boost::spirit::x3 phrase_parse doing arithmetic operations before pushing into vector

Question

I'm working on a project for my univertitiy studies. My goal is to read double numbers from a large file (2,6 GB) into a double vector.

I am working with the boost spirit x3 library with mmap. I have found some code in the net: https://github.com/sehe/bench_float_parsing which i am using.

Before pushing these double values into the vector i would like to do some arithmetic operations on these. So here i'm stuck. How can i do some artihmetic operations to double values before pushing them?

    template <typename source_it>
    size_t x3_phrase_parse<data::float_vector, source_it>::parse(source_it f, source_it l, data::float_vector& data) const {
        using namespace x3;
        bool ok = phrase_parse(f, l, *double_ % eol, space, data);
        if (ok)
            std::cout << "parse success\n";
        else
            std::cerr << "parse failed: '" << std::string(f, l) << "'\n";

        if (f != l) std::cerr << "trailing unparsed: '" << std::string(f, l) << "'\n";
        std::cout << "data.size(): " << data.size() << "\n";
        return data.size();
    }

Answer 1

I am sorry to not exactly answer your question. But boost spirit is not the appropriate tool. Spirit is a parser generator (as a subset is does of course also lexical analysis) . So, one level to high in the Chomsky hiearchy of languages. You do not need a parser but regular expressions: std:regex

A double can easily be found with a regular expression. In the attached code, I created a simple pattern for a doubles. And a regex can be used to search for it.

So, we will read from an istream (what can be a file, a stringstream, console input or whatever). We will read line by line, until the whole input is consumed.

For each line, we will check, if the input matches the expected pattern, being 1 double.

Then we read this double, do some calculations and then push it into the vector.

Please see the following very simple code.

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <regex>

std::istringstream input{R"(0.0
1.5
2.0
3.0
4.0
-5.0
)"};

using VectorDouble = std::vector<double>;
const std::regex reDouble{R"(([-+]?[0-9]*\.?[0-9]*))"};

std::istream& get(std::istream& is, VectorDouble& dd)
{
    // Reset vector to empty before reading
    dd.clear();

    //Read all data from istream
    std::string line{};
    while (getline(is, line)) {
        // Search for 2 doubles
        std::smatch sm;
        if (std::regex_search(line, sm, reDouble)) {
            // Convert found strings to double
            double d1{std::stod(sm[1])};
            // Do some calculations
            d1 = d1 + 10.0;
            // Push back into vector
            dd.emplace_back(d1);
        }
        else
            std::cerr << "Error found in line: " << line << "\n";
    }
    return is;
}

int main()
{
    // Define vector and fill it
    VectorDouble dd{};
    (void)get(input, dd);

    // Some debug output
    for (double& d : dd) {
        std::cout << d << "\n";
    }
    return 0;
}