how is an string being read?

Question

i want to write a class POLY for equations like aX^A+bX^B+cX^C... i want to get two inputs for the constructor like this:

POLY(int input_degree,string input_factor);

input_degree contains "A" as mentioned above , and input_factor contains factors like this example : "12 3 -2". i want to write some code which reads input_factor character by character and when there is an space it saves first factor(like "A")and goes for next factor. but i dont know how an string is read by computer (or do i have to know that?).

UPDATE: I answered my question. just check down below. full answer http://cpp.sh/8nriao

Answer 1

So for the community rules and to make it clear i want to answer my question.

#include <iostream>
#include <string>
#include <sstream>
#include <iterator>

using namespace std;

template <size_t N>
void splitString(string (&arr)[N], string str)
{
    int n = 0;
    istringstream iss(str);
    for (auto it = istream_iterator<string>(iss); it != istream_iterator<string>() && n < N; ++it, ++n)
        arr[n] = *it;
}

class Poly {
  public:
    string degree[10];
    string factor[10];
    
    Poly(string input_degree, string input_factor) {
        splitString(degree, input_degree);
        splitString(factor, input_factor);
        
        for (int i = 0; i < 10; i++){
            int this_degree = stoi(degree[i]);
            int this_factor = stoi(factor[i]);
            
            string this_sentence = "";
            
            if(this_degree != 1 && this_degree != 0 ){
                this_sentence = this_sentence + degree[i];
                
                if(this_factor != 0){
                    if(this_factor != 1){
                        this_sentence = this_sentence + "X" + factor[i];
                    }else{
                        this_sentence = this_sentence + "X";
                    }
                }
            }
            
            if(this_sentence != ""){
                cout << "(" << this_sentence << ")";
            }
            
            if(stoi(degree[i+1]) != 0 && stoi(degree[i+1]) != 1){
                cout << " + ";
            }
        }
    }
};

int main()
{
  Poly mypoly("2 1 -4", "1 3 5");
  return 0;
  
}

Answer 2

The process of reading a string and extracting information from it into some sort of structure is called parsing. There are many ways to do this, and which way is appropriate depends on exactly what you want to do, how quickly it needs to run, how much memory you've got available and various other things.

You can write a simple loop which steps over each character and decides what to do based on some variables that store current state - so you might have a flag that says you're in the middle of a number, you see another digit so you add that digit to another variable which is collecting the digits of the current number. When the current number completes (perhaps you find a character which is a space), you can take what's in the accumulator variable and parse that into a number using the standard library.

Or you can make use of standard library features more fully. For your example, you'll find that std::istringstream can do what you want, out of the box, just by telling it to extract ints from it repeatedly until the end of the stream. I'd suggest searching for a good C++ input stream tutorial - anything that applies to reading from standard input using std::cin will be relevant, as like std::istringstream, cin is an input stream and so has the same interface.

Or you could use a full-blown parsing library such as boost::spirit - total overkill for your scenario, but if you ever need to do something like parsing a structured configuration file or an entire programming language, that kind of tool is very useful.