I am having trouble in code:
typedef struct{
int a;
int b;
} Passenger;
typedef struct{
int amount;
Passenger bus_array[amount];
} Bus;
The amount undeclared here.
3 Answers
You have two alternatives, both involves dynamic memory allocation. The big difference is what structure you're allocating dynamically.
The first alternative is using a pointer instead of an array:
typedef struct{
int amount;
Passenger *bus_array;
} Bus;
Once you know the value of amount you can allocate memory for bus_array:
Bus my_bus;
bus.amount = get_passenger_amount();
bus.bus_array = malloc(bus.amount * sizeof(Passenger));
The second alternative is using an flexible array member (as I mentioned in a comment):
typedef struct{
int amount;
Passenger bus_array[];
} Bus;
Then you need to allocate the Bus structure dynamically:
int amount = get_passenger_amount();
Bus *my_bus = malloc(sizeof(Bus) + amount * sizeof(Passenger));
my_bus->amount = amount;
There are some differences between the two methods that might be worth to note. The most important is that the first method makes two separate and distinct allocations: One for the Bus structure and another for the bus_array. The second method there is only a single combined allocation for all of the Bus structure as well as bus_array.
answered Mar 23, 2019 at 16:42
Some programmer dude
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Adding to @Achai's correct answer, and because you asked, I would allocate memory for the passengers like this:
typedef struct{
int amount;
Passenger *bus_array;
} Bus;
Bus emptyBus = { 0, NULL}; // use this to initialize new buses so that pointer and amount are zero.
void changeAmount(Bus *bus, int amount)
{
bus->amount = amount;
free(bus->bus_array); // freeing a null pointer is OK.
bus->bus_array = malloc(sizeof(Passenger)*amount);
}
answered Mar 23, 2019 at 16:47
Peter - Reinstate Monica
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Comments
Here
typedef struct{
int amount;
Passenger bus_array[amount];
} Bus;
When compiler see the below statement
Passenger bus_array[amount];
it doesn't know how much memory to allocate for bus_array because amount is unknown for compiler at this stage. Hence it throws the error.
Instead of
Passenger bus_array[amount];
you can use
Passenger *bus_array;
and later on you can allocate memory for bus_array equal to amount bytes when compiler knows what is amount.
1 Comment
IMEI
Thank you! I will proceed using this method :)
amountin a function which setsammount[sic] and de-allocates the old memory and then allocates the new memory. That would be going in the direction of C++ (classes are structures of data with operations on them).mallocfor an array ofint, you usesizeof(int)for the element size right? It's the exact same thing with structures, but the type is e.g.struct Passanger, which means you usesizeof(struct Passanger).sizeof()is wrong (hey, it's asizeoff()!)? (Except with a zero size array.)