Making Conway "look and say" numbers

I was asked to write a small C ++ 11 program that would generate so called Conway numbers, or "watch and talk". Essentially, given the nth term, for example. 11112, the next is just the pronunciation of the first term; in our case 4112, because there were 4 1 and only one 2. Another example: from 13 to 1113.

Below is the complete source code of the program for completeness (omit stdafx.h include if not compile with MS Visual Studio):

#include "stdafx.h"
#include <iostream>
#include <algorithm>
#include <numeric>
#include <string>
#include <vector>

using namespace std;

void print_usage(){

    cout << endl << "Conway Number Generator";
    cout << endl << "Arguments: initializer followed by nth term to be printed" << endl;
}

string say(string s){

    // Returns string said phonetically
    // e.g. '111' becomes '31'

    unsigned int sz = s.size();

    if (sz == 1){
        s.insert(0, "1");
    }
    else{
        s.insert(0, to_string(sz));
        s.erase(2, sz-1);
    }
    return s;
}

string get_next(string x){

    // Returns next term in Conway sequence

    unsigned prev_point = 0;
    vector<string> grp;
    string tmp;
    tmp.resize(x.size());

    // Organize sequence in group of identical consecutive characters

    for (unsigned int i = 0; i < x.size(); ++i){

        if (i != x.size() - 1){

            if (x.at(i) != x.at(i + 1)){
                tmp.assign(x, prev_point, i - prev_point);
                grp.push_back(tmp);
                prev_point = i + 1;
                tmp.clear();
            }
        }
        else if (i != 0){

            if (x.at(i) != x.at(i - 1)){
                tmp = x.at(i);
                grp.push_back(tmp);
                tmp.clear();
            }
        }
    }

    // Phonetically say each group

    // Could use a lambda: transform(begin(grp), end(grp), begin(said_grp)[=](string s){return say(s);});
    // if I create a new vector<string> said_grp to copy in
    // That didn't help the runtime error

    for (auto& i : grp)
        i = say(i);

    // Join each vector<string> entry into a single string

    string next_seq;
    next_seq = accumulate(begin(grp), end(grp), string(""));
    return next_seq;
}

string conway(string init, int n){

    // Print out nth Conway term
    // starting sequence at init

    for (int i = 1; i <= n; ++i)
        init = get_next(init);
    return init;
}

int main(int argc, const char* argv[]){

    if (argc < 3){
        print_usage();
        return 0;
    }
    cout << endl << "Conway number: " << conway(argv[1], stoi(argv[2])) << endl;
    return 0;
}

      

        
        
        
      

    

General approach:

• Accept two arguments: the first member of the Conway sequence and an integer n
  
  select the calculation of the nth term of this sequence.
• The function conway(...)
  
  loops get_next()
  
  over the source string n
  
  times.
• The say()
  
  ' function speaks numbers.
• get_next()
  
  functions by breaking the whole number into consecutive identical numbers; each is then converted to say()
  
  .

The problem I am facing is the error std::out_of_range

, from the return statement of my function say()

. However, I can say()

only remember testing with different inputs, and this never threw an exception. I must be using it wrong somehow. As I noted in the code comments, I tried using STL conversion instead of looping for

with lambda before, same error.

Note: For those interested in Conway's sequence, see Conway's original article. They allow for some interesting properties, and they seem to generate a constant known as Conway's constant, which has been shown to be algebraic.