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基于C++语言实现的PL/0语言的算术表达式的自下而上的语法分析程序。该语言的其他语法实现思想与此一致，故不赘述。
运行此程序前，必须先将代码通过：【编译原理】c++实现词法分析器的词法分析，生成词法表（词法表是txt文件，为了语法分析成功，务必删除文件中最后空着的一行，即文件末尾不可以留空白行）。生成的该词法表为此程序的必要输入。
产生式：
S->X(AX)*|AX(AX)*
X->Y(MY)*
Y->I|N|(S)
A->+|-
M->*|/
C->=|#|<|<=|>|>=
本次的代码主要是在【编译原理】c++实现自下而上语法分析器的基础上，伴随着归约的过程，增加了生成四元式的过程，也就是一边归约一边生成中间代码。
Talk is cheap, show you my source code:
/* this code was first initiated by TZ,COI,HZAU contact email:[email protected] personal website:wnm1503303791.github.io personal blogs:www.cnblogs.com/acm-icpcer/ this code has been posted on my personal blog,checking url:www.cnblogs.com/acm-icpcer/p/9173880.html Copyright 2018/6/12 TZ. All Rights Reserved. */ #include<cstdio> #include<cstring> #include<algorithm> #include<iostream> #include<string> #include<vector> #include<stack> #include<bitset> #include<cstdlib> #include<cmath> #include<set> #include<list> #include<deque> #include<map> #include<queue> #include<fstream> using namespace std; //预处理函数 bool preproccess(char *a,char *b,char *c) { int i1=0,i2=1;//i2为原串指针 memset(b,1024,'\0'); while(a[i2]!=',') { b[i1]=a[i2]; ++i1,++i2; } b[i1]='\0'; i1=0;i2++; while(a[i2]!=')') { c[i1]=a[i2]; ++i1,++i2; } c[i1]='\0'; //cout<<b<<endl; return true; } fstream f2("stack.txt", ios::out);//打开文件，供写 fstream f3("quaternary.txt", ios::out);//打开文件，供写 static int mcount=1;//存储打印次数 //当移进或者归约时打印栈内情况，以供分析 bool outf(int head,char data[1024][1024],fstream &f) { f<<"times("<<mcount<<"),"; f<<"head is:"<<head<<endl; for(int i=head;i>=0;i--) { f<<data[i]<<endl; } mcount++; f<<endl; return true; } //四元式写入文件函数1 bool outf2(int top,char dt[1024][1024],fstream &f) { /* char arg1[1024],arg2[1024],op[1024]; memset(arg1,sizeof(arg1),'\0'); strcpy(arg1,dt[top]); memset(op,sizeof(op),'\0'); strcpy(op,dt[top-1]); memset(arg2,sizeof(arg2),'\0'); strcpy(arg2,dt[top-2]); f<<"("<<op<<","<<arg1<<","<<arg2<<","<<"T"<<")"<<endl; */ f<<"("<<dt[top-1]<<","<<dt[top]<<","<<dt[top-2]<<","<<"T"<<")"<<endl; return true; } //四元式写入文件函数2 bool outf3(int top,char dt[1024][1024],fstream &f) { f<<"("<<dt[top-1]<<","<<dt[top]<<","<<"-"<<","<<"T"<<")"<<endl; return true; } //四元式写入文件函数3 bool outf4(int top,char dt[1024][1024],fstream &f,char T) { f<<"("<<":="<<","<<dt[top]<<","<<"-"<<","<<T<<")"<<endl; return true; } //“策略”设计模式，面向对象方法 class presentation { private: char data[1024][1024];//栈 char dt[1024][1024];//四元式栈 fstream *infile;//词法分析表 int head,top;//两个栈的栈顶指针 public: //first initiated the object presentation(fstream *in_f) { this->infile=in_f; memset(data,sizeof(data),'\0'); memset(dt,sizeof(dt),'\0'); head=top=-1; } bool push() { head++; top++; infile->getline(data[head],1024); char t1[1024],t2[1024];//存放字符标志 preproccess(data[head],t1,t2); cout<<data[head]<<","<<t1<<endl; memset(data[head],1024,'\0'); strcpy(data[head],t1); memset(dt[top],1024,'\0'); strcpy(dt[top],t2); cout<<dt[top]<<","<<t2<<endl; } /* S->X(AX)*|AX(AX)* X->Y(MY)* Y->I|N|(S) A->+|- M->*|/ C->=|#|<|<=|>|>= */ //归约函数 bool reduce() { //S->X(AX)*|AX(AX)* if( head>=4&& (!strcmp(data[head],"X"))&& (!strcmp(data[head-1],"plus")||!strcmp(data[head-1],"minus"))&& (!strcmp(data[head-2],"X"))&& (!strcmp(data[head-3],"plus")||!strcmp(data[head-3],"minus"))&& (!strcmp(data[head-4],"X")) ) { memset(data[head],1024,'\0'); memset(data[head-1],1024,'\0'); memset(data[head-2],1024,'\0'); memset(data[head-3],1024,'\0'); memset(data[head-4],1024,'\0'); head=head-5+1; strcpy(data[head],"S");//归约 /* stack description: top-> arg op top-2-> arg op arg */ if(outf2(top,dt,f3)&&outf2(top-2,dt,f3)) { top==-1; memset(dt,sizeof(dt),'\0'); } return true; } if( head>=2&& (!strcmp(data[head],"X"))&& (!strcmp(data[head-1],"plus")||!strcmp(data[head-1],"minus"))&& (!strcmp(data[head-2],"X")) ) { memset(data[head],1024,'\0'); memset(data[head-1],1024,'\0'); memset(data[head-2],1024,'\0'); head=head-3+1; strcpy(data[head],"S"); if(outf2(top,dt,f3)) { top==-1; memset(dt,sizeof(dt),'\0'); } return true; } if( head>=2&&/*top>=3*/ (!strcmp(data[head],"plus")||!strcmp(data[head],"minus"))&& (!strcmp(data[head-1],"X"))&& (!strcmp(data[head-2],"plus")||!strcmp(data[head-2],"minus"))&& (!strcmp(data[head-3],"X")) ) { memset(data[head],1024,'\0'); memset(data[head-1],1024,'\0'); memset(data[head-2],1024,'\0'); memset(data[head-3],1024,'\0'); head=head-3+1; strcpy(data[head],"S"); if(outf3(top,dt,f3)&&outf3(top-2,dt,f3)) { top==-1; memset(dt,sizeof(dt),'\0'); } return true; } if( head>=1&& (!strcmp(data[head],"plus")||!strcmp(data[head],"minus"))&& (!strcmp(data[head-1],"X")) ) { memset(data[head],1024,'\0'); memset(data[head-1],1024,'\0'); head=head-2+1; strcpy(data[head],"S"); if(outf3(top,dt,f3)) { top==-1; memset(dt,sizeof(dt),'\0'); } return true; } //X->Y(MY)* if( head>=4&& (!strcmp(data[head],"Y"))&& (!strcmp(data[head-1],"times")||!strcmp(data[head-1],"slash"))&& (!strcmp(data[head-2],"Y"))&& (!strcmp(data[head-3],"times")||!strcmp(data[head-3],"slash"))&& (!strcmp(data[head-4],"Y")) ) { memset(data[head],1024,'\0'); memset(data[head-1],1024,'\0'); head=head-5+1; strcpy(data[head],"X"); /* current stack description: top-> arg op top-2-> arg op arg */ if(outf2(top,dt,f3)&&outf2(top-2,dt,f3)) { top==-1; memset(dt,sizeof(dt),'\0'); } return true; } if( head>=2&& (!strcmp(data[head],"Y"))&& (!strcmp(data[head-1],"times")||!strcmp(data[head-1],"slash"))&& (!strcmp(data[head-2],"Y")) ) { memset(data[head],1024,'\0'); memset(data[head-1],1024,'\0'); memset(data[head-2],1024,'\0'); head=head-3+1; strcpy(data[head],"X"); /* current stack description: top->arg op arg */ if(outf2(top,dt,f3)) { top==-1; memset(dt,sizeof(dt),'\0'); } return true; } if( head>=0&&(!strcmp(data[head],"Y")) ) { memset(data[head],1024,'\0'); head=head-1+1; strcpy(data[head],"X"); if(outf4(top,dt,f3,'X')) { top==-1; memset(dt,sizeof(dt),'\0'); } return true; } //Y->I|N|(S) if( head>=0&&(!strcmp(data[head],"ident")) ) { memset(data[head],1024,'\0'); head=head-1+1; strcpy(data[head],"Y"); if(outf4(top,dt,f3,'Y')) { top==-1; memset(dt,sizeof(dt),'\0'); } return true; } if( head>=0&&(!strcmp(data[head],"number")) ) { memset(data[head],1024,'\0'); head=head-1+1; strcpy(data[head],"Y"); if(outf4(top,dt,f3,'Y')) { top==-1; memset(dt,sizeof(dt),'\0'); } return true; } if( head>=2&& (!strcmp(data[head],"rparen"))&& (!strcmp(data[head-1],"S"))&& (!strcmp(data[head-2],"lparen")) ) { memset(data[head],1024,'\0'); memset(data[head-1],1024,'\0'); memset(data[head-2],1024,'\0'); head=head-3+1; strcpy(data[head],"Y"); return true; } return false; } //遍历栈 bool visit_data() { cout<<"current stack:"<<endl; for(int i=head;i>=0;i--) { cout<<data[i]<<endl; } return true; } //主控函数 bool mainf() { while(!infile->eof()) { push(); bool t=reduce(); outf(head,data,f2); //每当移进结束时就检查一下是否有可规约串 while(t)//防止规约嵌套 { t=reduce(); outf(head,data,f2); } //visit_data(); } visit_data(); bool flag=false; for(int i=head;i>=0;i--) { if(!strcmp(data[i],"S")) { flag=true; } if( strcmp(data[i],"S")&& strcmp(data[i],"X")&& strcmp(data[i],"A")&& strcmp(data[i],"Y")&& strcmp(data[i],"M")&& strcmp(data[i],"C") ) { return false; } } return flag; /* while(head>0) { bool t=reduce(); //每当移进结束时就检查一下是否有可规约串 while(t)//防止规约嵌套 { t=reduce(); } //visit_data(); outf(head,data,f2); } */ } }; int main() { fstream f1; f1.open("lexical.txt", ios::in);//打开词法分析表，供读 presentation* s1=new presentation(&f1); bool result=s1->mainf(); if(result) cout<<"ACCEPTED!"<<endl; else cout<<"ERROR!"<<endl; f1.close(); f2.close(); return 0; }
运行示例：
(1)合法的语句：
(2)不合法的语句：
tz@COI HZAU
2018/6/12