Accelerated C++ 7.4 生成句子

We'll wrap up this chapter with a slightly more complicated example: We can use a map to write a program that takes a description of a sentence structure—a grammar—and generates random sentences that meet that description. For example, we might describe an English sentence as a noun and a verb, or as a noun, a verb, and an object, and so on.

The sentences that we can construct will be more interesting if we can handle complicated rules. For example, rather than saying merely that a sentence is a noun followed by a verb, we might allow noun phrases, where a noun phrase is either simply a noun or an adjective followed by a noun phrase. As a concrete example, given the following input

Categories        Rules                        <noun>            cat<noun>            dog<noun>            table<noun-phrase>     <noun><noun-phrase>     <adjective> <noun-phrase><adjective>       large<adjective>       brown<adjective>       absurd<verb>            jumps<verb>            sits<location>        on the stairs<location>        under the sky<location>        wherever it wants<sentence>        the <noun-phrase> <verb> <location>

our program might generate

the table jumps wherever it wants

#include <iostream>#include <string>#include <cctype>#include <map>#include <vector>#include <algorithm>#include <cstdlib>using namespace std;typedef vector<string> Rule;typedef vector<Rule> Rule_collection;typedef map<string, Rule_collection> Grammar;bool space(char c){    return isspace(c);}bool not_space(char c){return !isspace(c);}vector<string> split(const string& str){    typedef string::const_iterator iter;    vector<string> ret;    iter i = str.begin();    while (i != str.end()) {        // ignore leading blanks        i = find_if(i, str.end(), not_space);        // find end of next word        iter j = find_if(i, str.end(), space);        // copy the characters in [i, j)        if (i != str.end())            ret.push_back(string(i, j));        i = j;    }    return ret;}// read a grammar from a given input stream Grammar read_grammar(istream& in){    Grammar ret;    string line;    // read the input    while (getline(in, line)){        // split the input into words         vector<string> entry = split(line);        if (!entry.empty())            // use the category to store the associated rule            ret[entry[0]].push_back(Rule(entry.begin() + 1, entry.end()));    }    return ret;}bool bracketed(const string& s){return s.size() > 1 && s[0] == '<' && s[s.size() - 1] == '>';}// return a random integer in the range [0, n)int nrand(int n){    if (n <= 0 || n > RAND_MAX)        throw domain_error("Argument to nrand is out of range");    const int bucket_size = RAND_MAX / n;    int r;    do r = rand() / bucket_size;    while (r >= n);    return r;}void gen_aux(const Grammar& g, const string& word, vector<string>& ret){    if (!bracketed(word)) {        ret.push_back(word);    } else {        // locate the rule that corresponds to word        Grammar::const_iterator it = g.find(word);        if (it == g.end())            throw logic_error("empty rule");        // fetch the set of possible rules        const Rule_collection& c = it->second;        // from which we select one at random        const Rule& r = c[nrand(c.size())];        // recursively expand the selected rule        for (Rule::const_iterator i = r.begin(); i != r.end(); ++i)            gen_aux(g, *i, ret);    }}vector<string> gen_sentence(const Grammar& g){    vector<string> ret;    gen_aux(g, "<sentence>", ret);    return ret;}int main(){Grammar g=read_grammar(cin);for(int ii=0;ii<6;ii++){vector<string> sentence = gen_sentence(g);vector<string>::const_iterator it = sentence.begin();if (!sentence.empty()) {cout << *it;++it;}// write the rest of the words, each preceded by a space while (it != sentence.end()) {cout << " " << *it;++it;}cout << endl;    }//// generate the sentence //   vector<string> sentence = gen_sentence(read_grammar(cin)); //   // write the first word, if any //   vector<string>::const_iterator it = sentence.begin(); //   if (!sentence.empty()) //{ //       cout << *it; //       ++it; //   } //   // write the rest of the words, each preceded by a space  //   while (it != sentence.end()) //{ //       cout << " " << *it; //       ++it; //   } //   cout << endl;    return 0;} 

输入：

<n> cat
<n> dog
<n> pig
<np> <n>
<np> <a> <np>
<a> beautiful
<a> big
<a> small
<v> jumps
<v> sits
<v> runs
<l> under the sky
<l> on the table
<l> in the computer
<sentence> the <np> <v> <l>
^Z

输出：

the beautiful big dog runs in the computer
the pig runs on the table
the cat sits under the sky
the big cat jumps on the table
the big beautiful big cat sits in the computer
the beautiful small small beautiful cat runs under the sky
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