Increment pointer inside C-linked list

Question

Good people, theoretically when we create a pointer and make it point to an array,

int a[10] = {0,1,2,3,4,5,6,7,8,9};
int *p = a;

then to increment the pointer to the next element we do something like

*(a+i)

But when it comes to a linked list, my code stops working. I have the following code

typedef struct lista carroPiloto, *pCarroPiloto;
struct lista {
    piloto piloto;
    carro carro;
    int *tempo;
    pCarroPiloto prox;
};

struct corrida {
    int voltas;     // entre 5 e 10
    int comprimento;    // entre 500 e 1000 (metros)
    int n_carros;   // numero maximo de carros a participar
};

/* ------------------ */

int tempos[caract.n_carros][caract.voltas];
pCarroPiloto aux = NULL;

for(j = 0; j < caract.voltas; j++) {
     tempos[i][j] = calculaSegundos(idade,peso,exp,potencia,caract.comprimento);     // matriz dos tempos
     *(aux->tempo + j) = tempos[i][j];
}

And when I do this the pointer won’t keep the valuables. And I can’t do

aux = aux->prox

because thus advance to the next node and my goal is to have a pointer with multiple times within each node of the linked list.

Answer 1

(...) my goal is to have a multi-stroke pointer within each linked list node

A pointer only tip to a memory zone, which leads to the first problem: Who defined this memory zone and put the values there ?

Here are two solutions:

• Have a pointer to the matrix tempos already in the correct element, and each time you want to navigate the matrix through the pointer you have to know how to do it.

  In this scenario I would start by manually assigning address of the first element of the appropriate matrix to the pilot.

  pCarroPiloto aux = malloc(sizeof(carroPiloto));
  
  aux->tempo = &(tempos[0][0]); //primeiro piloto aponta para o [0][0]
  

  And this puts his time pointer pointing to the address of tempos[0][0]. With this pointer you can always traverse all the elements of the matrix that matter. To traverse you only need to do the following:

  int *tempo_corrente = aux->tempo; //declarar outro ponteiro que aponta para o inicio das voltas
  for(j = 0; j < caract.voltas; j++) {
      printf("%d\n", *tempo_corrente); //utilizar tempo corrente
      tempo_corrente = tempo_corrente + caract.voltas; //passar para o próximo com aritmética de ponteiro
  }
  

  An array is stored as a contiguous zone of memory and each row is stored one after the other. So to take a pointer and move to the next line of the same matrix just advance the amount of elements that exist on a line, which in your example is caract.voltas.

  Note that you should not do aux->tempo = aux->tempo + caract.voltas else lose the reference for the first time of that car.

• Have a pointer to a memory zone defined by you, and built through malloc in which to replicate matrix values tempos in that new memory.

  In this solution the first step is to allocate space for the various car turns:

  pCarroPiloto aux = malloc(sizeof(carroPiloto));
  
  aux->tempo = malloc(sizeof(int) * caract.voltas);
  

  Now you can go through the matrix tempos and keep the values that matter:

  for(j = 0; j < caract.voltas; j++) {
      tempos[i][j] = calculaSegundos(idade,peso,exp,potencia,caract.comprimento);     
      aux->tempo[j] = tempos[i][j];
  }
  

  But where is the pointer notation ?

  Well you can use pointer notation if you want in this case, but it is totally unnecessary and complicates, but it would look the same as the question:

  for(j = 0; j < caract.voltas; j++) {
      tempos[i][j] = calculaSegundos(idade,peso,exp,potencia,caract.comprimento);     
      *(aux->tempo + j) = tempos[i][j];
  }
  

  However the biggest difference to your code is in the malloc I gave to the pointer tempo, because if you don’t, you’re trying to store elements in a pointer that doesn’t point to a location defined by you, and that represents undefined behavior, and a potential Segmentation fault.

  The same can be said for the variable aux which cannot have access to its fields if it is declared as NULL.