Code

 Header...
#define N 3

void normalise (double p[]);

 int main(){
»  double a[N]={2.7,5.3,8.3};
»  double b[N]={5.8,2.9,3.0};
»  double c[N]={-9.6,13.9,6.41};

»  normalise(a);
»  normalise(b);
»  normalise(c);
  
  // More stuff here..
  
»  return 0;
}
 ... 


// Normalise a vector (an array) of length N
 void normalise(double p[]){
»  double norm=0;
»  for ( int i=0;i<N; ++i) 
    norm+=pi*pi;

»  if (norm>0) {
»    norm=sqrt(norm);
»    for ( int i=0;i<N; ++i) 
    pi/=norm;
  }
}
 ...