64,654
社区成员
发帖
与我相关
我的任务
分享关于按位或运算
jd=j-e[k][1]; id =i-e[k][0];
if(flag[jd][id]==2|flag[jd][id]==3)
{
}
如上一段代码。已经知道flag[m][n]可以等于1/2/3/4，
请问下，怎么情况下才能运算大括号内的内容
...全文
请发表友善的回复…
发表回复
yayakfa 2019-08-15
- 打赏
- 举报
我给你分析分析，|符号是按位或，优先级低于==，先计算两个等式，flag[jd][id]==2和flag[jd][id]==3,最多只有一个等式成立，或者两个等式都不成立。
不成立的等式结果为负数，符号位1，成立的等式结果为非负数，符号位为0。按位或之后，符号位一定是1，得出来的数也一定是负数。if判断是不会成立的，所以说{}里面的程序一定不会执行的。
日立奔腾浪潮微软松下联想 2019-08-08
- 打赏
- 举报
注意我说的是“C中是可以用|替代||的”，并没有说两种运算符等价，否则何必设计两种运算符呢，C最初设计的时候并没有||、&&，就是用|、&替代逻辑运算符的，如果不能替代的话，那早期C中涉及逻辑运算的代码就没法写了，见：http://www.quut.com/c/，“Dennis Ritchie on the precedence of | and & vs. ==”那部分的解释（这可是1982年Dennis Ritchie在新闻组中的发言哟）
至于浮点和指针操作数，也很简单：
if (p && xxx) ...
只要写成
if (!!p & xxx) ...
就可以替代前面的逻辑运算
至于浮点和指针操作数，也很简单：
if (p && xxx) ...
只要写成
if (!!p & xxx) ...
就可以替代前面的逻辑运算
赵4老师 2019-08-08
- 打赏
- 举报
//C++ Operators
// Operators specify an evaluation to be performed on one of the following:
// One operand (unary operator)
// Two operands (binary operator)
// Three operands (ternary operator)
// The C++ language includes all C operators and adds several new operators.
// Table 1.1 lists the operators available in Microsoft C++.
// Operators follow a strict precedence which defines the evaluation order of
//expressions containing these operators. Operators associate with either the
//expression on their left or the expression on their right; this is called
//“associativity.” Operators in the same group have equal precedence and are
//evaluated left to right in an expression unless explicitly forced by a pair of
//parentheses, ( ).
// Table 1.1 shows the precedence and associativity of C++ operators
// (from highest to lowest precedence).
//
//Table 1.1 C++ Operator Precedence and Associativity
// The highest precedence level is at the top of the table.
//+------------------+-----------------------------------------+---------------+
//| Operator | Name or Meaning | Associativity |
//+------------------+-----------------------------------------+---------------+
//| :: | Scope resolution | None |
//| :: | Global | None |
//| [ ] | Array subscript | Left to right |
//| ( ) | Function call | Left to right |
//| ( ) | Conversion | None |
//| . | Member selection (object) | Left to right |
//| -> | Member selection (pointer) | Left to right |
//| ++ | Postfix increment | None |
//| -- | Postfix decrement | None |
//| new | Allocate object | None |
//| delete | Deallocate object | None |
//| delete[ ] | Deallocate object | None |
//| ++ | Prefix increment | None |
//| -- | Prefix decrement | None |
//| * | Dereference | None |
//| & | Address-of | None |
//| + | Unary plus | None |
//| - | Arithmetic negation (unary) | None |
//| ! | Logical NOT | None |
//| ~ | Bitwise complement | None |
//| sizeof | Size of object | None |
//| sizeof ( ) | Size of type | None |
//| typeid( ) | type name | None |
//| (type) | Type cast (conversion) | Right to left |
//| const_cast | Type cast (conversion) | None |
//| dynamic_cast | Type cast (conversion) | None |
//| reinterpret_cast | Type cast (conversion) | None |
//| static_cast | Type cast (conversion) | None |
//| .* | Apply pointer to class member (objects) | Left to right |
//| ->* | Dereference pointer to class member | Left to right |
//| * | Multiplication | Left to right |
//| / | Division | Left to right |
//| % | Remainder (modulus) | Left to right |
//| + | Addition | Left to right |
//| - | Subtraction | Left to right |
//| << | Left shift | Left to right |
//| >> | Right shift | Left to right |
//| < | Less than | Left to right |
//| > | Greater than | Left to right |
//| <= | Less than or equal to | Left to right |
//| >= | Greater than or equal to | Left to right |
//| == | Equality | Left to right |
//| != | Inequality | Left to right |
//| & | Bitwise AND | Left to right |
//| ^ | Bitwise exclusive OR | Left to right |
//| | | Bitwise OR | Left to right |
//| && | Logical AND | Left to right |
//| || | Logical OR | Left to right |
//| e1?e2:e3 | Conditional | Right to left |
//| = | Assignment | Right to left |
//| *= | Multiplication assignment | Right to left |
//| /= | Division assignment | Right to left |
//| %= | Modulus assignment | Right to left |
//| += | Addition assignment | Right to left |
//| -= | Subtraction assignment | Right to left |
//| <<= | Left-shift assignment | Right to left |
//| >>= | Right-shift assignment | Right to left |
//| &= | Bitwise AND assignment | Right to left |
//| |= | Bitwise inclusive OR assignment | Right to left |
//| ^= | Bitwise exclusive OR assignment | Right to left |
//| , | Comma | Left to right |
//+------------------+-----------------------------------------+---------------+
日立奔腾浪潮微软松下联想 2019-08-08
- 打赏
- 举报
瞎扯.多去读读书吧.[/quote]
第一，我有没有瞎扯~~~
第二，你是不是应该多去读读书~~~
赵4老师 2019-08-07
- 打赏
- 举报
|
和
||
不是一回事！
lin5161678 2019-08-07
- 打赏
- 举报
愚蠢的做法 毫无优势
lin5161678 2019-08-07
- 打赏
- 举报
flag[jd][id]==2 和 flag[jd][id]==3
2个表达式至少一个成立的时候
lin5161678 2019-08-07
- 打赏
- 举报
从结果看，的确是可以，0|0=0,0|1=1,1|0=1,1|1=1，结果没问题
不过逻辑上显得有些混乱
另外||和&&有短路计算，有些地方替换可能会导致出错
[/quote]不能替换的
就算不考虑短路
位运算只能是 整型
浮点和指针都歇菜 差远了
双杯献酒 2019-08-07
- 打赏
- 举报
C89就已经有 || 了。
编程不是考古， 不要把史前的东西拿来说。
nice_cxf 2019-08-07
- 打赏
- 举报
从结果看，的确是可以，0|0=0,0|1=1,1|0=1,1|1=1，结果没问题
不过逻辑上显得有些混乱
另外||和&&有短路计算，有些地方替换可能会导致出错
轻箬笠 2019-08-07
- 打赏
- 举报
与其纠结于运算符优先级读问题，还不如多加一些括号
日立奔腾浪潮微软松下联想 2019-08-07
- 打赏
- 举报
瞎扯.多去读读书吧.[/quote]
多去读读书吧，这是Dennis Ritchie在80年代回应C的运算符优先级设计问题（为什么& |的优先级比==低）时说的~~~
双杯献酒 2019-08-07
- 打赏
- 举报
瞎扯.多去读读书吧.
日立奔腾浪潮微软松下联想 2019-08-07
- 打赏
- 举报
C中是可以用|替代||的，C最初设计的时候并没有||、&&，就是用|、&
