1 fdtd code error,pls help me

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

#define ke 200

int main(void)
{
  float ex[ke], hy[ke];
  int n, k, kc, nsteps;
  float t;
  float to, spread, pulse;
  FILE *fp;

  kc = ke / 2;
  to = 40.0;
  spread = 12;
  t = 0;
  nsteps = 1;
  for (k = 1; k < ke; k++)
  {
    ex[k] = 0;
    hy[k] = 0;
  }
  kc = ke / 2;
  to = 40.0;
  spread = 12;
  t = 0;
  nsteps = 1;
  while (nsteps > 0)
  {
    printf("nsteps-->");
    scanf("%d", &nsteps);
    printf("%d\n", nsteps);
    n = 0;
    for (n = 1; n <= nsteps; n++)
    {
      t = t + 1;
      for (k = 1; k < ke; k++)
      {
        ex[k] = ex[k] + .5 * (hy[k - 1] - hy[k]);
      }
      pulse = exp(-.5 * (pow((to - t) / spread, 2)));
      ex[kc] = pulse;
      printf("%5.1f %6.2f\n", to - t, ex[kc]);
      for (k = 0; k < ke - 1; k++)
      {
        hy[k] = hy[k] + .5 * (ex[k] - ex[k + 1]);
      }
    }
    for (k = 1; k <= ke; k++)
    {
      printf("%3d %6.2f %6.2f\n", k, ex[k], hy[k]);
    }
    fp = fopen("ex", "w");
    for (k = 1; k <= ke; k++)
    {
      fprintf(fp, " %6.2f \n", ex[k]);
    }
    fclose(fp);
    fp = fopen("hy", "w");
    for (k = 1; k <= ke; k++)
    {
      fprintf(fp, " %6.2f \n", hy[k]);
    }
    fclose(fp);
    printf("t= %5f\n", t);
  }
  return 0;
}