how to code multi-dimensional array in a function prototype in c++

Question

When I create a multidimensional array like value[][],and pass it to the function.But I am quite confused about how to write a prototype for this function,because there is an error message like:

   error: declaration of 'value' as multidimensional array must have bounds for all dimensions except the first|


   //function prototype;
  void matrix(double s[],int j,int n,double value[][],double alpha[], double beta[], double gamma[]);
  //function
   void matrix(double s[],int j,int n,double value[][],double alpha[], double beta[],double gamma[]){...}

help,How to write it correctly?

Answer 1

The error message from the compiler is very telling. The argument double value[][] needs to be changed to double vale[][N] where N is a integer. Examples:

double value[][20] // A matrix in which each row has 20 columns.
double value[][10] // A matrix in which each row has 10 columns.

Answer 2

the compiler already told you error: declaration of 'value' as multidimensional array must have bounds for all dimensions except the first|

you need specify the length, eg: double s[3], double[3][3] etc.

Answer 3

The folks above have given the technical explanation.

The reason behind is is that C and C++ treat arrays as blocks of memory. When an array gets passed to a function, all that gets sent is a pointer to the array. This is in stark contrast to Ada, Pascal, and Fortran that send descriptors of the array to the function.

Your declaration must provide enough information to work with the array with just a pointer passed. That means is need all but the last (first specified) array dimension.

Multidimensional arrays of variable size are usually a sign of bad design. If you have a fixed array size (4x4 is common in 3D transforms), 2D array work out well. For something like general purpose matrix operations, 2D arrays do not work well.

For that you need to define a class that: 1) Manages a 1D array 2) Provides the mechanism for translating 2D references into 1D index.

What you would do is something like

class Matrix 
{
  unsigned int range1 ;
  unsigned int range2
  double *values ; // Could use a template

  double &valueAt (unsigned int x, unsigned int y) { return values [range1 * x + y] ; }
} ;