求算符文法的FIRSTVT集的算法

原理

数据结构

 G = {'key':[v1,v2,v3],'key':[v1,v2,v3]};
 VN = [];
 Vt = [];
 FirstVT = {'key':[v1,v2,v3],'key':[v1,v2,v3]};

也就是map里放list，同样将文法压缩，对于产生式相同的发到一个map元素里，追加到map元素对应的list后面

算法过程

1、 先求出直接满足A->a 或 A->Ba的文法的A的FIRSTVT集合

2、 扫描FIRSTVT集，将满足蔓延性公式的终结符加到非终结符的FIRSTVT集合中。蔓延性满足下面的条件

若a属于FIRSTVT(B) 且有产生式A->B..... 则a属于FIRSTVT(A)

输入

 8
 S->#E#
 E->E+T
 E->T
 T->T*F
 T->F
 F->P^F|P
 P->(E)
 P->i

完整算法

 #!/usr/bin/env python
 #-*-coding:utf8-*-

 #count = raw_input('Please input P count:');

 #print "Input all P:\n";
 f = open("./2.in", 'r',1);
 count = int(f.readline());
 #G = [];
 G = {};
 VN = [];
 Vt = [];
 FirstVT = {};
 for i in range(0,count):
     #key = raw_input("P key:");
     #value = raw_input("P value:");
     line = f.readline().strip();
     print line;
     arr = line.split("->");
     #P = {'key':key,'value':value};
     #P = {
         #'key':arr[0],
         #'value':arr[1]
     #};
     VN.append(arr[0]);

     #G.append();
     if arr[0] not in G:
         G[arr[0]] = [];
     G[arr[0]].append(arr[1]);
 #print G;

 #for p in G:
     #a  = '';
     #if (p['value'][0] not in VN):
         #a = p['value'][0];
     #elif (len(p['value']) >= 2 ) and ( p['value'][0] in VN):
         #a = p['value'][1];

 for k in G:
     vs = G.get(k);
     for v in vs:
         a = '';
         if v[0] not in VN:
             a = v[0];
         elif len(v) >= 2 and v[0] in VN and v[1] not in VN:
             a = v[1];

         if k not in FirstVT:
             FirstVT[k] = [];

         if a != '':
             #将形如 A->a 的 FirstVT[A] 添加进 a
             FirstVT[k].append(a);

 #print FirstVT;

 stack = [];

 for _k in FirstVT:
     _vs = FirstVT.get(_k);
     for _v in _vs:
         # 将 形如 A->a 的入栈
         stack.append([_k,_v]);

 #print stack;

 while len(stack) > 0:
     ij =  stack.pop();
     B = ij[0];
     a = ij[1];
     for A in G:
         vvs = G.get(A);
         for _vs in vvs:
             # 存在形式如  A->B &&  f[ia,ja]为假
             if _vs[0] == B and A != B and ( a not in FirstVT.get(A) ):
                 FirstVT[A].append(a);
                 stack.append([A,a]);

 #print FirstVT;

 print '------------------------------------------------'
 for fk in FirstVT:
     fv = FirstVT.get(fk);
     print 'FIRSTVT(',fk,')={',;
     for item in fv:
         if item != fv[-1] :
             print item,',',;
         else:
             print item,;
     print '}\n',;

运行结果