Tutorial :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


Solution:1

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.


Solution: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*.


Solution:3

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.


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