Code:
    /* dac.c - bipolar two's complement D/A converter */

    double dac(int* b, int B, double R) {
       
       double dac = 0;
       b[0] = 1 - b[0];
       
       for (int i=B-1; i>=0; i--) {
           dac = 0.5 * (dac + b[i]);
       }
       
       dac = R * (dac - 0.5);
       b[0] = 1 - b[0];

       return dac;
    }
       
Usage:
       double R = 10.0;
       int B = 4;
       int* b = new int[B];
       b[0] = 1; b[1] = 0; b[2] = 1; b[3] = 0;
       
       xQ = dac(b, B, R);