Thread: problems with matrix

Heys Pals,
Currently I have the next problem with my code, because of matrix somebody can watch it and help me later thx in advance

Code:

float x[max],y[max],f[max][max],u[max][max],v[max][max],c[max][max];

for(i=2;i<=imax-1;i++)
	{
		for(j=2;j<=jmax-1;j++)
		u[i][j]=f[i+1][j]-f[i-1][j]/x[i+1]-x[i-1];
		v[i][j]=(f[i][j+1]-f[i][j-1])/(y[j+1]-y[j-1]);
		c[i][j]=1.0-u[i][j]*u[i][j]+v[i][j]*v[i][j];	
		
	}

I dont know wether is valid or not because I dont get good results yet

well this is the whole code

Code:

/*Laplace equation 2D*/
/*Build the Mesh*/
#include<stdio.h>
#include<stdlib.h>
#include<math.h>
#define max 500
/*====================================================================
                   Defining function VOID
======================================================================*/
/*====================================================================
                     Function Contour
======================================================================*/
    void contour(float f[max][max],float x[max],float y[max],int jmax, int imax, float ui,
	 int i, int j, float il,float it)
	{
		
	for(j=1;j<=jmax;j++)
	{
		f[1][j]=ui*x[1];
		f[imax][j]=ui*x[imax];
	}
	for(i=1;i<=imax;i++)
	{
		f[i][jmax]=ui*x[i];
		if((i>=il) && (i<=it))
		f[i][1]=f[i][2]-0.05*(y[2]-y[1])*(1.0-2*x[i]);
		else
		f[i][1]=f[i][2];
	}
	}// end contour ()
/*====================================================================
                         end VOID
======================================================================*/
/*====================================================================
                      Function Res
======================================================================*/
void resi(float res[max][max],float x[max],float y[max],float f[max][max],int i, int j,
int imax, int jmax, float remax)	
{
	for (i=2;i<=imax-1;i++)
	{
		for(j=2;j<=jmax-1;j++)
		res[i][j]=(2/(x[i+1]-x[i-1]))*((f[i+1][j]-f[i][j])/(x[i+1]-x[i])-(f[i][j]-f[i-1][j])/(x[i]-x[i-1]))
		+(2/(y[j+1]-y[j-1]))*((f[i][j+1]-f[i][j])/(y[j+1]-y[j])-(f[i][j]-f[i][j-1])/(y[j]-y[j-1]));
	}
	remax=0;
	for(i=2;i<=imax-1;i++)
	{
		for(j=2;j<=jmax-1;j++)
		if (fabs(res[i][j])>=remax)
		remax=fabs(res[i][j]);
	}
}
/*===================================================================
                          End resi
=====================================================================*/
/*===================================================================
                     Main Body
=====================================================================*/

int 
main()
{
	FILE *mesh;
	float dx,x[max],y[max],f[max][max],u[max][max],v[max][max],c[max][max],df[max][max],res[max][max];
	float ys,xs,il,it,ui,dy,remax;
	
	int imax,jmax,nmax,n,i,j,s;
	mesh=fopen("mesh.data","w");
	
	il=11;
	it=31;
	imax=41;
	jmax=12;
	ys=1.24;
	xs=1.24;
	ui=1.0;
	nmax=10000;
	/*=========================================================
	               Generating the Mesh
	===========================================================*/
	dx=-1.0/(il-it);
	
	for (i=il;i<=it;i++)
	{
		x[i]=(i-il)*dx;
		
	}
	for (i=it+1;i<=imax;i++)
	{
		x[i]=x[i-1]+(x[i-1]-x[i-2])*xs;
		
	}
	for(i=il-1;i>=1;i--)
	{
	    x[i]=x[i+1]+(x[i+1]-x[i+2])*xs;
	    
	}
	y[1]=-dx/2;
	y[2]=dx/2;
	for(j=3;j<=jmax;j++)
	{
	    y[j]=y[j-1]+(y[j-1]-y[j-2])*ys;
	}
	/*===============================================================*/

	/*===============================================================
	                   Initial Conditions
	=================================================================*/
	for(i=1;i<=imax;i++)
	{
		for(j=1;j<=jmax;j++)
    	f[i][j]=ui*x[i];    
		
	}
	
	/*============================================================*/
	
	for(n=1;n<=nmax;n++)
	{
        contour( f, x, y,jmax,imax, ui,i, j, il,it);
		resi( res,x,y,f,i,j,imax,jmax,remax);
   
   
   
   for(i=2;i<=imax-1;i++)
   {
   	for(j=2;j<=jmax-1;j++)
   	dx=(x[i+1]-x[i-1])/2;
   	dy=(y[j+1]-y[j-1])/2;
   	df[i][j]=-res[i][j]/(-2/(dx*dx)-2/(dy*dy));
   }
   for(i=2;i<=imax-1;i++)
   {
   	for(j=2;j<=jmax-1;j++)
   	f[i][j]=f[i][j]+df[i][j];
   }
   
	}	
	
	/*=============================================================*/
	/*===============================================================
	                Calculus of Velocities
	=================================================================*/
	for(i=2;i<=imax-1;i++)
	{
		for(j=2;j<=jmax-1;j++)
		u[i][j]=f[i+1][j]-f[i-1][j]/x[i+1]-x[i-1];
		v[i][j]=(f[i][j+1]-f[i][j-1])/(y[j+1]-y[j-1]);
		c[i][j]=1.0-u[i][j]*u[i][j]+v[i][j]*v[i][j];	
		
	}  
	
	for(i=1;i<=imax;i++)
	{
		for(j=1;j<=jmax;j++)
		printf("%f\n",u[i][j]);
	}
	
	
	/*================================================================
                   	Print the Results
	==================================================================*/
	for(j=1;j<=jmax;j++)
	{
		for(i=1;i<=imax;i++)
		{
		/*fprintf(mesh,"%1.15f  %1.15f\n",u[i][j],v[i][j]);*/
		/*printf("%f\n",u[i][j]);*/
		
		}
		/*printf("\n");*/
	}
	/*fclose(mesh);*/	
	
	exit(0);
}

I have problems with u[i][j], coz the print out of u[i][j] is 0's, I dont expect it

I did it, but doesn't work :-((

Good indentation allows us to use our trained eyes to spot errors, much more quickly and easily.

You will see that same benefit increase, as you program and work with correcting code, more. You are underestimating the benefit as well as the importance of using a consistent and common style.

This part of your code, calculates velocities, but isn't really doing anything like what you make it look like:

Code:

	for(i=2;i<=imax-1;i++)
	{
		for(j=2;j<=jmax-1;j++)
		u[i][j]=f[i+1][j]-f[i-1][j]/x[i+1]-x[i-1];
		v[i][j]=(f[i][j+1]-f[i][j-1])/(y[j+1]-y[j-1]);
		c[i][j]=1.0-u[i][j]*u[i][j]+v[i][j]*v[i][j];	
		
	}  
	
	for(i=1;i<=imax;i++)
	{
		for(j=1;j<=jmax;j++)
		printf("%f\n",u[i][j]);
	}
	
	

//What it is doing is really like this:

	for(i=2;i<=imax-1;i++)
	{

/*Is this ( for j=2 ) loop what you wanted, or is this an error that the other two lines aren't
   included in the loop?


   Use your eyes - Luke - they're getting trained, *feel* your indentation *flow*. Help you 
   they will in the ways of the Code. :)  
*/
	   for(j=2;j<=jmax-1;j++)
	   	   u[i][j]=f[i+1][j]-f[i-1][j]/x[i+1]-x[i-1];

	   v[i][j]=(f[i][j+1]-f[i][j-1])/(y[j+1]-y[j-1]);
	   c[i][j]=1.0-u[i][j]*u[i][j]+v[i][j]*v[i][j];	
		
	}  
	
	for(i=1;i<=imax;i++)
	{
		for(j=1;j<=jmax;j++)
		   printf("%f\n",u[i][j]);
	}