How to pass double[][] into a method call?

Question

Hi I am trying to init an object with a double value in the format double filter[3][3]; but i keep getting the following error.

cannot convert 'double[3][3]' to 'double' in assignment.

in my header i have this

@interface filter : NSObject
{
    double **matrix;
}
@property(nonatomic)double **matrix;
-(id)initWithMatrix:(double**)filterMatrix;

inside my class i have this method.

-(id)initWithMatrix:(double**)filterMatrix
{
    matrix = filterMatrix;
    return self;
}

and i am trying to call it like this.

double filter[3][3] = {0,0,0,0,1,0,0,0,0};
MyMatrix *myMatrix = [[MyMatrix alloc] initWithMatrix:filter];

I now get the error.

Error: Cannot convert double[*][3] to double** in argument passing

Any help on this issue would be amazing. Thanks A

Answer 1

you are passing as a parameter a double 2d array(double[][]) when your method signature asks for a double (a primitive like 34.2). set the method declaration to

- (id)initWithMatrix:(double*) matrix;

this passes a pointer to your array (2d) to the method. edit: missed a semicolon.

Answer 2

That's because double** isn't the equivalent of double[*][*]. In fact, double[*][*] is an invalid type, because it leaves the stride undefined. double** is a pointer to a pointer to a double, or to put it another way, it's a pointer to an array of doubles. You should just use double* as your type.

Edit: To clarify, double[*][3] is still just an array of doubles, even though it has 2 dimensions. This is still the equivalent of double*.

Answer 3

A two-dimensional array is not the same thing as a pointer-to-a-pointer. You have two choices - change the filter class to contain a 2D array, or change your initialization to use pointer-to-pointers.

In choice #1, you're could keep a copy of the array in your filter instance, instead of just holding a pointer. You need to change the class interface:

@interface filter : NSObject
{
    double matrix[3][3];
}

-(id)initWithMatrix:(double[3][3])filterMatrix;

Then your implementation of initWithMatrix: can just do a memcpy() or the equivalent to copy the data into your instance.

Choice #2 is a bit different. Keep your other code the way it is, but change your initialization of filter:

double row0[3] = {0,0,0};
double row1[3] = {0,1,0};
double row2[3] = {0,0,0};
double **filter[3] = { row0, row1, row2 };

It's probably safer to malloc() all of those arrays, since otherwise you're going to end up with references to stack variables in your filter class, but I think you get the idea.