Average occurrence of the first word in sentence

average_fw.cpp

#include <iostream>
#include <regex>

using namespace std;

int main()
{
    regex sentense_regex("[^!?.]+");
    string all_str;
    string cmp_str;
    size_t num_sts;
    size_t num_cmp;
    double average_num;

    getline(cin,all_str);

    if((num_cmp = all_str.find(' ')) != string::npos)
    {
        cmp_str = all_str.substr(0, num_cmp);
    }
    else
    {
        cout << "have't string!"<< endl;
        return 0;
    }

    regex word_regex(cmp_str);
    auto sentence_begin = sregex_iterator(all_str.begin(), all_str.end(), sentense_regex);
    auto sentence_end = sregex_iterator();

    num_sts = distance(sentence_begin, sentence_end);
    for (sregex_iterator i = sentence_begin; i != sentence_end; ++i)
    {
        smatch match = *i;
        string match_str = match.str();
        auto words_begin = sregex_iterator(match_str.begin() + 1, match_str.end(), word_regex);
        auto words_end = sregex_iterator();
        average_num += static_cast<double>(distance(words_begin, words_end))/num_sts;

    }
    cout << average_num << endl;
    return 0;
}