Exercises

Exercise Solutions

Here are the basic classes.

class Sieve
{
public:
    virtual int NextNumber () = 0;
};

class SourceSieve: public Sieve
{
public:
    SourceSieve () : _i (1) {}
    int NextNumber ();
private:
    int _i;
};

class Sieve2: public Sieve
{
public:
    Sieve2 (Sieve & src) : _src (src) {}
    int NextNumber ();
private:
    Sieve & _src;
};

Here’s the implementation of NextNumber for SourceSieve and Sieve2:

int SourceSieve::NextNumber ()
{
    if (_i > 100)
        return -1; // end
    return _i++;
}

int Sieve2::NextNumber ()
{
    int i;
    do
    {
        i = _src.NextNumber ();
    } while (i % 2 == 0 && i != 2 && i != -1);
    return i;
}

This is main which prints all prime numbers less than 100:

int main ()
{
    SourceSieve src;
    Sieve2 s2 (src);
    Sieve3 s3 (s2);
    Sieve5 s5 (s3);
    Sieve7 s7 (s5);
    int i;
    for (;;)
    {
        i = s7.NextNumber ();
        if (i == -1)
            break;
        std::cout << i << " ";
    }    
}