What does this matrix code do?

[Matt]

Hello. I'm struggling to work out what bit of matrix manipulation the
following piece of code is doing to the 3x3 matrix given:

#include <stdio.h>

int main()

{

int check_mat[3][3] = { { 1 , 2 , 3 } , { 4 , 5 , 6 } , { 7 , 8 ,
9 } };
int im,jm;

/* Print starting values to screen */

for ( im = 0 ; im < 3 ; im++ )

{

for( jm = 0 ; jm < 3 ; jm++ )

{

printf("%d\t" , check_mat[im][jm] );

}

printf("\n");

}

/* Perform some kind of matrix manipulation */

for( im = 0 ; im <= 2 ; im++ )

{

for( jm = 0 ; jm <= im ; jm++ )

{

check_mat[jm][im] = check_mat[im][jm];

}

}

printf("\n");

/* Print final manipulated values to screen */

for ( im = 0 ; im < 3 ; im++ )

{

for( jm = 0 ; jm < 3 ; jm++ )

{

printf("%d\t" , check_mat[im][jm] );

}

printf("\n");

}

return 0;

}
Kind Regards,

Matt

[Ben Bacarisse]

Matt <ma*****@hotmai l.comwrites:

Hello. I'm struggling to work out what bit of matrix manipulation the
following piece of code is doing to the 3x3 matrix given:

I am not sure what *kind* of answer you want. The program shows you,
at some level, exactly what is done to the matrix.

Other related questions such as "what is this transformation called?"
and "why would one do it?" are not C questions and you'll get fuller
answers elsewhere (though I can't point to a good group).

<code snipped>

--
Ben.

[Jens Thoms Toerring]

Matt <ma*****@hotmai l.comwrote:

Hello. I'm struggling to work out what bit of matrix manipulation the
following piece of code is doing to the 3x3 matrix given:

#include <stdio.h>

int main()
{
int check_mat[3][3] = { { 1 , 2 , 3 } , { 4 , 5 , 6 } , { 7 , 8 , 9 } };
int im,jm;
/* Print starting values to screen */

for ( im = 0 ; im < 3 ; im++ )
{
for( jm = 0 ; jm < 3 ; jm++ )
{
printf("%d\t" , check_mat[im][jm] );
}
printf("\n");
}
/* Perform some kind of matrix manipulation */
for( im = 0 ; im <= 2 ; im++ )
{
for( jm = 0 ; jm <= im ; jm++ )

{
check_mat[jm][im] = check_mat[im][jm];
}
}

Actually

for ( im = 1; im < 3; im++ )
for ( jm = 0; jm < im; jm++ )
check_mat[ jm ][ im ] = check_mat[ im ][ jm ];

should be enough, saving a few operations.

printf("\n");
/* Print final manipulated values to screen */
for ( im = 0 ; im < 3 ; im++ )
{
for( jm = 0 ; jm < 3 ; jm++ )
{
printf("%d\t" , check_mat[im][jm] );
}
printf("\n");
}
return 0;
}

Well, you seem to have a working program here, so you can compare
the matrix before the operation and afterwards. That should tell
you immediately what operation is done on the matrix. If you still
don't see it get out your book about linear algebra and read what
the "transpose" of a matrix is.
Regards, Jens
--
\ Jens Thoms Toerring ___ [e-mail]
\______________ ____________ http://toerring.de

[Keith Thompson]

[e-mail] (Jens Thoms Toerring) writes:

Matt <ma*****@hotmai l.comwrote:

>Hello. I'm struggling to work out what bit of matrix manipulation the
following piece of code is doing to the 3x3 matrix given:

[...]

Well, you seem to have a working program here, so you can compare
the matrix before the operation and afterwards. That should tell
you immediately what operation is done on the matrix. If you still
don't see it get out your book about linear algebra and read what
the "transpose" of a matrix is.

The posted code doesn't transpose the matrix, though I can believe
(barely) that it was intended to.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[CBFalconer]

Ben Bacarisse wrote:

Matt <ma*****@hotmai l.comwrites:

>Hello. I'm struggling to work out what bit of matrix manipulation the
following piece of code is doing to the 3x3 matrix given:

I am not sure what *kind* of answer you want. The program shows you,
at some level, exactly what is done to the matrix.

Other related questions such as "what is this transformation called?"
and "why would one do it?" are not C questions and you'll get fuller
answers elsewhere (though I can't point to a good group).

<code snipped>

But when you do ask elsewhere, dispense with the double spacing of
the code portion.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

--
Posted via a free Usenet account from http://www.teranews.com

[Dik T. Winter]

In article <5j************ *@[e-mail] (Jens Thoms Toerring) writes:

Matt <ma*****@hotmai l.comwrote:

....

for( im = 0 ; im <= 2 ; im++ )
{
for( jm = 0 ; jm <= im ; jm++ )

{
check_mat[jm][im] = check_mat[im][jm];
}
}

Actually
for ( im = 1; im < 3; im++ )
for ( jm = 0; jm < im; jm++ )
check_mat[ jm ][ im ] = check_mat[ im ][ jm ];
should be enough, saving a few operations.

Well, you seem to have a working program here, so you can compare
the matrix before the operation and afterwards. That should tell
you immediately what operation is done on the matrix. If you still
don't see it get out your book about linear algebra and read what
the "transpose" of a matrix is.

Only, it is not the transpose. When doing a transpose you interchange
elements.
--
dik t. winter, cwi, kruislaan 413, 1098 sj amsterdam, nederland, [phone]
home: bovenover 215, 1025 jn amsterdam, nederland; http://www.cwi.nl/~dik/

[Thad Smith]

Matt wrote:

Hello. I'm struggling to work out what bit of matrix manipulation the
following piece of code is doing to the 3x3 matrix given:

(code snipped)

When presented with a problem like this, I try to visualize the
operations. You already have one handy tool: the program prints the
matrix before and after. Run it and study the output. See if you can
discern the general pattern.

When I looked at your code, I used another tool: playing computer. I
played computer by writing down the values of the matrix on a piece of
paper in the standard row by column arrangement. As I mentally executed
each statement, I drew arrows to show how the data was moved (usually I
scratch out old values and write in the new ones, but arrows worked fine
for your code). When I was finished, I could clearly see the data
pattern illustrating the transformation.

Playing computer is a powerful tool for me when I am working with
algorithms that shuffle data. Examples of code that I have developed
using this are
1) implementing a data heap structure
2) rotating an array of values by a given number of positions, moving
each item directly to its destination.
3) tree construction and balancing operations.

Try it!

--
Thad

[David Mathog]

Matt wrote:

Hello. I'm struggling to work out what bit of matrix manipulation the
following piece of code is doing to the 3x3 matrix given:

/* Perform some kind of matrix manipulation */

for( im = 0 ; im <= 2 ; im++ ) {
for( jm = 0 ; jm <= im ; jm++ ){
check_mat[jm][im] = check_mat[im][jm];
}
}

This is slightly off topic, and it won't hurt anything I suppose,
but when the inner loop reaches jm==im all it does is read a value from
one cell of that array and then write it back into the same cell.
Assuming the compiler doesn't optimize this away, it's a wasted operation.
Regards,

David Mathog

[Barry Schwarz]

On Wed, 22 Aug 2007 15:12:47 -0700, Matt <ma*****@hotmai l.comwrote:

>Hello. I'm struggling to work out what bit of matrix manipulation the
following piece of code is doing to the 3x3 matrix given:

#include <stdio.h>

int main()

{

snip double spaced code

>
}

Why not just execute it and see for yourself.
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