QPSK BER in C++

qpskber.cpp

#include <math.h>
#include <complex>
#include <ctime>
#include <stdio.h>
#include <cstdlib>
#include <random>
struct result
{
 double snr[20];
 double ber[20];
};


result simulate(int n ,bool runtype);
double seqlink(double k);
int main()
{
 
 double snr[20];
 double ber[20];
 struct result ans;
 
 clock_t start = clock();
 
 // program goes here
 ans=simulate(20,false);
 
 clock_t ends = clock();
 printf("Time elapsed %fms",(float) 1000*(ends - start) / CLOCKS_PER_SEC);
 printf("\nSNR = [");
 for(int k=0;k<20;k++)  printf("%5.2f ",ans.snr[k]);printf("]");   
 printf("\nBER= [");
 for(int k=0;k<20;k++)  printf("%f ",ans.ber[k]);printf("]");   
 
 printf("\n Done..\n");
}



result simulate(int n ,bool runtype)
{
 
 result ans;
 double f[n];
 double snr[n];
 double initSNR= 0;
 
  for (int k = 0; k < n; k++) {
   double currentSNR=initSNR+double(k);
   ans.snr[k]=currentSNR;
   ans.ber[k]=seqlink(currentSNR);
  }
   
 }
 
return ans;

}
 

double seqlink(double k)  {
 int Nbits=1024*100; //*(50-int(k))
 std::complex <double> symbols[4];
 symbols[0]=std::complex <double> (0.7,0.7); //,+0.7-0.7i,-0.7+0.7i,-0.7-0.7i}
 symbols[1]=std::complex <double> (0.7,-0.7); //,+0.7-0.7i,-0.7+0.7i,-0.7-0.7i}
 symbols[2]=std::complex <double> (-0.7,0.7); //,+0.7-0.7i,-0.7+0.7i,-0.7-0.7i}
 symbols[3]=std::complex <double> (-0.7,-0.7); //,+0.7-0.7i,-0.7+0.7i,-0.7-0.7i}
 
 int txbits[Nbits];
 for (int i=0;i<Nbits;i++){
   txbits[i]=rand() % 4;
 }
 
 std::complex<double> txsymbols[Nbits];
 std::complex<double> noise[Nbits];
 std::complex<double> rxnoise[Nbits];
 
 double lsnr=double(pow(10,(k/10)));
 double Es=1;
 double noisevar=Es/lsnr;
 int errorbits=0;
 std::default_random_engine generator;
 std::normal_distribution<double> distribution;
 
 
 for (int i=0;i<Nbits;i++){
  txsymbols[i]=symbols[txbits[i]]; 
  double re=distribution(generator);
  double im=distribution(generator);
  noise[i]=std::complex<double>(re*sqrt(noisevar*0.5),im*sqrt(noisevar*0.5));
  std::complex <double> tmp=txsymbols[i]+noise[i];
  
  {
   if(real(tmp)>0 && imag(tmp)>0) rxnoise[i]=symbols[0];
   if(real(tmp)>0 && imag(tmp)<0)rxnoise[i]=symbols[1]; 
   if(real(tmp)<0 && imag(tmp)>0)rxnoise[i]=symbols[2]; 
   if(real(tmp)<0 && imag(tmp)<0)rxnoise[i]=symbols[3]; 
  } 
  if (rxnoise[i]!=txsymbols[i]){
   errorbits++;  
  }
 }
 double ber =double(errorbits)/double(Nbits);
 return ber;
}

Time elapsed 1403.682007ms
SNR = [ 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 ]
BER= [0.295244 0.251191 0.201748 0.152852 0.113027 0.076904 0.046738 0.025527 0.013203 0.005537 0.001846 0.000469 0.000068 0.000010 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 ]
Done..
I am getting output BER as zeroes for last values of SNR but wouldn't be zero right when comparing it with the theorectical BER vs SNR