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2D-array pointer as a struct member
I have an array of struct:
static struct basket baskets[MAX_ITEMLEN + 1];
struct basket {
char *items; // malloc(itemlen * itemcount)
int itemcount;
};
char *items does all I need to do with some pointer-gymnastics, but at times it gets a little ugly.
I would prefer a 2D-array pointer allowing items[item_index][char_index] navigation by default, but all my attempts to get a 2D-array pointer as struct member have failed miserably.
If it is indeed possible: how?
Details (in case relevant):
#1 Specs:
#define MAX_ITEMLEN 20
#define MAX_ITEMCOUNT 16000
#2 itemcount(s) & the char content of each item are not known at compile-time but are derived at start-up by parsing two user-provided .txt files
#3 The code will not interrogate baskets[x] having itemcount == 0
#4 *items is initialised: |←item_chars→|←item_chars→| ...itemcount times
...with no delimiters ('\0' or otherwise)
#5 After initialsation, basket[] is invariant until exit()
2 answers
If you don't mind the extra memory, you can do it with an extra array:
struct basket {
char *item_memory;
char **items;
int itemcount;
};
/* I omitted any error handling */
void initialize(basket *b, int itemlen, int itemcount) {
int item_index;
b->item_memory = malloc(itemlen * itemcount);
b->items = malloc(sizeof(char*) * itemcount);
b->itemcount = itemcount;
for (item_index = 0; item_index < itemcount; ++item_index)
b->items[item_index] = item_memory + itemlen ** item_index;
}
Of course here the actual storage is not in *items but in *item_memory. You cannot avoid that if you want to use double index on items; the only thing in C you can apply the index operator to are pointers, therefore *index must be a pointer.
Thanks @celtschk for your suggestion: essentially a lookup-table into a (renamed) char *items. It would work, BUT
Ignoring the inevitable memory inflation (237% in a simple test-case), the array-of-pointers look-up is not fast! Speed is a concern since the code must look (and re-look) millions of times at items.
wasizy
Newbie on this site: don't know the protocol for OP response to a proposed answer: if this should be in a (new?) comment thread: tell me.
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