Code

 Header...
// Demonstrate a growable array 
// - NB This code contains an unnecessary call to malloc()
// - to simulate the behavior of realloc() in a larger, 
// - more realistic program whether there is other 
// - dynamic memory allocation

# define GROWBY 4
// Growable array of floats
typedef struct floatarray {
  float *values;
  int nvals;
  int buflen;
}Floatarray;

// Allocate a new growable array
 Floatarray* newfloatarray(void){
   ... 

»  Floatarray *f;

»  f=xmalloc(sizeof *f);// NB xmalloc checks for NULL
»  f->buflen=f->nvals=0;
»  f->values=NULL;

»  return f;
}

// Add a value to an array, extending it if necessary
 void addtoarray(Floatarray *f,float value){
   ... 

  // Check to see if we need to extend the array
»  if (f->nvals==f->buflen) {
»    f->buflen+=GROWBY;
»    f->values=realloc(f->values, f->buflen*sizeof f->values);
  }
»  f->values(f->nvals)=value;
»  f->nvals++;
}

void print2 (Floatarray **ar2);

 int main(){
   ... 

»  Floatarray **fars=NULL;
»  float value;
»  int kmax=-1, k;
  
»  printf("Just keep typing in pairs of numbers!\n");
  
»  while (scanf("%d %g", &k, &value)==2) {
»    if (k>kmax) {
»      fars=realloc(fars, (k+2)*sizeof *fars);
      
»      while (kmax<k) {
»  ++kmax;
»  farskmax=newfloatarray();
  }
»      fars(kmax+1)=NULL;
      
}
»    addtoarray(farsk, value);
  }

»  print2(fars);
»  return 0;
}

// Print one "array" (accepts Floatarray)
 void print1(Floatarray *far){
   ... 

»  printf("The array (%p) has %d elements\n", far, far->nvals);
»  for ( int i=0;i<far->nvals; ++i) 
    printf("%d: %g\n", i, far->values[i]);
}

// Print out a 2-D "array"
 void print2(Floatarray **ar2){
   ... 

»  while (*ar2) {
»    print1(*ar2);
»    ar2++;
»    printf("\n");
  }
}