implementation of push() and pop() using stack.

#define MAXVAL    100  /*maximum depth of val stack */

int sp = 0;           /*next free stack position */
double val[MAXVAL];   /*value stack */

/*push: push f onto value stack */
void push(double f){
     if (sp < MAXVAL)
         val[sp++] = f;
     else
        printf("error:stack full, cant push %g\n", f);
}

/*pop: pop and return top value from stack */
double pop(void){
      if(sp > 0)
         return val[--sp];
      else{
        printf("error: stack empty\n");
        return 0.0;
      }
}