Accelerated C++ 7 generate sentence

split.h
see Note 6
grammar.txt

<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>

main.cpp

#include <iostream>#include <map>#include <string>#include <vector>#include <stdlib.h>#include <stdexcept>#include <fstream>#include "split.h"using std::cout; using std::endl; using std::domain_error;using std::string; using std::vector; using std::istream;using std::map; using std::ifstream;    using std::logic_error;using std::cerr;typedef vector<string> Rule;typedef vector<Rule> Rule_collection;typedef map<string, Rule_collection> Grammar;Grammar read_grammar(ifstream& infile) {    Grammar ret;    string line;    while(getline(infile, line)) {        vector<string> entry = split(line);        if(!entry.empty()) {            Rule rules(entry.begin() + 1, entry.end());            ret[entry[0]].push_back(rules);        }    }    return ret;}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;}bool bracketed(const string& s) {    return s[0] == '<' && s[s.size() - 1] == '>';}void gen_aux(const Grammar& g, const string& s, vector<string>& v) {    if (bracketed(s)){        Grammar::const_iterator it = g.find(s);        if (it == g.end())            throw logic_error("empty error");        const Rule_collection& rc = it->second;        const Rule r = rc[nrand(rc.size())];        for (Rule::size_type i = 0; i != r.size(); ++i)            gen_aux(g, r[i], v);    } else         v.push_back(s);}vector<string> gen_sentence(const Grammar& g) {    vector<string> ret;    gen_aux(g, "<sentence>", ret);    return ret;}int main() {    string ifile = "grammar.txt";    ifstream infile(ifile.c_str());    // check that the open succeede    if (!infile) {        cerr << "error: unable to open input file: "            << ifile << endl;        return -1;    }    // generate the sentence    Grammar g = read_grammar(infile);    vector<string> sentence = gen_sentence(g);    // write the first word, if any    vector<string>::const_iterator it = sentence.begin();    if(!sentence.empty()) {        cout << *it;        ++it;    }    // write the rest of the word, each preceded by a space    while (it != sentence.end()) {        cout << " " << *it;        ++it;    }    cout << endl;    return 0;}