指定的位数	a >> 2 = 15 (means 0000 1111)

例

#!/usr/bin/python

a = 60            # 60 = 0011 1100 
b = 13            # 13 = 0000 1101 
c = 0

c = a & b;        # 12 = 0000 1100
print "Line 1 - Value of c is ", c

c = a | b;        # 61 = 0011 1101 
print "Line 2 - Value of c is ", c

c = a ^ b;        # 49 = 0011 0001
print "Line 3 - Value of c is ", c

c = ~a;           # -61 = 1100 0011
print "Line 4 - Value of c is ", c

c = a << 2;       # 240 = 1111 0000
print "Line 5 - Value of c is ", c

c = a >> 2;       # 15 = 0000 1111
print "Line 6 - Value of c is ", c

执行

Line 1 - Value of c is 12
Line 2 - Value of c is 61
Line 3 - Value of c is 49
Line 4 - Value of c is -61
Line 5 - Value of c is 240
Line 6 - Value of c is 15

逻辑

Python 语言支持以下逻辑运算符。假设变量 a 持有 10，变量 b 持有 20

Operator	Description	Example
and Logical AND	如果两个操作数都为真，则条件为真。	(a and b) is true.
or Logical OR	如果两个操作数有一个非零，则条件为真.	(a or b) is true.
not Logical NOT	用于反转其操作数的逻辑状态。	Not(a and b) is false.

成员

Python 的成员资格运算符测试序列中的成员资格，例如字符串、列表或元组。有两个会员运营商，如下所述 -

Operator	Description	Example
in	如果在指定序列中找到变量，则计算为true，否则为false。	x in y, here in results in a 1 if x is a member of sequence y.
not in	如果在指定序列中没有找到变量，则计算为true，否则为false。	x not in y, here not in results in a 1 if x is not a member of sequence y.

例

#!/usr/bin/python

a = 10
b = 20
list = [1, 2, 3, 4, 5 ];

if ( a in list ):
   print "Line 1 - a is available in the given list"
else:
   print "Line 1 - a is not available in the given list"

if ( b not in list ):
   print "Line 2 - b is not available in the given list"
else:
   print "Line 2 - b is available in the given list"

a = 2
if ( a in list ):
   print "Line 3 - a is available in the given list"
else:
   print "Line 3 - a is not available in the given list"

执行

Line 1 - a is not available in the given list
Line 2 - b is not available in the given list
Line 3 - a is available in the given list

身份

恒等运算符比较两个对象的内存位置。有两个身份运算符

Operator	Description	Example
is	如果操作符两边的变量指向同一个对象，则计算为true，否则为false。	x is y, here is results in 1 if id(x) equals id(y).
is not	如果操作符两边的变量指向同一个对象，则计算为false，否则为true。	x is not y, here is not results in 1 if id(x) is not equal to id(y).

例

#!/usr/bin/python

a = 20
b = 20

if ( a is b ):
   print "Line 1 - a and b have same identity"
else:
   print "Line 1 - a and b do not have same identity"

if ( id(a) == id(b) ):
   print "Line 2 - a and b have same identity"
else:
   print "Line 2 - a and b do not have same identity"

b = 30
if ( a is b ):
   print "Line 3 - a and b have same identity"
else:
   print "Line 3 - a and b do not have same identity"

if ( a is not b ):
   print "Line 4 - a and b do not have same identity"
else:
   print "Line 4 - a and b have same identity"

执行

Line 1 - a and b have same identity
Line 2 - a and b have same identity
Line 3 - a and b do not have same identity
Line 4 - a and b do not have same identity

优先级

下表列出了从最高优先级到最低优先级的所有运算符

Operator	Description
**	Exponentiation (raise to the power)
~ + -	Complement, unary plus and minus (method names for the last two are +@ and -@)
* / % //	Multiply, divide, modulo and floor division
+ -	Addition and subtraction
>> <<	Right and left bitwise shift
&	Bitwise ‘AND’td>
^ |	Bitwise exclusive OR' and regular OR’
<= < > >=	Comparison operators
<> == !=	Equality operators
= %= /= //= -= += *= **=	Assignment operators
is is not	Identity operators
in not in	Membership operators
not or and	Logical operators

运算符优先级影响表达式的计算方式。例如，x = 7 + 3 * 2；在这里，x 被赋值为 13，而不是 20，因为运算符的优先级高于 +，所以它首先将 32 相乘，然后加到 7。这里，优先级最高的运算符出现在表的顶部，最低的出现在表的顶部底部。

例

#!/usr/bin/python

a = 20
b = 10
c = 15
d = 5
e = 0

e = (a + b) * c / d       #( 30 * 15 ) / 5
print "Value of (a + b) * c / d is ",  e

e = ((a + b) * c) / d     # (30 * 15 ) / 5
print "Value of ((a + b) * c) / d is ",  e

e = (a + b) * (c / d);    # (30) * (15/5)
print "Value of (a + b) * (c / d) is ",  e

e = a + (b * c) / d;      #  20 + (150/5)
print "Value of a + (b * c) / d is ",  e

执行

Value of (a + b) * c / d is 90
Value of ((a + b) * c) / d is 90
Value of (a + b) * (c / d) is 90
Value of a + (b * c) / d is 50