使用位操作提取位 [英] Extracting bits using bit manipulation

问题描述

我有一个32位无符号整数，我需要在给定位置提取位，并从这些位中提取一个新的数字.例如，如果我有一个0xFFFFFFFF并且想要位0,10,11，我的结果将是7(111b).

I have a 32-bit unsigned int and I need to extract bits at given positions and make a new number out of those bits. For example, if I have a 0xFFFFFFFF and want bits 0,10,11 my result will be 7 (111b).

这是我的尝试，它会正确提取位，但不会产生正确的结果.我将结果左移1位，并与提取的位进行与"运算，显然这是不正确的吗?

This is my attempt, it extracts the bits correctly but doesn't create the correct result. I'm shifting the result 1 place left and ANDing it with my extracted bit, apparenlty this is incorrect though?

我还可以确定有一种更优雅的方法吗?

I'm also sure there is probably a much more elegant way to do this?

#define TEST 0xFFFFFFFF

unsigned int extractBits(unsigned short positions[], unsigned short count, unsigned int bytes)
{
    unsigned int result = 0;
    unsigned int bitmask = 0;
    unsigned short bit = 0;
    int i = 0;

    for(i = 0; i < count; i++) {
        bitmask = 2 << (positions[i] -1);
        if(bitmask == 0) bitmask = 1;

        bit = bytes & bitmask;
        bit = bit >> positions[i];

        result = result << 1;
        result = result & bit;  
    }

    if(result != 31) {
        printf("FAIL");
    }

    return result;
}

int main(void)
{
    unsigned short positions[5] = {8, 6, 4, 2, 0};
    unsigned int result = extractBits(positions, 5, TEST);

    printf("Result: %d\n", result);

    return 0;
}

推荐答案

由于您要选择单个位，因此没有理由将位掩码设置为变量.只需将所需的位转换为单位位，并使用1的掩码即可.例如:

Since you are picking off individual bits, there's no reason to make the bit mask a variable; just shift the desired bit into the units bit, and use a mask of 1. E.g.:

...
result = (2*result) | ((bytes >> positions[i]) & 1);
...

许多编译器会为2*result和result<<1生成相同的代码，因此请随意使用.

Many compilers generate the same code for 2*result and result<<1, so use whichever you like.

请注意，如果您正在设计接口，并且没有充分理由对positions[]和count使用short整数，那么就不要这样做.保持一致，并以相同的方式指定所有整数.

Note, if you are designing the interface and don't have good reasons for using short integers for positions[] and count as you do, then don't. Be consistent and specify all the integers the same way.

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