What does using ampersand (&) with minus operator achieve?

Question

I'm preparing for a programming contest and I have stumbled upon the question below.

void main(){
    int number, result;
    scanf("%d",&number);
    result=number-(number&-number); //this is what i'm struggling to understand
    printf("%d",result);  
}

Note the use of "&-" in the commented line. I couldn't figure out its function. I tried googling and reverse engineering but I couldn't find anything.

Also, the question itself is not about the exact output because the variable "number" is dynamic. I just need to learn what the "&-" part do. Thanks!

It's just an operator-think unary minus and take it from there. — Dave Newton, May 02 '18 at 17:46
This is achieving -10 into the karma of the author of this code. — Eugene Sh., May 02 '18 at 17:46
isn't that logical AND (which is supposed to return binary results)? I am getting 4 when I put in 4. — Sinan Yumurtacı, May 02 '18 at 17:47
@EugeneSh. A temporary is not an lvalue either, no? What I meant was, doing `&-x` causes undefined behavior. No? — DeiDei, May 02 '18 at 18:02
@DeiDei What's "temporary", anyway? The address can be taken only for result of `*`, `[]` or an lvalue as defined [here](http://port70.net/~nsz/c/c11/n1570.html#6.5.3.2p1) — Eugene Sh., May 02 '18 at 18:04
In this context, `&` is the binary "and" operator. In other contexts, the same symbol is the unary address operator. Incidentally, `void main()` should be `int main(void)`. If you have a textbook that tells you to use `void main()`, its author doesn't know C well enough to be writing books about it. — Keith Thompson, May 02 '18 at 18:29

score 6 · Answer 1 · answered May 02 '18 at 18:25

6

This is a binary AND operator. The expression x & -x zeroes out all but the rightmost 1-bit in the number x. Then x - (x & -x) zeroes only that bit, so the result is the original number with the rightmost 1-bit cleared.

answered May 02 '18 at 18:25

Lee Daniel Crocker

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score 1 · Accepted Answer · answered May 02 '18 at 17:52

1

The ampersand is a binary AND operator. The minus is just a regular minus. Thanks to @tkausl

answered May 02 '18 at 17:52

Sinan Yumurtacı

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score 0 · Answer 3 · answered May 02 '18 at 18:20

0

(number&-number) means (number & (-number))

answered May 02 '18 at 18:20

Kamil

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Please provide a small description along with the code – GPrathap May 02 '18 at 18:46

What does using ampersand (&) with minus operator achieve?

3 Answers3