Using constant to declare an array of item - objective-c

Question

Is it possible to use some thing like this in Objective-C:
#define number_of_items 10
and then using it as:
int arr[number_of_items];

Answer 1

Yes, assuming you mean Objective C. It's pretty much a superset of "proper" C so this is perfectly okay. It's also okay in both C and C++.
You can see that it works in the following transcript:
pax> cat qq.m
#import <objc/Object.h>
// First method.
#define number_of_items 10
int arr[number_of_items];
// Second method.
#define NUMBER_OF_ROWS 10
#interface test : Object{ int xyzzy[NUMBER_OF_ROWS]; }
#end;
pax> vi qq.m ; gcc -o qq.o -c qq.m -lobjc
pax> # no errors occurred
And, now that we've finally seen what you're actually using:
#define IS_IPAD (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad)
#define NUMBER_OF_ROWS_ (IS_IPAD? 18: 18)
NUMBER_OF_ROWS_ is not a constant, since it depends on the return value of the function UI_USER_INTERFACE_IDIOM().
In other words, it cannot be calculated at compile time. That's why you're getting the error. You can see this by compiling the following code:
#define IS_IPAD (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad)
#define NUMBER_OF_ROWS_ (IS_IPAD ? 18: 20)
int UI_USER_INTERFACE_IDIOM(void) {return 20;}
int UIUserInterfaceIdiomPad;
int main (void) {
int arr[NUMBER_OF_ROWS_];
return 0;
}
Under gcc --pedantic, you get:
qq.m: In function ‘main’:
qq.m:8: warning: ISO C90 forbids variable length array ‘arr’
You either need to use a dynamically adjustable collection like NSMutableArray or use an array of the maximum size desired and only use what you need of that.