[C] Problem with pointers and arrays

Hi everyone! I wrote this code to calculate a polynomy's derivative:

[CODE]
typedef int *poly;
typedef poly *der;

der derivative(poly P, int N, int k);
int calcolaDerivata(int coeff, int grado);

der derivative(poly P, int N, int k)
{
   int i, j, f;
   der d;
   poly p2;
   poly pp;

   d = calloc(N, sizeof(poly));
   p2 = calloc(k+1, sizeof(int));

   for( i=0; i   {
      p2 = calloc(k+1, sizeof(int));

      for( j=0; j<=k; j++ )
      {
         p2[j] = calcolaDerivata(P[j], j+1);
         printf("%d: %d -- %d\n",j,P[j],p2[j]);
      }

      k -= 1;
     [B] d[i] = p2;[/B]
      pp=d[i];

      printf("i: %d\nd: %d %d %d %d\n",i, pp[0], pp[1], pp[2], pp[3] );
      getch();

      for( f=0; f<=k; f++ )
         P[f]=p2[f+1];
      //free(p2);
   }

   pp=d[0];

      printf("\n\nCycle:\n\ni: %d\nd: %d %d %d %d",i, pp[0], pp[1], pp[2], pp[3] );
      getch();
   return d;
}

int calcolaDerivata(int coeff, int grado)
{
   if( grado == 1 )
      return 0;
   else
      return coeff*(grado-1);
}

[/CODE]

I don't want to talk about the algorithm, I just want to understand why the bolded code (d[i] = p2) every time copies the value of p2 into EVERY elements of d.

In other words:
1 iteration --> d[0] = p2 OK (at the end of the cycle, I of course recalculate p2 value)
2 iteration --> d[1] = p2 and I have d[0] AND d[1] with the same value!!! Why the assignment of d[1] changes the value of d[0] too???

Of course, each iteration copies the last value in EVERY elements of d.

Could you please help me? Thanks a lot!!!

2 Replies

Know the answer? Post it — somebody with the same question will find it here.

marco_brovelli15y ago
Thanks for your answer! Actually, the first calloc is useless ;)
I solved this one, commenting ther free(p2) line.
Sam Hobbs15y ago
First, I want to say that the first calloc for p2, the one preceding the for loop, is superfluous; you do not need it, correct? That is not causing the problem however.

In the line that has "d[i] = p2;" you do not have an index so I assume p2 has the address; the return value of calloc. I assume that is not what you want but I am not sure. When you do a calculation for p2, you have an index as in "p2[j]".

It is difficult for me to know what it is that you need to do since the code you posted is C code written in a manner that is intended to be difficult to understand. On top of that, I do not know what the code is supposed to be doing.