Changing bits in an int in C?

问题

So I have a 16 bit number. Say that the variable name for it is Bits. I want to make it so that Bits[2:0] = 001, 100, and 000, without changing anything else. I'm not sure how to do it, because all I can think of is ORing the bit I want to be a 1 with 1, but I'm not sure how to clear the other bits so that they're 0. If anyone has advice, I'd appreciate it. Thanks!

回答1:

To clear certain bits, & with the inverse of the bits to be cleared. Then you can | in the bits you want.

In this case, you want to zero out the lower three bits (111 in binary or 7 in decimal), so we & with ~7 to clear those bits.

Bits = (Bits & ~7) | 1; // set lower three bits of Bits to 001

回答2:

union structures allow the ability to specify the number of bits a variable has and address each bit of a bigger variable individually.

union {
    short value;
    struct {
        unsigned char bit0 : 1;
        unsigned char bit1 : 1;
        unsigned char bit2 : 1;
        unsigned char bit3 : 1;
        unsigned char bit4 : 1;
        unsigned char bit5 : 1;
        unsigned char bit6 : 1;
        unsigned char bit7 : 1;
        unsigned char bit8 : 1;
        unsigned char bit9 : 1;
        unsigned char bit10 : 1;
        unsigned char bit11 : 1;
        unsigned char bit12 : 1;
        unsigned char bit13 : 1;
        unsigned char bit14 : 1;
        unsigned char bit15 : 1;
    } bits;
} var;

Now you have a variable named var that hold a 16-bit integer which can be referenced by var.value, and you have access to each individual bit of this variable by acessing var.bits.bit0 through var.bits.bit15.

By setting var.value = 0; all bits are set to 0 too. By setting var.bits.bit0 = 1; you automatically change the value of var.value to 0x8000, or as seen in binary, 1000000000000000.

If you intention is change only the 3 last bits, you can simplify the structure to something more like this:

union {
    short value;
    struct {
        unsigned short header : 13;
        unsigned char bit13 : 1;
        unsigned char bit14 : 1;
        unsigned char bit15 : 1;
    } bits;
} var;

Now you have the var.bits.header, that is a 13-bits variable, and 3 other 1-bit variables you can play with.

But notice C++ does not support this kind of structure, so for best C to C++ portability you might prefer to use bitwise operations instead as proposed by @nneonneo.

来源：https://stackoverflow.com/questions/15915198/changing-bits-in-an-int-in-c

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int

bit