Swap an array...

•

What is DaniWeb IT Discussion Community?

You're currently browsing the C section within the Software Development category of DaniWeb, a massive community of 427,101 software developers, web developers, Internet marketers, and tech gurus who are all enthusiastic about making contacts, networking, and learning from each other. In fact, there are 2,176 IT professionals currently interacting right now! Registration is free, only takes a minute and lets you enjoy all of the interactive features of the site.

Join our community!

Please support our C advertiser: Programming Forums

Thread Tools Display Modes

Views: 512 | Replies: 8 | Solved

•

Join Date: Nov 2006

Location: Athens, Greece

Posts: 198

Reputation: n.aggel is an unknown quantity at this point

Rep Power: 2

Solved Threads: 9

n.aggel

Offline

Junior Poster

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"

•

Join Date: Feb 2008

Location: Sivakasi, Tamilnadu, India

Posts: 459

Reputation: selvaganapathy is an unknown quantity at this point

Rep Power: 1

Solved Threads: 79

selvaganapathy selvaganapathy is offline

Offline

Posting Pro in Training

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

KSG

•

Join Date: Nov 2006

Location: Athens, Greece

Posts: 198

Reputation: n.aggel is an unknown quantity at this point

Rep Power: 2

Solved Threads: 9

n.aggel

Offline

Junior Poster

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"

•

Join Date: Aug 2006

Location: Noida, India

Posts: 158

Reputation: Luckychap is an unknown quantity at this point

Rep Power: 3

Solved Threads: 17

Luckychap

Offline

Junior Poster

Re: Swap an array...

May 16th, 2008

may this will work:

Help with Code Tags

(Toggle Plain Text)

function swap_array(int **after, int **before) {
int **temp = after;
after = before;
before = temp;

}

I have not checked it. Hope it will work.

•

Join Date: Nov 2006

Location: Athens, Greece

Posts: 198

Reputation: n.aggel is an unknown quantity at this point

Rep Power: 2

Solved Threads: 9

n.aggel

Offline

Junior Poster

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 11:01 am.

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"

•

Join Date: Dec 2006

Location: india

Posts: 1,074

Reputation: vijayan121 is a name known to all

Rep Power: 9

Solved Threads: 161

vijayan121 vijayan121 is offline

Offline

Veteran Poster

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

c99 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] ) ; 
}

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

Help with Code Tags

c99 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 ) ;
	}
}

•

Join Date: Oct 2007

Location: Cherry Hill, NJ

Posts: 1,876

Reputation: Duoas is a splendid one to behold

Rep Power: 11

Solved Threads: 193

Duoas

Offline

Posting Virtuoso

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

  ...
  }

Hope this helps.

•

Join Date: Nov 2006

Location: Athens, Greece

Posts: 198

Reputation: n.aggel is an unknown quantity at this point

Rep Power: 2

Solved Threads: 9

n.aggel

Offline

Junior Poster

Re: Swap an array...

May 18th, 2008

thanks vijayan121, this solved my problem.

Duoas, i wanted to swap the arrays inside the function...

-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"

•

Join Date: Nov 2006

Location: Athens, Greece

Posts: 198

Reputation: n.aggel is an unknown quantity at this point

Rep Power: 2

Solved Threads: 9

n.aggel

Offline

Junior Poster

Re: Swap an array...

May 18th, 2008

Sorry for the double post, it was an accident.

Last edited by n.aggel : May 18th, 2008 at 8:18 am.

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"