Question

Trying to implement a function to return the two’s complement of a string of bits. I’ve tried two varieties and get odd results.

Version 1 (does the inversion but not the “+1”):
string twosComp(signed int number) {

string twosComp(signed int number) {

     if ( number == 0 ) { return "1"; }
     if ( number == 1 ) { return "0"; }

     if ( number % 2 == 0 ) {
         return twosComp(number / 2) + "1";
     }
     else {
         return twosComp(number / 2) + "0";
     }
}

Version 2 (inverts and attempts “+1” but doesn’t always get it right)

string twosComp(signed int number) {

    bool bit = 0;
    int size = 3; // not sure what to do about this, value could be -32768 to 32767
    string twos;
    number = ~abs(number) + 1;

    for(int i = 0; i < size; i++) {

        //Get right-most bit
        bit = number & 1;
        if(bit) {
            twos += '1';
        }
        else {
            twos += '0';
        }

        //Shift all bits right one place
        number >>= 1;
    }

    return twos;
} // end twosComp

I’ve been trying various iterations of both of these functions. I’m running out of steam on this. If anyone has a better option — I’m VERY open to suggestions at this point.

Answer 1

how about (abs(number) ^ 0xffffffff) + 1, and then turning that value into a string?

edit: also, why is size = 3? ints are 32 bits, usually