# 求算符文法的FIRSTVT集的算法

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## 数据结构

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

## 算法过程

1、 先求出直接满足A->a 或 A->Ba的文法的AFIRSTVT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

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

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);
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:");
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] 添加进 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 的入栈
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]为假
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',;

## 运行结果

9432 0

10823 0

13106 0

6876 0
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