otvet.mail.ru

#include <iostream>
using namespace std;

bool isnumber(const char *p)
{
    for (; *p != 0; *p++)
        if (*p < '0' || *p > '9')
            return false;
    return true;
}

void main()
{
    char str[20];
    while (1)
    {
        cout << "type anything: ";
        cin >> str;
        cout << "is a number ~ " << (isnumber(str) ? "true" : "false") << "\n\n";
    }
}