Trouble with nested array of structs

Question

I am having trouble getting a nested array of structs to retain its members after returning from a function call.

Here is what I have defined in the header files (condensed struct members for readability):

typedef struct {
    char *name;
} weapon;

#define MAX_SHOP_ITEMS 10
struct shop {
    weapon items[MAX_SHOP_ITEMS];
};

typedef struct {
    char *name;
    struct shop shop;
} location;

I use these functions to add members to the shop.items[] array of structs. The beginnerWeapon[] is type weapon and has elements 0 and 1 declared in another file:

void PopulateShop(location loc)
{
    printf("DEBUG: character location pointer: %s\n\n\n", loc.name);

    loc.shop.items[0] = beginnerWeapons[0];
    loc.shop.items[1] = beginnerWeapons[1];

    printf("DEBUG: loc.shop.items[0].name: %s\n\n\n", loc.shop.items[0].name);
}

void ShowItems(location loc)   
{
    printf("DEBUG: location pointer in ShowItems(): %s\n\n\n", loc.name);
    printf("DEBUG: loc.shop.items[0].name: %s\n\n\n", loc.shop.items[0].name);
}

Now, when I pass a location to each function from another file through this function:...

void ShopMenu(location loc)
{
    PopulateShop(loc);
    ShowItems(loc);
}

...the debugs show the location name correctly in both functions, the correct weapon name shows up in PopulateShop(), but the shop.items[] array changes don't stick by the time I call ShowItems(). I'm not sure if I need to allocate memory for .items[], or externalize it, or pass pointers instead (which I'm not entirely sure how to do with subscripting), or perhaps I'm defining shop incorrectly. Any help is appreciated!

Common problem. Function parameters in C are pass by value. Changes inside a function to a function parameter have no effect on the caller's values. Pass in a pointer if you want to change the original struct. — kaylum, Jul 02 '17 at 02:35
Possible duplicate of [Passing struct to function](https://stackoverflow.com/questions/10370047/passing-struct-to-function) — kaylum, Jul 02 '17 at 02:39

Jahni Slim · Answer 1 · 2017-07-02T13:38:26.927

0

Fixed by using pointers as suggested by Kaylum --

void PopulateShop(location *locIn)
{
    location loc = *locIn;

    loc.shop.items[0] = beginnerWeapons[0];
    loc.shop.items[1] = beginnerWeapons[1];

    *locIn = loc;
}

Took me a minute to realize I needed to save the local variable back into the pointer. Thanks for the help!

Edit:

BE CAREFUL -- this only works for structs due to the copy constructor for structures doing a copy of members... David C. Rankin

Good to know.

locIn->shop.items[0] = beginnerWeapons[0];
locIn->shop.items[1] = beginnerWeapons[1];

which I also learned is equivalent to

(*locIn).shop.items[0] = myArr[i];

As I mentioned at the top, I wasn't sure how to use pointers with struct subscripting. Now I have another feather in my learning cap, thanks folks!

edited Jul 02 '17 at 13:38

answered Jul 02 '17 at 03:59

Jahni Slim

21
6

BE CAREFUL -- this only works for structs due to the *copy constructor* for structures doing a copy of members. If `loc` was any other locally declared compound type, e.g. an array, you would be assigning the address of values local to `PopulateShop` to `*locIn` which would be destroyed on function return when the function stack memory is released for reuse. – David C. Rankin Jul 02 '17 at 04:29
3

Or just drop `loc` and use `locIn->` instead of `loc.`. – alk Jul 02 '17 at 08:06

Trouble with nested array of structs

1 Answers1