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I have written a C++ application and within it, I need to call a math function that was written in C. The prototype looks like:

void Jacobi_Cyclic_Method(double *eigenvalues, double *eigenvectors, double *A, int n);

My problem is that I can't seem to pass the function double * (for instance both eigenvectors and A are multi-dimensional arrays. The C++ way to pass those things seems to be 

double [][size]

I have read about extern C but I don't think it applies here since I am not interfacing with an object but with source itself. How can I send that C function my multi-dimensional arrays defined as such:

double [100][100] A;
double [100][100] eigenvectors;
double [100] eigenvalues;

Trying to compile I get:

error: no matching function for call to ‘MathEngine::Jacobi_Cyclic_Method(double 
[100], double [100][100], double [100][100], int)’
mathEngine.h:9: note: candidates are: static void    
MathEngine::Jacobi_Cyclic_Method(double*, double*, double*, int)
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  • Are you re-compiling everything, including your math function's implementation, or are you linking to an existing compiled library? That matters a lot as to whether extern "C" matters or not Commented Apr 25, 2012 at 3:35
  • The same way that you would do it had your arrays been defined in C. Commented Apr 25, 2012 at 3:36
  • You should just be able to pass these values in directly. A double[100] is a double *, and so is a double[100][100]. What goes wrong when you try? Commented Apr 25, 2012 at 3:37
  • 7
    Your C prototype requires one-dimension arrays for eigenvalues, eigenvectors, and A. Am I missing something? Commented Apr 25, 2012 at 3:37
  • I am recompiling from it's source. When I try to compile I edited my post with what I get when I try to compile Commented Apr 25, 2012 at 3:39

2 Answers 2

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Probably the problem is that your Jacobi_Cyclic_Method function requires the matrix to be in either column or row major format, where every column/row is stored consecutively in a single, one-dimensional array. E.g. for a row major matrix of size m x n, the elements in any given row are stored contiguously and the item in row i and column j would be at position i*n+j (for zero-based indices i and j). If the matrix is column-major, it would be at position i+j*m.

Using multi-dimensional arrays in C/C++ is often not what you want because something like

double A[100][100];

is not a two-dimensional array, but a single array of length 100 containing pointers to arrays of length 100. Consequently, the rows in A are not stored contiguously.

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4 Comments

Well, the method is defined here: code.google.com/p/biaxmc2/source/browse/trunk/montecarlo2/mc2/… It appears that it does indeed accept a double[][].
That's what the comments say, but the code says differently. If it accepted double[][], the argument declaration for the matrix should be double** A, not double* A. -- BTW, what kind of a person are you to chose such a disrespectful, stupid user-name?!
Wow! You know I took the name from someone I saw on a game I used to play a while back and certainly wasn't aware of the chemical warfare aspect. I was quite confused at your comment here until I looked it up. I had an avatar of a guy in an orange suit etc, etc. Certainly no offense intended, I apologize. Thank you for pointing that out.
;-) Glad you didn't choose that name knowing its meaning. Apologies too, for my harsh reaction. Any ways, I generally prefer it if people choose display names related to their real names.
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I'm assuming that your C function requires multi-dimensional arrays for some of its parameters and that the prototype is written with pointers to doubles for the array/matrix parameters where the integer parameter indicates the size of each dimension (I guess the matrices are square?). If this is the case then you can pass a pointer to the first element of each array/matrix like this:

Jacobi_Cyclic_Methods(&eigenvalues[0], &eigenvectors[0][0], &A[0][0], 100);

Your initial attempt doesn't work because the type of, for instance, eigenvectors is double[100][100] which decays to double (*)[100], not double *.

This post addresses the issue of pointers and multi-dimensional arrays.

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4 Comments

I wouldn't try this unless the function documentation states that a double[100][100] is the right format. Check if the function requires row-major or column-major matrix layout.
This makes a lot of sense, I will try it out when I get home. Thanks
This has the exact effect of casting directly to (double *). Both methods let me compile but I still had issues with how the C function thought the memory was laid out vs how it actually was. That said, this works as a solution to my original post.
When operating with matrices it's worth remembering that C and C++ store them in row- major order. Iterating over matrices in column-major order can create huge performance problems. Unless the whole matrix fits in a single cache-line you can take a cache-miss every time you increment the column index.

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