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passing a matrix

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passing a matrix

Nov 6th, 2008

Can somebody explain whats wrong in the following code ? I am trying to
pass a matrix and the called function is a double pointer.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <iostream>
 
void func(int **);
 
int main(int argc,char *argv[])
{
	int A[2][2]={1,2,3,4};
	func(A);
}
void func(int **A)
{
}
#include <iostream>

void func(int **);

int main(int argc,char *argv[])
{
	int A[2][2]={1,2,3,4};
	func(A);
}
void func(int **A)
{
}

Best,
Pradeep

Last edited by drdolittle; Nov 6th, 2008 at 8:56 pm.

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Re: passing a matrix

Nov 6th, 2008

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

Klatu Barada Nikto

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Re: passing a matrix

Nov 6th, 2008

•

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

Nov 7th, 2008

 Help with Code Tags 
 c++ Syntax (Toggle Plain Text) 
//--------------------------------------
 
int A[2][2]={1,2,3,4}; //--overflow
func(A);
 
//--------------------------------------
 
int A[1][4];
A[0][0] = 1;
A[0][1] = 2;
A[0][3] = 3;
A[0][4] = 4;
 
//--------------------------------------
//--------------------------------------

int A[2][2]={1,2,3,4}; //--overflow
func(A);

//--------------------------------------

int A[1][4];
A[0][0] = 1;
A[0][1] = 2;
A[0][3] = 3;
A[0][4] = 4;

//--------------------------------------

Last edited by cikara21; Nov 7th, 2008 at 3:11 am.

$.:-cikara21-:.$

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Re: passing a matrix

Nov 7th, 2008

•

Originally Posted by cikara21

Help with Code Tags

c++ Syntax (Toggle Plain Text)
//--------------------------------------
 
int A[2][2]={1,2,3,4}; //--overflow
func(A);
 
//--------------------------------------
 
int A[1][4];
A[0][0] = 1;
A[0][1] = 2;
A[0][3] = 3;
A[0][4] = 4;
 
//--------------------------------------
//-------------------------------------- int A[2][2]={1,2,3,4}; //--overflow func(A); //-------------------------------------- int A[1][4]; A[0][0] = 1; A[0][1] = 2; A[0][3] = 3; A[0][4] = 4; //--------------------------------------

What? Where is overflow?

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drdolittle drdolittle is offline

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Re: passing a matrix

Nov 7th, 2008

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

Nov 7th, 2008

Try

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
void func(int (*)[2]);   // declaration
 
void func(int (*x)[2])
{
    // whatever
}
void func(int (*)[2]);   // declaration

void func(int (*x)[2])
{
    // whatever
}

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;

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
int A[2][2]={{1,2},{3,4}};
int A[2][2]={{1,2},{3,4}};

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Re: passing a matrix

Nov 7th, 2008

•

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.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
//either
void func(int [][2]);
 
int main(int argc,char *argv[])
{
	int A[2][2]={1,2,3,4};
 
	func(A);
}
void func(int A[][2])
{
}
 
// --- OR ---
 
void func(int **);
 
int main(int argc,char *argv[])
{
	int **A;
        A = new int *[2];
        A[0] = new int[2];
        A[1] = new int[2];
        //now go store data in the matrix
 
	func(A);
}
void func(int **A)
{
}
//either
void func(int [][2]);

int main(int argc,char *argv[])
{
	int A[2][2]={1,2,3,4};

	func(A);
}
void func(int A[][2])
{
}

// --- OR ---

void func(int **);

int main(int argc,char *argv[])
{
	int **A;
        A = new int *[2];
        A[0] = new int[2];
        A[1] = new int[2];
        //now go store data in the matrix

	func(A);
}
void func(int **A)
{
}

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Re: passing a matrix

Nov 7th, 2008

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

read from page 230

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cikara21

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Re: passing a matrix

#10

Nov 7th, 2008

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

 Help with Code Tags 
 c++ Syntax (Toggle Plain Text) 
#include <iostream>
 
void func(int **);
 
int main(int argc,char *argv[])
{
	int A[2][2]={1,2,3,4};
//--reinterpret_cast
      func(reinterpret_cast<int**>(A));
//--c style cast
      func((int**)A);	//--func(A);
}
void func(int **A)
{
}
#include <iostream>

void func(int **);

int main(int argc,char *argv[])
{
	int A[2][2]={1,2,3,4};
//--reinterpret_cast
      func(reinterpret_cast<int**>(A));
//--c style cast
      func((int**)A);	//--func(A);
}
void func(int **A)
{
}