从 C 字节数组中提取最后 N 位

Question

我在 C: 中有一个无符号字符数组

unsigned char array[] = { 0xF0, 0xCC, 0xAA, 0xF0}; 
/* Represented as binary: 11110000 11001100 10101010 11110000 */

我想从这个数组中提取最后 N 位并将它们存储在一个整数中。例如，如果我想提取最后 5 位，结果应该是：

int i = 32; /* Represented as binary: 10000 */

我尝试使用 BIGNUM 库，但我发现它有点矫枉过正，而且对于这个目的来说有点慢。在 C 中是否有更有效的方法来实现这一目标？

附代码：

unsigned char array[] = { 0xF0, 0xCC, 0xAA, 0xF0}; 
int i = 0; 
int j;
int totalBits = sizeof(array) * 8;  
int startBit = totalBits - 5;  

for (j = startBit; j < totalBits; j++) 
{
            i = i << 1;
            i = i | (array[j] & 1);
}    

Answer 1

勇敢的努力！感谢您展示您的尝试。

我在您的版本中添加了一些评论：

unsigned char array[] = { 0xF0, 0xCC, 0xAA, 0xF0}; 
int i = 0; 
int j;
int totalBits = sizeof(array) * 8;  // currently meaning 32
int startBit = totalBits - 5;  // meaning 27 where '5' is the magic number wanted

for (j = startBit; j < totalBits; j++) 
{
            i = i << 1;
            i = i | (array[j] & 1); // Oops! there is no array[27]! Top element is array[3]!
}    

下面是一个似乎有效的版本的草稿：

int main(void) {
    unsigned char arr[] = { 0xF0, 0xCC, 0xAA, 0xF0 }; // Your array (add more! Try it out!)

    union { // providing for up to N = 64bits (on my system)
        unsigned char c[8];
        unsigned long l;
    } foo;

    foo.l = 0; // initialise

    size_t sz = sizeof arr / sizeof arr[0]; // source byte count
    size_t n = 0; // destination byte count

    // wastefully copy as many bytes as are available until 'foo' is a full as possible
    // Notice the 'endian-ness' of index n going from 0 to 8.
    // Other hardware could change this to count downward, instead.
    for( size_t i = sz; i && n < sizeof foo; ) {
        foo.c[ n++ ] = arr[ --i ]; // grab one byte
        printf( "%x\n", foo.l ); // debugging
    }

    int N = 5;
    foo.l &= (1<<N)-1; // Mask off the low order N bits from that long

    printf( "%x\n", foo.l );

    return 0;
}

希望这有帮助。

后期警告：在这段代码中，

N

的最大值是

63

...应该添加检查

N

不超过累加器

foo

的宽度，并且如果

N

恰好是该数字，绕过移位和掩码操作...

PS：在进行位操作时，使用

unsigned

数据类型通常更安全。一些 C 实现显然反对符号位被不当修改。