Unlimited matrix

#include <iostream>
#include <vector>
#include <cstdlib>
#include <cctype>
#include <cstring>
using namespace std;

int main()
{
    vector<vector<double> > rows;
    vector<double> columns;
    char str[80], *ptr;

    cout << "Enter value of matrix element or key-letters\n"
        << "[N] to start typing next matrix line\n"
        << "[Q] to stop typing\n\n"
        << "First line:\n";

    do {
        cout << ">> ";
        cin >> str;
        if (strtod(str, &ptr))
            columns.push_back(strtod(str, &ptr));
        else if (strchr("n", tolower(str[0])) && !isalpha(tolower(str[1]))) {
            cout << "Next line:\n";
            rows.push_back(columns);
            columns.clear();
        }
        else if (strchr("q", tolower(str[0])) && !isalpha(tolower(str[1]))) {
            cout << "Quitting...\n";
            rows.push_back(columns);
            break;
        }
        else {
            cout << "Enter only element value or key-letters!\n";
            continue;
        }

    } while (1);

    unsigned int i, j;

    cout << "\t\t\tEntered matrix:\n\n";

    for (i = 0; i < rows.size(); i++) {
        if (rows[i].size() == 0) {
            cout << "\t[empty line]\n";
            continue;
        }
        for (j = 0; j < rows[i].size(); j++)
            cout << rows[i][j] << "\t";
        cout << "\n\n";
    }
    cout << "\n\n";

    //system("pause"); // - В Бубунтах пауза не та, но и так сойдёт
    return 0;
}