Extract substring from string C++

Question

I have a text file with GCode commands. When I encounter a specific command, I want to place a new line after it with a new command. That part is not an issue. The issue I have is that the command has the following form G0/G1 Xnnn Ynnn Znnn where the X/Y/Z params may or may not be present (at least one is required, but not all). What I want are the numbers after the X/Y/Z. My current working solution would involve using substring(start + 1, end - 1) and find(X/Y/Z) , then checking for all combinations as:

size_t xPos = str.find("X");
size_t yPos = str.find("Y");
size_t zPos = str.find("Z");
float x, y, z;
if(xPos != std::string::npos && yPos != std::string::npos) 
    x = std:stof(str.substring(xPos + 1, yPos - 1);

Is this an appropriate method of doing this? Am I overcomplicating things?

Edit: So, an input would look like G0 X101.1 Y102.3 or G0 Y122 or G0 X55 Z123 , so the n refers to a digit and the command is to tell something like a 3D printer how many units to move in a given direction, so it may need to make a move in one, two, or three directions.

Answer 1

Each time I need to parse something in C++, I consider using Boost.Spirit. It is suitable for both trivial and quite complex parsing tasks.

With Boost.Spirit, it would look like this :

#include <boost/config/warning_disable.hpp>
#include <boost/spirit/include/qi.hpp>

#include <iostream>
#include <string>
#include <vector>

int main()
{
  namespace qi = boost::spirit::qi;
  namespace ascii = boost::spirit::ascii;
  std::string input = "G0/G1 X001 Y123 Z321";
  int x, y, z;
  bool result = qi::phrase_parse(input.begin(), input.end(),
     qi::lit("G0/G1") >> "X" >> qi::int_ >> "Y" >> qi::int_ >> "Z" >> qi::int_ , ascii::space, x, y, z);
  if (result)
  {
      std::cout << "X:" << x << std::endl;
      std::cout << "Y:" << y << std::endl;
      std::cout << "Z:" << z << std::endl;
  }
}

Note that the coordinates are parsed directly into int s!

UPDATE : A bit more complicated parsing , according to your updated question:

#include <boost/config/warning_disable.hpp>
#include <boost/spirit/include/qi.hpp>

#include <iostream>
#include <string>
#include <vector>
#include <boost/optional.hpp>
#include <boost/optional/optional_io.hpp>

int main()
{
  namespace qi = boost::spirit::qi;
  namespace ascii = boost::spirit::ascii;
  std::string input = "G0 X001 Z321";
  boost::optional<float> x, y, z;
  int cmd;
  using namespace qi;
  bool result = phrase_parse(input.begin(), input.end(), qi::lexeme[lit("G") >> int_] >> -(lit("X") >> float_) >> -(lit("Y") >> float_) >> -(lit("Z") >> float_) , ascii::space, cmd, x, y, z);
  if (result)
  {
      std::cout << "Command number:" << cmd << std::endl;
      std::cout << "X:" << x << std::endl;
      std::cout << "Y:" << y << std::endl;
      std::cout << "Z:" << z << std::endl;
  }
}

Answer 2

Using std::stringstream

int x, y, z = -1;

std::stringstream ss("G0/G1 X123 Y456");
std::string str;
while (ss>>str) {
    if (str.at(0) == 'X') {
        std::stringstream(str.substr(1)) >> x;
    } else if (str.at(0) == 'Y') {
        std::stringstream(str.substr(1)) >> y;
    } else if (str.at(0) == 'Z') {
        std::stringstream(str.substr(1)) >> z;
    }
}