【实验目的】
通过上机实验，加深对语法制导翻译的理解，掌握将语法分析所识别的语法成分变换为中间代码的语义翻译方法。
【实验内容】
对给定的程序通过语义分析器能够判断语句串是否正确。正确则输出三地址指令形式的四元式代码，错误则抛出错误信息。
【设计思想】
（1）输入待分析的字符串。
语法如下：
a.关键字：begin,if,then,while,do,end.
b.运算符和界符：:= + - * / < <= > >= <> = ; ( ) #
c.其他单词是标识符(ID)和整形常数(NUM)：ID=letter(letter|digit)*，NUM=digitdigit*
d.空格由空白、制表符和换行符组成。空格一般用来分隔ID、NUM、运算符、界符和关键字，词法分析阶段通常被忽略。
（2）扫描字符串，采用递归向下进行分析。
主要函数如下：
a.scaner()//词法分析函数，char token[8]用来存放构成单词符号的字符串;
b.parser()//语法分析，在语法分析的基础上插入相应的语义动作：将输入串翻译成四元式序列。只对表达式、赋值语句进行翻译。
c.emit(char *result,char *arg1,char *op,char *ag2)//该函数功能是生成一个三地址语句返回四元式表中。
d.char *newtemp()//该函数返回一个新的临时变量名，临时变量名产生的顺序为T1,T2,…。
【实验要求】
四元式表的结构如下：
struct {char result[8];
char ag1[8];
char op[8];
char ag2[8];
}quad[20];
（3）输出为三地址指令形式的四元式序列。
例如：语句串begin a:=2+3*4;x:=(a+b)/c;end#，
输出的三地址指令如下：
t1=3*4
t2=2+t1
a=t2
t3=a+b
t4=t3/c
x=t4
代码：
#include "stdio.h"
#include "string.h"
char prog[100], token[8], ch;
char *rwtab[6] = { "begin", "if", "then", "while", "do", "end" };
int syn, p, m, n, sum, q;
int kk;
struct {
char result1[8];
char ag11[8];
char op1[8];
char ag21[8];
} quad[20];
char *factor();
char *expression();
int yucu();
char *term();
int statement();
int lrparser();
char *newtemp();
scaner();
emit(char *result, char *ag1, char*op, char *ag2);
main()
{
int j;
q = p = kk = 0;
printf("
**************************************");
printf("
please input a string (end with'#'): ");
do
{
scanf_s("%c", &ch);
prog[p++] = ch;
} while (ch != '#');
p = 0;
scaner();
lrparser();
if (q>19)printf(" too longsentense!
");
else for (j = 0; j<q; j++)printf(" %s = %s%s %s
", quad[j].result1, quad[j].ag11, quad[j].op1, quad[j].ag21);
getchar();
}
int lrparser()
{
int schain = 0;
kk = 0;
if (syn == 1)
{
scaner();
schain = yucu();
if (syn == 6)
{
scaner();
if ((syn == 0) && (kk == 0)) printf("Success!
");
}
else {
if (kk != 1)printf("short of 'end'!
");
kk = 1;
getchar();
exit(0);
}
}
else {
printf("short of 'begin' !
");
kk = 1;
getchar();
exit(0);
}
return (schain);
}
int yucu()
{
int schain = 0;
schain = statement();
while (syn == 26)
{
scaner();
schain = statement();
}
return (schain);
}
int statement()
{
char tt[8], eplace[8];
int schain = 0;
if (syn == 10)
{
strcpy_s(tt, 255, token);
scaner();
if (syn == 18)
{
scaner();
strcpy_s(eplace, 255, expression());
emit(tt, eplace, "", "");
schain = 0;
}
else {
printf("short of sign ':='!
");
kk = 1;
getchar();
exit(0);
}
return (schain);
}
}
char *expression()
{
char *tp, *ep2, *eplace, *tt;
tp = (char *)malloc(12);
ep2 = (char *)malloc(12);
eplace = (char *)malloc(12);
tt = (char *)malloc(12);
strcpy_s(eplace, 255, term());
while ((syn == 13) || (syn == 14))
{
if (syn == 13)strcpy_s(tt, 255, "+");
else strcpy_s(tt, 255, "-");
scaner();
strcpy_s(ep2, 255, term());
strcpy_s(tp, 255, newtemp());
emit(tp, eplace, tt, ep2);
strcpy_s(eplace, 255, tp);
}
return (eplace);
}
char *term()
{
char *tp, *ep2, *eplace, *tt;
tp = (char *)malloc(12);
ep2 = (char *)malloc(12);
eplace = (char *)malloc(12);
tt = (char *)malloc(12);
strcpy_s(eplace, 255, factor());
while ((syn == 15) || (syn == 16))
{
if (syn == 15)strcpy_s(tt, 255, "*");
else strcpy_s(tt, 255, "/");
scaner();
strcpy_s(ep2, 255, factor());
strcpy_s(tp, 255, newtemp());
emit(tp, eplace, tt, ep2);
strcpy_s(eplace, 255, tp);
}
return (eplace);
}
char *factor()
{
char *fplace;
fplace = (char *)malloc(12);
strcpy_s(fplace, 255, "");
if (syn == 10)
{
strcpy_s(fplace, 255, token);
scaner();
}
else if (syn == 11)
{
itoa(sum, fplace, 10);
scaner();
}
else if (syn == 27)
{
scaner();
fplace = expression();
if (syn == 28) scaner();
else {
printf("error on ')'!
");
kk = 1;
getchar();
exit(0);
}
}
else {
printf("error on '(' !
");
kk = 1;
getchar();
exit(0);
}
return (fplace);
}
char *newtemp()
{
char *p;
char m[8];
p = (char *)malloc(8);
kk++;
itoa(kk, m, 10);
strcpy_s(p + 1, 255, m);
p[0] = 't';
return(p);
}
scaner()
{
sum = 0;
for (m = 0; m<8; m++)token[m++] = NULL;
m = 0;
ch = prog[p++];
while (ch == ' ')ch = prog[p++];
if (((ch <= 'z') && (ch >= 'a')) || ((ch <= 'Z') && (ch >= 'A')))
{
while (((ch <= 'z') && (ch >= 'a')) || ((ch <= 'Z') && (ch >= 'A')) || ((ch >= '0') && (ch <= '9')))
{
token[m++] = ch;
ch = prog[p++];
}
p--;
syn = 10;
token[m++] = ' ';
for (n = 0; n<6; n++)
if (strcmp(token, rwtab[n]) == 0)
{
syn = n + 1;
break;
}
}
else if ((ch >= '0') && (ch <= '9'))
{
while ((ch >= '0') && (ch <= '9'))
{
sum = sum * 10 + ch - '0';
ch = prog[p++];
}
p--;
syn = 11;
}
else switch (ch)
{
case '<':m = 0;
ch = prog[p++];
if (ch == '>')
{
syn = 21;
}
else if (ch == '=')
{
syn = 22;
}
else
{
syn = 20;
p--;
}
break;
case '>':m = 0;
ch = prog[p++];
if (ch == '=')
{
syn = 24;
}
else
{
syn = 23;
p--;
}
break;
case ':':m = 0;
ch = prog[p++];
if (ch == '=')
{
syn = 18;
}
else
{
syn = 17;
p--;
}
break;
case '+': syn = 13; break;
case '-': syn = 14; break;
case '*': syn = 15; break;
case '/': syn = 16; break;
case '(': syn = 27; break;
case ')': syn = 28; break;
case '=': syn = 25; break;
case ';': syn = 26; break;
case '#': syn = 0; break;
default: syn = -1; break;
}
}
emit(char *result, char *ag1, char*op, char *ag2)
{
strcpy_s(quad[q].result1, 255, result);
strcpy_s(quad[q].ag11, 255, ag1);
strcpy_s(quad[q].op1, 255, op);
strcpy_s(quad[q].ag21, 255, ag2);
q++;
}
运行截图：
