Storing and Accessing a Variable 3-D C Array in Objective C on iOS

Question

Currently, I have a need to store a potentially large amount of data (as doubles) in memory. I had originally created a NSObject container to store the data and added those to a NSMutableArray. However, this meant creating potentially thousands (or even hundreds of thousands) of NSObjects, the allocation of which was slowing everything down a lot. Moreover, all of those objects were eating up more memory than I'd like. This same data can be "easily" represented by a 3 dimensional c array to the effect of:

double data[fieldCount][dataAggregateCount][recordCount];
//Aggregate count is actually a static 6 at the moment, so it could be initialized:
double data[fieldCount][6][recordCount];

However, I can't really store that as an instance variable (that I know of). At present I do this:

@implementation MyClass{
    double *_data;
}

And then malloc the data:

_data = malloc(sizeof(double) * fieldCount * 6 * recordCount);

However, to access the data, I've gotta do some particularly verbose math:

double value = _data[x + (y * fieldCount) + (z * fieldCount * aggregateCount)];

What I really want is to access the data like so: _data[x][y][z] but, of course, the compiler bitterly complains about that sort of notation on a double *.

Am I being unreasonable to expect I could store a variable multidimensional c-array as an instance variable without resorting to a continuous chunk of memory and manual offset calculations?

Mostly I'm concerned about speed, and secondarily memory. Creating a hundred thousand NSNumber objects takes a whole lot longer than malloc(sizeOf(double) * 100000). While NSNumber can alloc on the stack (using that neat pointer storage trick), it's unlikely to with the numbers I'm dealing with. Thus, I'm alloc'ing 100,000 NSObjects. I'm trying to avoid this. — Aaron Hayman
– Aaron Hayman, Commented Apr 23, 2014 at 14:03

Michael · Accepted Answer · 2014-04-23 14:10:05Z

1

Just allocate a three-dimensional array...

/// Allocates a 2-dimensional double array.
double **doubleArray2(int y, int z)
{
    double **a = malloc(sizeof(double *)*y);
    if(!a) {
        [NSException raise:@"no memory" format:@"haha-1"];
    }
    for(int i=0; i<y; i++)
    {
        a[i] = malloc(sizeof(double)*z);
        if(!a[i]) {
            [NSException raise:@"no memory" format:@"haha-2"];
        }
    }
    return a;
}

/// Allocates a 3-dimensional double array.
double ***doubleArray3(int x, int y, int z)
{
    double ***b = malloc(sizeof(double **)*x);
    if(!b) {
        [NSException raise:@"no memory" format:@"haha-3"];
    }
    for(int i=0; i<x; i++)
    {
        double **a = doubleArray2(y, z);
        if(!a) {
            [NSException raise:@"no memory" format:@"haha-4"];
        }
        b[i] = a;
    }
    return b;
}

have fun...

EDIT: and don't forget to deallocate the array properly when you're done ;)

answered Apr 23, 2014 at 14:10

Michael

6,5115 gold badges33 silver badges54 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Aaron Hayman Over a year ago

Hmmm... yeah, that would work. Constructing and deconstructing the array is a bit of a pain, but I'd only have to abstract that out once. I can't help but thinking that calculated offsets aren't more efficient... although, at this point I may be over engineering my solution here.

Collectives™ on Stack Overflow

Storing and Accessing a Variable 3-D C Array in Objective C on iOS

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related