~ operator in C

Question

The output of this program is -13. I have never fully understood ~ operator in C. Why does it give -13 as output? How to limit ~ operator to just 4 bits of a number?

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Answer 1

To limit the effect to a specified number of bits, just use bitwise masks, e.g.:

#include 

int main(void) {
    int a = 16;             /* 10000 in binary */
    int b = ~a;             /* Will interpret b as -17 in two's complement */
    int c = (a & ~0xF) | (~a & 0xF); /* Will limit operator to rightmost 4 bits,
                                        so 00000 becomes 01111, and c will become
                                        11111, not 11...101111, so c will be 31     */

    printf("a is %d, b is %d, c is %d\n", a, b, c);
    return 0;
}

Outputs:

paul@local:~/src/c/scratch$ ./comp
a is 16, b is -17, c is 31
paul@local:~/src/c/scratch$