求算符文法的FIRSTVT集的算法

原理

資料結構

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

也就是map裡放list，同樣将文法壓縮，對于産生式相同的發到一個map元素裡，追加到map元素對應的list後面

算法過程

1、 先求出直接滿足A->a 或 A->Ba的文法的A的FIRSTVT集合

2、 掃描FIRSTVT集，将滿足蔓延性公式的終結符加到非終結符的FIRSTVT集合中。蔓延性滿足下面的條件

若a屬于FIRSTVT(B) 且有産生式A->B..... 則a屬于FIRSTVT(A)

輸入

1 8
2 S->#E#
3 E->E+T
4 E->T
5 T->T*F
6 T->F
7 F->P^F|P
8 P->(E)
9 P->i      

完整算法

1 #!/usr/bin/env python
 2 #-*-coding:utf8-*-
 3 
 4 #count = raw_input('Please input P count:');
 5 
 6 #print "Input all P:\n";
 7 f = open("./2.in", 'r',1);
 8 count = int(f.readline());
 9 #G = [];
10 G = {};
11 VN = [];
12 Vt = [];
13 FirstVT = {};
14 for i in range(0,count):
15     #key = raw_input("P key:");
16     #value = raw_input("P value:");
17     line = f.readline().strip();
18     print line;
19     arr = line.split("->");
20     #P = {'key':key,'value':value};
21     #P = {
22         #'key':arr[0],
23         #'value':arr[1]
24     #};
25     VN.append(arr[0]);
26 
27     #G.append();
28     if arr[0] not in G:
29         G[arr[0]] = [];
30     G[arr[0]].append(arr[1]);
31 #print G;
32 
33 #for p in G:
34     #a  = '';
35     #if (p['value'][0] not in VN):
36         #a = p['value'][0];
37     #elif (len(p['value']) >= 2 ) and ( p['value'][0] in VN):
38         #a = p['value'][1];
39 
40 for k in G:
41     vs = G.get(k);
42     for v in vs:
43         a = '';
44         if v[0] not in VN:
45             a = v[0];
46         elif len(v) >= 2 and v[0] in VN and v[1] not in VN:
47             a = v[1];
48     
49         if k not in FirstVT:
50             FirstVT[k] = [];
51         
52         if a != '':
53             #将形如 A->a 的 FirstVT[A] 添加進 a
54             FirstVT[k].append(a);
55 
56 #print FirstVT;
57 
58 stack = [];
59 
60 for _k in FirstVT:
61     _vs = FirstVT.get(_k);
62     for _v in _vs:
63         # 将 形如 A->a 的入棧
64         stack.append([_k,_v]);
65         
66 
67 #print stack;
68 
69 while len(stack) > 0:
70     ij =  stack.pop();
71     B = ij[0];
72     a = ij[1];
73     for A in G:
74         vvs = G.get(A);
75         for _vs in vvs:
76             # 存在形式如  A->B &&  f[ia,ja]為假
77             if _vs[0] == B and A != B and ( a not in FirstVT.get(A) ):
78                 FirstVT[A].append(a);
79                 stack.append([A,a]);
80 
81 
82 
83 #print FirstVT;
84 
85 print '------------------------------------------------'
86 for fk in FirstVT:
87     fv = FirstVT.get(fk);
88     print 'FIRSTVT(',fk,')={',;
89     for item in fv:
90         if item != fv[-1] :
91             print item,',',;
92         else:
93             print item,;
94     print '}\n',;      

運作結果