Thread: Dynamic array of structures with a dynamic arrays inside of it

You've messed up more than your malloc() calls.

In init(), words is an uninitialised pointer. The first thing you then do is access words[i]. That gives undefined behaviour.

You need to make up your mind about whether you want words to of type storage * (only one asterix) or storage ** (which is what you have). Your usage of the words is inconsistent.

Originally Posted by mdarda

Honestly, I've got a big problem with when i need to use * and when **. My college teacher tought me that when I'm creating a function, which creates a table i need to use ** and then return *.

That's rubbish. Sometimes you need to do that, but most times, it is unnecessary.

From information, you've provided, declaring words as storage * and returning words will work. You will need to change how you set up elements of the dynamically allocated array (using . rather than ->, etc).

Rule of thumb: use no more levels of indirection than necessary. "Indirection" means levels of pointer to pointer to ....

Originally Posted by mdarda

Code:

    for(i = 0; i < argc - 2; i++) {
        words = malloc(sizeof(storage));
        words++;
    }

Would you care to explain what, exactly, you think this loop achieves?

In your code, words should never need to be incremented, and realloc() is not needed. Use pointer arithmetic or array indexing. Not both together.

Sheesh, are CT and grumpy jacking you around?

Originally Posted by mdarda

I thought that with the for statement i can alloc space for argc-2 pointers (words pointers) thats why i'm incementing word++ that i could use them as a static array with [i] index, am I wrong?

You do allocate the space, and that would be okay except:

1) words itself is supposed to be an array of pointers, but you never initialize it as such. Not only does every pointer in the array need to be allocated dynamically, but so does the array itself (see below).
2) You increment words in the initial for loop, so at the end, words does not point to the same place anymore. You would then need to rewind it:

Code:

words -= i; // pointer arithmetic uses unit size of type

However, that is sort of an unorthodox method. Your original for(i) loop in post #1 was correct, just you did not allocate for the array before you allocated its members.

Here's an example of part of what you are trying to do. It reads a list of words from standard input into an array of structs, dynamically allocated. You must specify how many words via a command line parameter, eg:

echo "one two three four five" | ./a.out 5

Code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

typedef struct {
	char *word;
	int count;
} storage;


storage *addWord (char *input) {
// allocate, initialize, and return a new struct
	storage *rv = malloc(sizeof(storage));
	rv->word = malloc(strlen(input));
	strcpy(rv->word, input);
	rv->count = 1;
	return rv;
}

void freeStorage (storage **p) {
// deallocate struct
	free((*p)->word);
	free(*p);
}

int main(int argc, char *argv[]) {
	int i, total = strtol(argv[1], NULL, 0);
// declare AND INITIALIZE words properly
	storage **words = malloc(total * sizeof(storage*));
	char data[1024];

// simultate file input
	for (i = 0; i < total; i++) {
	// check for error when reading
		if (fscanf(stdin, "%s", data) != 1) break;   
		words[i] = addWord(data);
	}

// check
	for (i = 0; i < total; i++)
		printf("%s (%d)\n", words[i]->word, words[i]->count);

// cleanup
	for (i = 0; i < total; i++) freeStorage(&words[i]);
	free(words);

	return 0;
}

So there is your **pointer array, and a function which returns a *pointer designed for it. Notice the malloc() on words is for sizeof(storage*) not sizeof(storage) -- the second one is used to allocate structs in the for loop, but the first one is for the pointers to the structs. Make sense?

Output:

one (1)
two (1)
three (1)
four (1)
five (1)

Originally Posted by mdarda

I thought that with the for statement i can alloc space for argc-2 pointers (words pointers) thats why i'm incementing word++ that i could use them as a static array with [i] index, am I wrong?

Absolutely and utterly wrong.

If you want to allocate an array of n Foo's, change the value you supply to malloc(). For example, Foo *p = malloc(n * sizeof(Foo)). You don't call malloc(sizeof(Foo)) n times and you certainly don't increment the pointer.

Originally Posted by MK27

Sheesh, are CT and grumpy jacking you around?

No we are not. The OP has showed evidence of a deep misunderstanding of what he is doing. I was trying to understand the extent of that misunderstanding in order to correct it.

Incidentally, you have the lengths of strings wrong in your example.

Originally Posted by grumpy

Incidentally, you have the lengths of strings wrong in your example.

Good catch, too bad I can't edit it. So line 14 from post #9:

Code:

    rv->word = malloc(strlen(input));
// should be
    rv->word = malloc(strlen(input)+1);