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Nov 6th, 2008
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passing a matrix

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Can somebody explain whats wrong in the following code ? I am trying to
pass a matrix and the called function is a double pointer.
C++ Syntax (Toggle Plain Text)
  1. #include <iostream>
  2.  
  3. void func(int **);
  4.  
  5. int main(int argc,char *argv[])
  6. {
  7. 	int A[2][2]={1,2,3,4};
  8. 	func(A);
  9. }
  10. void func(int **A)
  11. {
  12. }


Best,
Pradeep
Last edited by drdolittle; Nov 6th, 2008 at 8:56 pm.
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Re: passing a matrix

My first thought is because int** is not the same as int[][], though the syntactic use of each is quite similar.
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Re: passing a matrix

Click to Expand / Collapse  Quote originally posted by Lerner ...
My first thought is because int** is not the same as int[][], though the syntactic use of each is quite similar.
You're quite right. The int** is a pointer that would be allocated an array of pointers, each of which point to an array of ints. int[][] is a contiguous block of memory, which can only be accessed correctly when the column dimension of the argument array and the column size in the function's formal parameter are the same.
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Re: passing a matrix

c++ Syntax (Toggle Plain Text)
  1. //--------------------------------------
  2.  
  3. int A[2][2]={1,2,3,4}; //--overflow
  4. func(A);
  5.  
  6. //--------------------------------------
  7.  
  8. int A[1][4];
  9. A[0][0] = 1;
  10. A[0][1] = 2;
  11. A[0][3] = 3;
  12. A[0][4] = 4;
  13.  
  14. //--------------------------------------
Last edited by cikara21; Nov 7th, 2008 at 3:11 am.
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Re: passing a matrix

Click to Expand / Collapse  Quote originally posted by cikara21 ...
c++ Syntax (Toggle Plain Text)
  1. //--------------------------------------
  2.  
  3. int A[2][2]={1,2,3,4}; //--overflow
  4. func(A);
  5.  
  6. //--------------------------------------
  7.  
  8. int A[1][4];
  9. A[0][0] = 1;
  10. A[0][1] = 2;
  11. A[0][3] = 3;
  12. A[0][4] = 4;
  13.  
  14. //--------------------------------------
What? Where is overflow?
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Re: passing a matrix

okay, but nobody has suggested how to correct the error. I think its the function declaration which has an error.
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Re: passing a matrix

Try
C++ Syntax (Toggle Plain Text)
  1. void func(int (*)[2]);   // declaration
  2.  
  3. void func(int (*x)[2])
  4. {
  5.     // whatever
  6. }
In this exmaple, func accepts an argument of type "pointer to array of two ints".

The initialisation of A in your example should be something like;
C++ Syntax (Toggle Plain Text)
  1. int A[2][2]={{1,2},{3,4}};
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Re: passing a matrix

Click to Expand / Collapse  Quote originally posted by drdolittle ...
okay, but nobody has suggested how to correct the error. I think its the function declaration which has an error.
The solution is to decide how you want to allocate the matrix, and use function parameter that is formatted the same.
C++ Syntax (Toggle Plain Text)
  1. //either
  2. void func(int [][2]);
  3.  
  4. int main(int argc,char *argv[])
  5. {
  6. 	int A[2][2]={1,2,3,4};
  7.  
  8. 	func(A);
  9. }
  10. void func(int A[][2])
  11. {
  12. }
  13.  
  14. // --- OR ---
  15.  
  16. void func(int **);
  17.  
  18. int main(int argc,char *argv[])
  19. {
  20. 	int **A;
  21.         A = new int *[2];
  22.         A[0] = new int[2];
  23.         A[1] = new int[2];
  24.         //now go store data in the matrix
  25.  
  26. 	func(A);
  27. }
  28. void func(int **A)
  29. {
  30. }
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Re: passing a matrix

http://www.scribd.com/doc/3048219/ex..._related_doc=1

read from page 230
kux
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Re: passing a matrix

Use reinterpret_cast or c style cast to casting..
But the matrix will become 1 dimension matrix..

c++ Syntax (Toggle Plain Text)
  1. #include <iostream>
  2.  
  3. void func(int **);
  4.  
  5. int main(int argc,char *argv[])
  6. {
  7. 	int A[2][2]={1,2,3,4};
  8. //--reinterpret_cast
  9.       func(reinterpret_cast<int**>(A));
  10. //--c style cast
  11.       func((int**)A);	//--func(A);
  12. }
  13. void func(int **A)
  14. {
  15. }


Best,
Pradeep[/QUOTE]
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