call dynamic c array values

Question

I am trying to access all values that I call. I'm not sure if I use array values correctly. In code below I'm trying to write values in file to memory and then call them by address. what is the problem? Am I overwriting on same memory adresses or I call them wrongly? Any help please?

my text file:

0 0 100 500 player1

0 1 400 450 player2

1 1 300 600 player3

FILE *fp;
struct ob {
  int x;
  int y;
  int c_hp;
  int max_hp;
  char name[100];
  int p_id;
};

struct ob *ptr;
int count = 0;
int main(int argc, char *argv[]) {
  if (argc != 2) {
    printf("Usage: program <File name>\n");
    exit(1);
  }
  read_player_values();
}

void read_player_values() {
  ptr = (struct ob *)malloc(sizeof(struct ob));

  if (ptr == NULL)
    printf("out of memory");
  else {
    while (!feof(fp)) {
      count++;
      fscanf(fp, "%d%d%d%d%s", &ptr->x, &ptr->y, &ptr->c_hp, &ptr->max_hp,
             ptr->name, &ptr->p_id);
      ptr->p_id = count;
      printf("%d %d %d %d %s %d", ptr->x, ptr->y, ptr->c_hp, ptr->max_hp,
             ptr->name, ptr->p_id);
      printf("\n");
    }
    fclose(fp);

    printf("------\n");
    for (int i = 0; i < count; i++) {
      /* here is my problem */
      /*trying to print all player names here*/
      printf("%s\n", (ptr - i)->name);
    }
  }
}

Where do you assign fp? In main you check whether filename was passed, but don't use it. — fas
– fas, Commented Apr 14, 2020 at 11:29
ptr is not a pointer to an ob array but a pointer to a single ob struct, but you seem to assume that its an array with the printing part. You just rewriting this single ob element with every loop iteration and not building an array as you might want. — Eraklon
– Eraklon, Commented Apr 14, 2020 at 11:33
I'm calling read_player_values() method after that and assing fp in fscanf . I'm reading file values and assing them to struct variables — hhilal
– hhilal, Commented Apr 14, 2020 at 11:35
@Eraklon how can build an array to contain all my file values? — hhilal
– hhilal, Commented Apr 14, 2020 at 11:37
after count++ you can do something like ptr = realloc(ptr, count * sizeof(struct ob)); and use ptr like &ptr[count-1].x in every place. — Eraklon
– Eraklon, Commented Apr 14, 2020 at 11:40

Hitokiri · Accepted Answer · 2020-04-14 20:21:09Z

1

Firstly, you have to open the file in main function:

fp = fopen(argv[1],"r");
if (!fp)
   return -1;

You can use fgets function to get each line of the file and get all values from this line by using sscanf. After each iteration, you have to re-allocate for ptr. ptr here likes an array that has count elements.

char line_buf[256];
count = 1;
while (fgets(line_buf, sizeof(line_buf), fp)) {
      ptr = realloc(ptr, count * sizeof(struct ob));
      if(!ptr)
         return;
      sscanf(line_buf, "%d%d%d%d%s\n", &ptr[count-1].x, &ptr[count-1].y, &ptr[count-1].c_hp, &ptr[count-1].max_hp,
             ptr[count-1].name);
      ptr[count-1].p_id = count;
      printf("%d %d %d %d %s %d", ptr[count-1].x, ptr[count-1].y, ptr[count-1].c_hp, ptr[count-1].max_hp,
             ptr[count-1].name, ptr[count-1].p_id);
      printf("\n");
      count++; 
}

For printing all names:

    printf("------\n");
    for (int i = 0; i < count-1; i++) {
      printf("%s\n", ptr[i].name);
    }

the result after testing:

#cat text.txt
0 0 100 500 player1
0 1 400 450 player2
1 1 300 600 player3

./test text.txt
0 0 100 500 player1 1
0 1 400 450 player2 2
1 1 300 600 player3 3
------
player1
player2
player3

edited Apr 14, 2020 at 20:21

answered Apr 14, 2020 at 12:37

Hitokiri

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2 Comments

Eraklon Over a year ago

The small problem with the order of realloc is that you will have one extra element in the array when the loop exits.

Hitokiri Over a year ago

I got it, so we can reallocate before increasing the count value. I will update it as soon as possible and use ptr[count-1], init count = 1. Thanks

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