negative x for jacobi

[john2010]

Dear all,
hope you could help me verifying my code. Because my result of x's are negative. I want to ask if negative x's are ok? or not correct? i want it positive. thanks.

#include <stdio.h>
#include <conio.h>
#include <math.h>
#include <stdlib.h>

/* n = nmber of interior points (equations) */
#define n 2500

FILE *foutput;
/* define variables and constants */
double tol=0.000001;
double a[n][n],b[n],h,
x[n],xnew[n],
d[n],e[n],f[n],g[n],L2;
int k,i,j;

main()
{
/* h = spacing between points */
h=1.0/(double)(n+1);

/* initialize the A matrix */

for(i=1;i<=n+1;i++)
for(j=1;j<=n+1;j++)
a[i][j]=0.0;

/* The matrix generating routine.
d[], e[], and f[] are vectors of non-zero diags for tri-diag matrices */

a[1][1]=-(2.0+4.0*h*h); e[1]=a[1][1];
a[1][2]=1.0; f[1]=a[1][2];

/* generate the main body */
for(i=2;i<=n-1;i++)
{
a[i][i-1]=1.0; d[i]=a[i][i-1];
a[i][i]=-(2.0+4.0*h*h); e[i]=a[i][i];
a[i][i+1]=1.0; f[i]=a[i][i+1];
}
a[n][n-1]=1.0; d[n]=a[n][n-1];
a[n][n]=-(2.0+4.0*h*h); e[n]=a[n][n];

/* now generate the right hand side of vector of values */
for(i=1;i<=n-1;i++)
b[i]=h*h;
b[n]=h*h-1.0;

/* Initial guess of vector x is zero */

for(i=1;i<=n;i++)
x[i]=0.0;

/* performing the iteration */

/* Initialize count of number of iterations k */
k=0;

/* main body of iteration */
do
{
/* count up k */
k+=1;

/* by using Jacobi Iteration*/

xnew[1]=(b[1]-f[1]*x[2])/e[1];
for(i=2;i<=n-1;i++)
xnew[i]=(b[i]-d[i]*x[i-1]-f[i]*x[i+1])/e[i];
xnew[n]=(b[n]-d[n]*x[n-1])/e[n];

/* convergence test */
L2=0.0;
for(i=1;i<n;i++)
{
/* applying the L2 exit criterion */
g[i] = pow((fabs(fabs(xnew[i])-fabs(x[i]))),2);
L2 = L2+g[i];

}
L2=pow((L2),0.5);
printf("iter %d %.6lf\n",k,L2);
/* put xnew as x */
for(i=1;i<=n;i++)
x[i]=xnew[i];
}
while(L2>tol);

/* for print output */
foutput=fopen("prob1jacobioutput.txt","w");
printf ("Number of Iteration = %d\n",k);
fprintf(foutput,"Number of Iteration = %d\n",k);
for (i=2300;i<=2310;i++)
{
printf("x[%d]=%.6lf\n",i,x[i]);
fprintf(foutput,"x[%d]=%.6lf\n",i,x[i]);
}
fclose(foutput);
system("PAUSE");

return 0;
}