带整数的C ++矢量积

Question

I have a weird Limit with my Datatype. Without explaining much, I will just show you.

This is the Class.

class vecmath
{
private:
    std::vector<int> avec;
public:
    vecmath(std::string b);
    vecmath(std::vector<int>);
    ~vecmath();
    vecmath add(vecmath b);
    vecmath mult(vecmath b);
    vecmath prod(vecmath b);
    void read();
    std::string write();
    std::vector<int> getVec();
    vecmath operator+(vecmath a);
    vecmath operator*(vecmath a);
};

This is the constructor which is being used.

vecmath::vecmath(std::string b) {
    for (unsigned long long i = 0; i < b.length(); ++i) {
        avec.push_back((int)(b[i]) - 48);
    }
}

This is the Add Function. It is being called in the operator+ function which just returns it.

vecmath vecmath::add(vecmath b) {
    std::vector<int> cbvec = b.getVec();
    std::vector<int> cavec = avec;
    if (cavec.size() > cbvec.size()) {
        std::swap(cbvec, cavec);
    }

    std::vector<int> ret;
    unsigned long long asize = cavec.size();
    unsigned long long bsize = cbvec.size();

    std::reverse(cavec.begin(), cavec.end());
    std::reverse(cbvec.begin(), cbvec.end());

    int carry = 0;
    for (unsigned long long i = 0; i < asize; ++i) {

        int sum = (cavec[i] + cbvec[i] + carry);
        ret.push_back(sum % 10);

        carry = sum / 10;
    }

    for (unsigned long long i = asize; i < bsize; ++i) {
        int sum = (cbvec[i] + carry);
        ret.push_back(sum % 10);
        carry = sum / 10;
    }

    if (carry) {
        ret.push_back(carry);
    }

    std::reverse(ret.begin(), ret.end());

    return vecmath(ret);
}

This is the function which calculates a Product from 2 Vectors (1 is in the Class itself).

vecmath vecmath::prod(vecmath b) {
    vecmath ret("0");
    std::vector<int> cbvec = b.getVec();
    std::vector<int> cavec = avec;

    if(cavec.size() > cbvec.size()) {
        std::swap(cavec, cbvec);
    }

    unsigned long long asize = cavec.size();
    unsigned long long bsize = cbvec.size();

    std::reverse(cavec.begin(), cavec.end());
    std::reverse(cbvec.begin(), cbvec.end());

    for(unsigned long long i = 0; i < asize; ++i) {
        for (unsigned long long j = 0; j < bsize; ++j) {
            ret = ret + vecmath(std::to_string(cavec[i] * cbvec[j] * nfzehnhoch(i) * nfzehnhoch(j)));
        }
    }

    return ret;
}

This is the nfzehnhoch function.

unsigned long long nfzehnhoch(unsigned int n) {
    unsigned long long ret = 1;
    for(unsigned int i = 0; i < n; ++i) {
        ret *= 10;
    }
    return ret;
}

This is my main Function.

int main(void) {
    vecmath a("2");
    vecmath c("2");

    for(int i = 1; i <= 100; ++i) {
        std::cout << c.write() << ", " << std::endl;
        c = c * a;
    }

    std::string end;
    std::getline(std::cin, end);
}

This is what I get when I run it.

2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, 262144, 524288, 1048576, 2097152, 4194304, 8388608, 16777216, 33554432, 67108864, 134217728, 268435456, 536870912, 1073741824, 2147483648, 4294967296, 8589934592, 17179869184, 34359738368, 68719476736, 137438953472, 274877906944, 549755813888, 1099511627776, 2199023255552, 4398046511104, 8796093022208, 17592186044416, 35184372088832, 70368744177664, 140737488355328, 281474976710656, 562949953421312, 1125899906842624, 2251799813685248, 4503599627370496, 9007199254740992, 18014398509481984, 36028797018963968, 72057594037927936, 144115188075855872, 288230376151711744, 576460752303423488, 1152921504606846976, 2305843009213693952, 4611686018427387904, 9223372036854775808, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 184467440737095 51616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616, 18446744073709551616,

As you see, it's stuck at 18446744073709551616. Maybe someone will just take a glance at it and find out.

Thanks for any help.

Answer 1

The value you get stuck at is 2^64. The underlying library is probably detecting the overflow and not doing the multiplication.