How to copy bits from one variable to another?

问题

Let's say I have this int variable v1:

1100 1010

And this variable int v2:

1001 1110

I need to copy the last four bits from v2 to the last four bits of v1 so that the result is:

1100 1110
^    ^ last four bits of v2
|
| first four bits of v1

How would I got about doing this in C or C++? I read a few articles about bitwise operations but I couldn't find any information specifically about this.

回答1:

Bitwise operations were the right things to look for.

v1 = (v1 & ~0xf) | (v2 & 0xf);

Is there something specific you didn't understand from the articles you read?

回答2:

How about

v1 = (v1 & 0xf0) | (v2 & 0xf);

If the value of "v1" has more bits, you'd want to use a bigger mask:

v1 = (v1 & 0xfffffff0) | (v2 & 0xf);

回答3:

Try:

v1 = (v2 & 0x0F) | (v1 & 0xF0);

回答4:

The most readable way to write it, in my opinion:

v1 &= ~0x0F;       // clear least sig. nibble of v1
v1 |= v2 & 0x0F;   // copy least sig. nibble of v2 into v1

来源：https://stackoverflow.com/questions/14391707/how-to-copy-bits-from-one-variable-to-another