How can I iterate through an array within an array of a struct using pointers

Question

I'm given a struct:

struct Agency {
    char name[255];
    int zip[5];
    RentalCar inventory[5];     // array of RentalCar objects
};

I need to retrieve data from a file to fill an array of 3 Agency objects. To set the values for the first item in the inventory array is easy,

Agency ag[3];
for (int i = 0; i < 3; i++) {
ag->inventory->setWhatever();
ag++;     // easy to iterate through this array
}

But It's not that easy to iterate through the inventory array within the agency array.

inventory++;        // doesn't know what object this is associated with
ag->inventory++;    // also doesn't work

Can you not do ag[0].inventory[1] for example to access the second RentalCar from the first Agency? Would it be ok or do you need to do this through pointers? — Attersson
– Attersson, Commented Jun 14, 2018 at 23:01
@Beta yes, and I can write over the first one in the array as many times as I want, I just can't get to the second or anything passed that. — griffin175
– griffin175, Commented Jun 14, 2018 at 23:44

Attersson · Accepted Answer · 2018-06-15 00:08:47Z

2

Agency ag[3];
Agency *ag_ptr= &ag[0]; // or: Agency *ag_ptr = ag; //as this decays to pointer
for (int i = 0; i < 3; i++) {
    Rentalcar *rental_ptr = ag_ptr->inventory; //rental_ptr now points at the first RentalCar
    rental_ptr->setWhatever(); //set first rental car
    rental_ptr++;
    rental_ptr->setWhatever(); //set second rental car
    ag_ptr++;
}

Arrays can be accessed through index, for example ag[0] ag[1] ag[2] but this is illegal: ag++. The array name ag decays to pointer to first element when used alone, but it is not a pointer.

Pointers, on the other hand can scroll memory address with + and - operations. Be wary, because they could easily point to memory not reserved for your program.

edited Jun 15, 2018 at 0:08

answered Jun 14, 2018 at 23:04

Attersson

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3 Comments

griffin175 Over a year ago

I can't use the array index operator [ ], only for declaring array. I know it's stupid but that's what's required. But if I were able to use ptr->inventory[1]->setWhatever(); wouldn't it be ptr->inventory[1].setWhatever(); since inventory[1] is a variable not a pointer?

user4581301 Over a year ago

@griffin175 Agency *ptr = &ag[0]; and Agency *ptr = ag; result in the same thing. That should eliminate all of the []s used to index.

Attersson Over a year ago

@user4581301 Sure thing.

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