C code - Poisson Equation by finite difference method

//program to solve Poisson's equation uxx+uyy=g(x,y)

// by Mahesha M G, MIT

#include <stdio.h>

#define n 5  //dimension rows

#define m 4  //dimension columns

#define gx(x,y) -4

main()

{

int i,j,k;

float a[10][10],g[10][10],h,xl,xh,yl,x;

FILE *fp;

clrscr();

fp=fopen("c:\\poisson.dat","w");

printf("enter xl: ");

scanf("%f",&xl);

printf("enter xh: ");

scanf("%f",&xh);

printf("enter yl: ");

scanf("%f",&yl);

h=(xh-xl)/(n-1);

for(i=1;i<=n;i++)

   {

    x=xl;

     for(j=1;j<=m;j++)

      {

      g[i][j]=gx(x,yl);

      x+=h;

      }

   yl+=h;

   }

printf("enter boundary conditions\n");

            for(i=1;i<=n;i++)

            {

               for(j=1;j<=m;j++)

               {

               if(i==1||i==n||j==1||j==m)

               {

               printf("a[%d,%d] = ",i,j);

               scanf("%f",&a[i][j]);

               }

               }

            }

            for(i=2;i<n;i++)

              for(j=2;j<m;j++)

               a[i][j]=a[n][j];   //initialization ends

  for(k=0;k<100;k++)

  {

               for(i=2;i<n;i++)

            {

               for(j=2;j<m;j++)

               {

                a[i][j]=(a[i-1][j]+a[i+1][j]+a[i][j-1]+a[i][j+1]-(h*h*g[i][j]))/4;

               }

            }

   }                     //calculation by Gauss-Seidel Method

            for(i=1;i<=n;i++)

            {

               for(j=1;j<=m;j++)

               fprintf(fp,"%f\t",a[i][j]);

            fprintf(fp,"\n");

            }

}