星期四, 1月 13, 2011

Lex 練習

看 lex & yacc 第一章的練習,然後改用 glib 的 GList 來做:
%{
#include <glib.h>
#include <stdlib.h>

/* token recognizer */
enum {
LOOKUP = 0, /* default is lookup, not add */
VERB,
ADJ,
ADV,
NOUN,
PREP,
PRON,
CONJ
};

int state;

int add_word( int type, char* word );
int lookup_word( char* word );
int print_word( gpointer data, gpointer userdata );
%}

%%
\n {state=LOOKUP;} /* go back to default */
^verb {state=VERB;}
^adj {state=ADJ; }
^adv {state=ADV; }
^noun {state=NOUN; }
^prep {state=PREP; }
^pron {state=PRON; }
^conj {state=CONJ; }
[a-zA-Z]+ {
if( state!=LOOKUP ) {
add_word( state, yytext );
}
else {
switch( lookup_word( yytext ) ) {
case VERB:
printf("%s: verb\n", yytext);
break;
case ADJ:
printf("%s: adjective\n", yytext);
break;
case ADV:
printf("%s: adverb\n", yytext);
break;
case NOUN:
printf("%s: noun\n", yytext);
break;
case PREP:
printf("%s: preposition\n", yytext);
break;
case PRON:
printf("%s: pronoun\n", yytext);
break;
case CONJ:
printf("%s: conjuction\n", yytext);
break;
default:
printf("%s: don't recognize\n", yytext );
break;
}
}
}
. /* ignore other characters. */
%%

GList* list=NULL;

int main( int argc, char* argv[] )
{
yylex();
g_list_foreach(list, (GFunc)print_word, NULL);
g_list_free(list);
}

struct word {
GString* word_name;
gint word_type;
};

int print_word( gpointer data, gpointer userdata ) {
struct word* word=(struct word*)data;
g_printf( "word=%s type=%d\n", word->word_name->str, word->word_type );
return 0;
}

int add_word( int type, char* word) {
struct word* wp;
if( lookup_word( word )!=LOOKUP ) {
printf("!!! Warning: word %s already defined.\n", word );
return 0;
}

wp = g_new0(struct word, 1);
wp->word_name=g_string_new( word );
wp->word_type=type;
list = g_list_append(list, wp);
return 1;
}

gint compare_word( gconstpointer a, gconstpointer b) {
struct word *p1=(struct word*)a;
GString* s=g_string_new( (char*)b );
int ret=0;
if( !g_string_equal( p1->word_name, s ) )
ret=1;
g_string_free(s, FALSE);
return ret;
}

int lookup_word( char* word ) {
GList* found=g_list_find_custom( list, word, compare_word );
if( found )
return ((struct word*)found->data)->word_type;
return LOOKUP;
}


Lex 是用來 tokenize 輸入用的,也就是用來辨識說輸入裡有哪些東西是你要的。這個例子書上沒針對怎麼使用做解說,乍看之下,不容易懂。其實使用上就是打 verb is,表明說 is 是 verb,你可以一直輸入 verb read、verb write、noun book、noun dog....程式就會把你輸入的這些內容分類、放到list裏面去,之後你輸入 read、write、book 時,程式就會告訴你 read、write 是 verb,book 是 noun 。

改用 glib,是因為想練習 glib。g_list_find_custom令人意外地沒範例,不過用法挺簡單,主要變化在第二個參數,第二個參數是一個 function。該 function 第一個參數是 list 裡的元素,第二個參數就是 g_list_find_custom 裡的第二個參數,你可以參考 compare_word,應該是不難懂。

參考資料:

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