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:
";
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 '}
',;

运行结果