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Swap an array...

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n.aggel

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Swap an array...

May 16th, 2008

hi guys, it's been a while since i used c for anything so here goes a question tha may seem silly.

I have 2 variable length arrays....

 Help with Code Tags 
 c Syntax (Toggle Plain Text) 
 
int main()
{
	int n = 5;
	int m = 6;
 
	int before[n][m];
	int after[n][m];
	array_function( 5, 6, before, after );
}
 
void array_function(int rows, int cols, float after[rows][cols] , float before[rows][cols])
{
	//for some iterations	
	for(;;)
	{
 
		//for all the elements int the array
		for (i=1; i<rows; i++)
			for(j=1; j<cols; j++)
				after[i][j] = 4 * before[i][j]
 
		//afterwards prepare for the next iteration [1]
		for (i=1; i<rows; i++)
			for(j=1; j<cols; j++)
				before[i][j]=after[i][j];
	}
}
int main()
{
	int n = 5;
	int m = 6;

	int before[n][m];
	int after[n][m];
	array_function( 5, 6, before, after );
}

void array_function(int rows, int cols, float after[rows][cols] , float before[rows][cols])
{
	//for some iterations	
	for(;;)
	{

		//for all the elements int the array
		for (i=1; i<rows; i++)
			for(j=1; j<cols; j++)
				after[i][j] = 4 * before[i][j]

		//afterwards prepare for the next iteration [1]
		for (i=1; i<rows; i++)
			for(j=1; j<cols; j++)
				before[i][j]=after[i][j];
	}
}

...What i want to do is find a way to swap the pointers to the arrays. So that i can get rid of the last loop.
I figured that since i only need to calculate "after" based on "before" and then make "after"->"before" and recalculate "after".
It would save a lot of time if i just swapped the 2 pointers.

i tried the following but with no luck:

 Help with Code Tags 
 c Syntax (Toggle Plain Text) 
//define function like this:
void array_function(int rows, int cols, float &after[rows][cols] , float &before[rows][cols])
 
//call it like this:
array_function( 5, 6, &before, &after );
//define function like this:
void array_function(int rows, int cols, float &after[rows][cols] , float &before[rows][cols])

//call it like this:
array_function( 5, 6, &before, &after );

thanks in advance
-nicolas

Two roads diverged in a wood, and I— I took the one less traveled by, and that has made all the difference.

by Robert Frost the "The Road Not Taken"

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selvaganapathy selvaganapathy is offline

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Re: Swap an array...

May 16th, 2008

Hi
U can try Memory functions like memcopy () .... something like that for quick swapping. I dont much about memory functions. but it may helpful to fast swapping

Selva

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n.aggel

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Re: Swap an array...

May 16th, 2008

the way i think it, i try to avoid copying anything than 2 pointers... maybe the thread title is a bit misleading

maybe a moderator could change to something like "swap the pointers to 2 arrays" or anything better!

Two roads diverged in a wood, and I— I took the one less traveled by, and that has made all the difference.

by Robert Frost the "The Road Not Taken"

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Luckychap

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Re: Swap an array...

May 16th, 2008

may this will work:

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
function swap_array(int **after, int **before) {
int **temp = after;
after = before;
before = temp;
 
}
function swap_array(int **after, int **before) {
int **temp = after;
after = before;
before = temp;

}

I have not checked it. Hope it will work.

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n.aggel

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Re: Swap an array...

May 16th, 2008

nop man this won't work.... because i am asking about variable lenght arrays....

check this out::

http://www.ddj.com/cpp/184401444

Last edited by n.aggel; May 16th, 2008 at 12:01 pm.

Two roads diverged in a wood, and I— I took the one less traveled by, and that has made all the difference.

by Robert Frost the "The Road Not Taken"

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vijayan121 vijayan121 is offline

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Re: Swap an array...

May 16th, 2008

> i try to avoid copying anything than 2 pointers...
swapping pointers is easy; swap pointers to arrays just like you swap any other pointers.

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
#include <stdio.h>
 
void swap( int rows, int cols, float (**after)[rows][cols] , 
           float (**before)[rows][cols] )
{
  float (*temp)[rows][cols] = *after ;
  *after = *before ;
  *before = temp ;
}
 
int main()
{
  int rows = 3, cols = 4 ;
  float one[rows][cols] ; one[0][0] = 1.1 ;
  float two[rows][cols] ; two[0][0] = 2.2 ;
  float (*p1)[rows][cols] = &one ; 
  float (*p2)[rows][cols] = &two ; 
  printf( "%f\t%f\n", (*p1)[0][0], (*p2)[0][0] ) ; 
  swap( rows, cols, &p1, &p2 ) ;
  printf( "%f\t%f\n", (*p1)[0][0], (*p2)[0][0] ) ; 
}
#include <stdio.h>

void swap( int rows, int cols, float (**after)[rows][cols] , 
           float (**before)[rows][cols] )
{
  float (*temp)[rows][cols] = *after ;
  *after = *before ;
  *before = temp ;
}

int main()
{
  int rows = 3, cols = 4 ;
  float one[rows][cols] ; one[0][0] = 1.1 ;
  float two[rows][cols] ; two[0][0] = 2.2 ;
  float (*p1)[rows][cols] = &one ; 
  float (*p2)[rows][cols] = &two ; 
  printf( "%f\t%f\n", (*p1)[0][0], (*p2)[0][0] ) ; 
  swap( rows, cols, &p1, &p2 ) ;
  printf( "%f\t%f\n", (*p1)[0][0], (*p2)[0][0] ) ; 
}

this means that you will have to write the array_function in terms of pointers to arrays:

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
void array_function( int rows, int cols, float (*after)[rows][cols],
                     float (*before)[rows][cols] )
{
	//for some iterations	
	for(;;)
	{
		int i, j ;
		//for all the elements int the array
		for (i=1; i<rows; i++)
			for(j=1; j<cols; j++)
				(*after)[i][j] = 4 * (*before)[i][j] ;
 
		//afterwards prepare for the next iteration
		swap( rows, cols, &after, &before ) ;
	}
}
void array_function( int rows, int cols, float (*after)[rows][cols],
                     float (*before)[rows][cols] )
{
	//for some iterations	
	for(;;)
	{
		int i, j ;
		//for all the elements int the array
		for (i=1; i<rows; i++)
			for(j=1; j<cols; j++)
				(*after)[i][j] = 4 * (*before)[i][j] ;

		//afterwards prepare for the next iteration
		swap( rows, cols, &after, &before ) ;
	}
}

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Duoas

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Re: Swap an array...

May 16th, 2008

Good grief!

n.aggel, you had the right idea to begin with, only your syntax needs help. Don't change the definition of the function, but just pass the appropriate pointer.

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
void array_function(int rows, int cols, float after[rows][cols] , float before[rows][cols])
  {
  ...
  }
 
int main()
  {
  int n = 5;
  int m = 6;
 
  int before[n][m];
  int after[n][m];
 
  array_function( 5, 6, before, after );  // this works
  array_function( 5, 6, after, before );  // this works
 
  ...
  }
void array_function(int rows, int cols, float after[rows][cols] , float before[rows][cols])
  {
  ...
  }

int main()
  {
  int n = 5;
  int m = 6;

  int before[n][m];
  int after[n][m];

  array_function( 5, 6, before, after );  // this works
  array_function( 5, 6, after, before );  // this works

  ...
  }