Code

 Header...
// Shrink or extend a symmetric array
 float * * resizesymarray(float * *p,int oldm,int newm){
   ... 

»  int i, siz;
»  float *dat=NULL;
  
»  siz=newm*(newm+1)/2;

»  if (p!=NULL) 
    dat=p0;

»  dat=realloc(dat, siz*sizeof *dat);
»  p=realloc(p, newm*sizeof *p);

»  for (i=0; i<newm; ++i) 
    pi=dat+i*(i+1)/2;
  
»  return p;
}

// Create a new symmetric array
 float * * newsymarray(int m){
   ... 

»  return resizesymarray(NULL, 0, m);
}

// Free a symmetric array
 void freesymarray(float * *p,int m){
   ... 

»  resizesymarray(p, m, 0);
}

 int main(){
   ... 

»  float * *p=NULL;
»  int n, newn;

»  printf("Size of array (>0)?\n");
»  scanf("%i", &n);

»  p=newsymarray(n);
»  p[0]0=17;
»  p[n-1]1=11.3;

»  printf("New size of array (>0)?\n");
»  scanf("%i", &newn);

»  p=resizesymarray(p, n, newn);
  // Do stuff here

»  freesymarray(p, newn);
  // Do more stuff not requiring p

»  return 0;
}