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  }
  if (c == '\n')
  lineno++;
  return c;
 }

 yyerror(s)
  char *s;
 {
  warning(s, (char *)0);
 }

 warning(s, t)
  char *s, *t;
 {
  fprintf(stderr, "%s: %s", progname, s);
  if (t && *t)
  fprintf(stderr, "%s", t);
  fprintf(stderr, " near line %d\n", lineno);
 }

3.2 hoc2

3.2.1 hoc.y

 %{
 double mem[26]; /* memory for variables 'a'..'z' */
 %}
 %union { /* stack type */
  double val; /* actual value */
  int index; /* index into mem[] */
 }
 %token <val> NUMBER
 %token <index> VAR
 %type <val> expr
 %right '='
 %left '+' '-'
 %left '*' '/'
 %left UNARYMINUS
 %%
 list: /* nothing */
  | list '\n'
  | list expr '\n' { printf("\t%.8g\n\ $2); }
  | list error '\n' { yyerrok; }
  ;
 expr: NUMBER
  | VAR { $$ = mem[$1]; }
  | VAR '=' expr { $$ = mem[$1] = $3; }
  | expr '+' expr { $$ = $1 + $3; }
  | expr '-' expr { $$ = $1 - $3; }
  | expr '*' expr { $$ = $1 * $3; }
  | expr '/' expr {
  if ($3 == 0.0)
  execerror("division by zero", "");
  $$ = $1 / $3;
  }
  | '(' expr ')' { $$ = $2; }
  | '-' expr %prec UNARYMINUS { $$ = -$2; }
  ;
 %%
 /* end of grammar */

 #include <stdio.h>
 #include <ctype.h>
 char *progname;
 int lineno = 1;
 #include <signal.h>
 #include <setjmp.h>
 jmp_buf begin;

 main(argc, argv) /* hoc2 */
  char *argv[];
 {
  int fpecatch;

  progname = argv[0];
  setjmp(begin);
  signal(SIGFPE, fpecatch);
  yyparse;
 }

 yylex /* hoc2 */
 {
  int c;

  while ((c=getchar) == ' ' || c == '\t')
  ;
  if (c == EOF)
  return 0;
  if (c == '.' || isdigit(c)) { /* number */
  ungetc(c, stdin);
  scanf("%lf", &yylval.val);
  return NUMBER;
  }
  if (islower(c)) {
  yylval.index = c - 'a'; /* ASCII only */
  return VAR;
  }
  if (c == '\n')
  lineno++;
  return c;
 }

 yyerror(s) /* report compile-time error */
  char *s;
 {
  warning(s, (char*)0);
 }

 execerror(s, t) /* recover from run-time error */
  char *s, *t;
 {
  warning(s, t);
  longjmp(begin, 0);
 }

 fpecatch /* catch floating point exceptions */
 {
  execerror("floating point exception", (char*)0);
 }

 warning(s, t) /* print warning message */
  char *s, *t;
 {
  fprintf(stderr. "%s: %s", progname, s);
  if (t && *t)
  fprintf(stderr, " %s , t);
  fprintf(stderr, " near line %d\n", lineno);
 }

3.2.2 makefile

 hoc2: hoc.o
  cc hoc.o -o hoc2

3.3 hoc3

3.3.1 makefile

 YFLAGS = -d # force creation of y.tab.h
 OBJS = hoc.o init.o math.o symbol.o # abbreviation

 hoc3: $(OBJS)
  cc $(OBJS) -lm -o hoc3

 hoc.o: hoc.h

 init.o symbol.o: hoc.h y.tab.h

 pr:
  @pr hoc.y hoc.h init.c math.c symbol.c makefile