Bitwise operator for simply flipping all bits in an integer?

The ~ unary operator is bitwise negation. If you need fewer bits than what fits in an int then you'll need to mask it with & after the fact.

Simply use the bitwise not operator ~.

int flipBits(int n) {
    return ~n;
}

To use the k least significant bits, convert it to the right mask.
(I assume you want at least 1 bit of course, that's why mask starts at 1)

int flipBits(int n, int k) {
    int mask = 1;
    for (int i = 1; i < k; ++i)
        mask |= mask << 1;

    return ~n & mask;
}

As suggested by Lưu Vĩnh Phúc, one can create the mask as (1 << k) - 1 instead of using a loop.

int flipBits2(int n, int k) {
    int mask = (1 << k) - 1;
    return ~n & mask;
}

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