Code

 Header...
#define NA 2
#define NB 3
#define NC 4

void normalise (float p[],int n);

 int main(){
   ... 

»  float a[NA]={2.7,5.3};
»  float b[NB]={5.8,2.9,3.0};
»  float c[NC]={-9.6,1.4,13.9,6.41};

»  normalise(a, NA);
»  normalise(b, NC);// Oops!
»  normalise(c, NC);
  
  // More stuff here..
  
»  return 0;
}

// Normalise a vector (an array) of length n
 void normalise(float p[],int n){
   ... 

»  float norm=0;
»  int i;

»  for (i=0; i<n; ++i) 
    norm+=pi*pi;
»  norm=sqrt(norm);

»  if (norm>0) {
»    for (i=0; i<n; ++i) 
    pi/=norm;
  }
}