如何设置、清除和切换一点?
当前回答
下面是我使用的一些宏:
SET_FLAG(Status, Flag) ((Status) |= (Flag))
CLEAR_FLAG(Status, Flag) ((Status) &= ~(Flag))
INVALID_FLAGS(ulFlags, ulAllowed) ((ulFlags) & ~(ulAllowed))
TEST_FLAGS(t,ulMask, ulBit) (((t)&(ulMask)) == (ulBit))
IS_FLAG_SET(t,ulMask) TEST_FLAGS(t,ulMask,ulMask)
IS_FLAG_CLEAR(t,ulMask) TEST_FLAGS(t,ulMask,0)
其他回答
这是我最喜欢的位算术宏,它适用于从无符号字符到size_t的任何类型的无符号整数数组(这是应该有效使用的最大类型):
#define BITOP(a,b,op) \
((a)[(size_t)(b)/(8*sizeof *(a))] op ((size_t)1<<((size_t)(b)%(8*sizeof *(a)))))
要设置位:
BITOP(array, bit, |=);
要清除一点:
BITOP(array, bit, &=~);
要切换一位,请执行以下操作:
BITOP(array, bit, ^=);
要测试一点:
if (BITOP(array, bit, &)) ...
etc.
对于初学者,我想用一个例子解释一下:
例子:
value is 0x55;
bitnum : 3rd.
使用&运算符检查位:
0101 0101
&
0000 1000
___________
0000 0000 (mean 0: False). It will work fine if the third bit is 1 (then the answer will be True)
切换或翻转:
0101 0101
^
0000 1000
___________
0101 1101 (Flip the third bit without affecting other bits)
|运算符:设置位
0101 0101
|
0000 1000
___________
0101 1101 (set the third bit without affecting other bits)
另一个选项是使用位字段:
struct bits {
unsigned int a:1;
unsigned int b:1;
unsigned int c:1;
};
struct bits mybits;
定义了一个3位字段(实际上是三个1位字段)。比特操作现在变得简单了一点(哈哈):
设置或清除一位:
mybits.b = 1;
mybits.c = 0;
要切换一位,请执行以下操作:
mybits.a = !mybits.a;
mybits.b = ~mybits.b;
mybits.c ^= 1; /* all work */
检查一点:
if (mybits.c) //if mybits.c is non zero the next line below will execute
这仅适用于固定大小的位字段。否则,您必须求助于前面文章中描述的比特旋转技术。
如果你在做很多无聊的事情,你可能会想使用口罩,这会让整个事情变得更快。以下函数速度非常快,而且仍然灵活(它们允许在任何大小的位图中进行位旋转)。
const unsigned char TQuickByteMask[8] =
{
0x01, 0x02, 0x04, 0x08,
0x10, 0x20, 0x40, 0x80,
};
/** Set bit in any sized bit mask.
*
* @return none
*
* @param bit - Bit number.
* @param bitmap - Pointer to bitmap.
*/
void TSetBit( short bit, unsigned char *bitmap)
{
short n, x;
x = bit / 8; // Index to byte.
n = bit % 8; // Specific bit in byte.
bitmap[x] |= TQuickByteMask[n]; // Set bit.
}
/** Reset bit in any sized mask.
*
* @return None
*
* @param bit - Bit number.
* @param bitmap - Pointer to bitmap.
*/
void TResetBit( short bit, unsigned char *bitmap)
{
short n, x;
x = bit / 8; // Index to byte.
n = bit % 8; // Specific bit in byte.
bitmap[x] &= (~TQuickByteMask[n]); // Reset bit.
}
/** Toggle bit in any sized bit mask.
*
* @return none
*
* @param bit - Bit number.
* @param bitmap - Pointer to bitmap.
*/
void TToggleBit( short bit, unsigned char *bitmap)
{
short n, x;
x = bit / 8; // Index to byte.
n = bit % 8; // Specific bit in byte.
bitmap[x] ^= TQuickByteMask[n]; // Toggle bit.
}
/** Checks specified bit.
*
* @return 1 if bit set else 0.
*
* @param bit - Bit number.
* @param bitmap - Pointer to bitmap.
*/
short TIsBitSet( short bit, const unsigned char *bitmap)
{
short n, x;
x = bit / 8; // Index to byte.
n = bit % 8; // Specific bit in byte.
// Test bit (logigal AND).
if (bitmap[x] & TQuickByteMask[n])
return 1;
return 0;
}
/** Checks specified bit.
*
* @return 1 if bit reset else 0.
*
* @param bit - Bit number.
* @param bitmap - Pointer to bitmap.
*/
short TIsBitReset( short bit, const unsigned char *bitmap)
{
return TIsBitSet(bit, bitmap) ^ 1;
}
/** Count number of bits set in a bitmap.
*
* @return Number of bits set.
*
* @param bitmap - Pointer to bitmap.
* @param size - Bitmap size (in bits).
*
* @note Not very efficient in terms of execution speed. If you are doing
* some computationally intense stuff you may need a more complex
* implementation which would be faster (especially for big bitmaps).
* See (http://graphics.stanford.edu/~seander/bithacks.html).
*/
int TCountBits( const unsigned char *bitmap, int size)
{
int i, count = 0;
for (i=0; i<size; i++)
if (TIsBitSet(i, bitmap))
count++;
return count;
}
注意,要在16位整数中设置位“n”,请执行以下操作:
TSetBit( n, &my_int);
这取决于你确保比特数在你通过的位图范围内。请注意,对于字节、字、dword、qword等的小端处理器,它们在内存中正确地相互映射(小端处理器比大端处理器“更好”的主要原因,啊,我感觉到一场火焰大战即将到来…)。
尝试使用C语言中的以下函数之一更改n位:
char bitfield;
// Start at 0th position
void chang_n_bit(int n, int value)
{
bitfield = (bitfield | (1 << n)) & (~( (1 << n) ^ (value << n) ));
}
Or
void chang_n_bit(int n, int value)
{
bitfield = (bitfield | (1 << n)) & ((value << n) | ((~0) ^ (1 << n)));
}
Or
void chang_n_bit(int n, int value)
{
if(value)
bitfield |= 1 << n;
else
bitfield &= ~0 ^ (1 << n);
}
char get_n_bit(int n)
{
return (bitfield & (1 << n)) ? 1 : 0;
}