% cat  heatbar.c

/* Program heatbar - Calculate heat flows in a 100CM bar

* Bar initially starts out at 30 degrees C
* At 1 CM there is a 100 degree C heat source
* At 63 CM there is a 0 degree heat source
* At 100 CM there is a 60 degree source

* The program will print out the temperature at 10 50 and 80 CM
* After 1, 10, and 100 time steps

* Written by - Charles Severance - Tue Dec  7 17:51:34 EST 1993
*/

main() {

  float bar[100];
  int time,pos,i;

/* Initialize the bar */

  for( i=0;i<100;i++ ) {
   bar[i] = 30.0;
  }
  bar[0] = 100.0;
  bar[62] = 0.0;
  bar[99] = 60.0;

/*  Loop through the time steps.  For each time step the new temperature */
/*  is calculated at each point in the bar.  Make sure we don't recalc */
/*  the fixed temperature positions */

  for(time=1;time<=100;time++) {
    for( pos=1; pos<62; pos++ ) {
      bar[pos] = ( bar[pos-1] + bar[pos] + bar[pos+1] ) / 3.0;
    }
    for( pos=63; pos<99;pos++ ) {
      bar[pos] = ( bar[pos-1] + bar[pos] + bar[pos+1] ) / 3.0;
    }

    if ( time == 1 || time == 10 || time == 100 ) {
      printf("time step  %d\n",time);
      printf("at bar[10] =  %f\n",bar[10]);
      printf("at bar[50] =  %f\n",bar[50]);
      printf("at bar[80] =  %f\n",bar[80]);
    }
  }
}
% cc  heatbar.c
% a.out 
time step  1
at bar[10] =  30.001186
at bar[50] =  30.000000
at bar[80] =  30.000000
time step  10
at bar[10] =  30.622185
at bar[50] =  30.000000
at bar[80] =  29.999048
time step  100
at bar[10] =  52.957127
at bar[50] =  23.488363
at bar[80] =  28.873075
%