Code

 Header...
 int main(){
   ... 

»  int m, n;
»  float * *p=NULL;// This will become the array of pointers

»  printf("How many vectors?\n");
»  scanf("%d", &m);

»  p=xmalloc(m*sizeof *p);// Allocate the array of pointers

»   printf("Size of each vector?\n");
»  scanf("%d", &n);// Find the row size

»  for ( int i=0;i<m; ++i) 
    pi=xmalloc(n*sizeof *p[i]);// Allocate rows

»  p[0]1=3.14159;
  // Use p for something 

»  printf("New size of first vector?\n");
»  scanf("%d", &n);

  // For variety we are making the vectors different sizes
»  for ( int i=0;i<m; ++i) 
    pi=realloc(pi, (n+i)*sizeof *p[i]);

  // Do something else

»  return 0;
}