Class specific arrays

Question

I'm making a simple opengl-engine and I need to be able to store series of integers for every class, containing animation frames.

Example: I create the class Dog. The Dog has 3 animations consisting of a number of frame indexes, representing the frame's position in the texture atlas. In the class file I want to define these animations like this:

const int animations[3] = {
    {1,2,3}, //walk
    {4,5,6}, //jump
    {1,7,8,9,10,11} //take a wiz
}

@implementation...

This is ofc not proper Obj-C, but is there a way to do something similar? Every instance of Dog will have the same animations so I dont want to waste memory by storing the animations in instance variables. If possible, I'd like to avoid using NSArray's.

Is there a way to store 2 dimensional, variable length arrays of ints like this at class level?

David Gelhar · Accepted Answer · 2011-03-11 17:32:26Z

1

You can certainly declare a static 2-dimensional array of int's. For example:

static int animations[3][3] = {{1,2,3},{4,5,6},{7,8,9}};

Because this is declared "static", animations will not be visible outside the scope of the source file that declares it, and there'll be only one copy of the array.

The trick is, C arrays have to be rectangular, and your example shows rows of different width.

To do that, you could use an array of pointers:

static int walk[] = {1,2,3};
static int run[] = {4,5,6};
static int wiz[] = {1,2,3,4,5,6};
static int *animations[3] = {walk,run,wiz};

(In that case, of course, you'd need some other means to know when you'd come to the end of a row, since they're not all the same length. For example, maybe the end of a row is marked with a '0' or '-1' value.)

answered Mar 11, 2011 at 17:32

David Gelhar

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3 Comments

oskob Over a year ago

That did the trick! But now, I want to find out the length of each animation. Doing sizeof animations[0] simply returns 8, i guess that the pointer adress is 8 bits long? If I do sizeof walk i get 24. Is there a way to get the size of the array the pointer is pointing to instead of the pointer itself?

David Gelhar Over a year ago

No. An array in C does not know its length; you have to keep track yourself. One approach (as suggested in the answer) is to use a "sentinel" value at the end, such as -1 (assuming that -1 is not a valid frame index). Alternatively, you wrap the array in some kind of object that records the length (at which point you're well on your way to re-inventing NSArray, so maybe you should just use that :-)).

oskob Over a year ago

Yeah, I tohught about that, it's just that NSArray has such dirty syntax. [[NSArray alloc] initWithObjects:[NSNumber numberWithInt:0], [NSNumber numberWithInt:1], [NSNumber numberWithInt:2], nil]. vs int arr[3] = {0, 1, 2} is a bit of a deal breaker :/ The sentinel value thingie seems like a good enough solution tohugh, thanks!

Chuck · Accepted Answer · 2011-03-11 17:26:53Z

0

It's possible to store two-dimensional arrays at any level you want, but 2D arrays where the rows have variable length are not generally all that useful except in special circumstances.

Anyway, the traditional way to pretend like Objective-C has class variables is just to make a static variable in the class's implementation file and create class methods to access it.

answered Mar 11, 2011 at 17:26

Chuck

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Anomie · Accepted Answer · 2011-03-11 17:27:37Z

0

Declare a static global variable in the .m file, just as you would do in C. If you need to access it outside of the class, create a class method (e.g. + (const int *)animations) to use as an accessor.

answered Mar 11, 2011 at 17:27

Anomie

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