Question

I made a MegaInt class that could handle very large numbers and overloaded a few operators. Unfortunately I’ve gotten stuck and my code just crashes.

Each MegaInt ojb has a vector value and an bool sign. Each position of a number is placed in the vector (i.e. 4325 is vector value = {5,2,3,4}) and whether its sign (+ or -) is 1 or 0.

here’s a bit of the code…

        #include <vector>
        #include <string>
        using namespace std;

        class MegaInt{
            friend class main;
            private:
                vector<int> value;
                bool sign; //true is pos, false is neg

            public:
                MegaInt(vector <int> x, bool y);
                MegaInt(string s);
                MegaInt(const MegaInt& m);
                MegaInt & operator*=(const MegaInt &rhs);

                #include <iostream>
                using namespace std;
                #include "MegaInt.h"
                #include <math.h>

                MegaInt::MegaInt(string s){
                    int pos = s.length()-1;

                    while (pos >= 0){
                        if(pos == 0 && s[pos] == '-'){
                            sign = false;
                            break;
                        }
                        else if (pos == 0 && s[pos] == '+'){
                            sign = true;
                            break;
                        }
                        else{
                            sign = true;
                        }
                        if(s[pos] >= 48 && s[pos] <= 57)
                            value.push_back(s[pos]-48);
                        else{
                            value.clear();
                            break;
                        }
                        pos --;
                    }
                    chopoffleadingOs();
                }
                MegaInt::MegaInt(const MegaInt& m){
                    value = m.value;
                    sign = m.sign;
                }

                MegaInt operator*(const MegaInt& x, const MegaInt& y){
                    int multi = 0;
                    int temp;
                    vector<int> total;

                    for(int i = x.value.size()-1; i>=0; --i){
                        for(int j = y.value.size()-1, k = 0; j>=0; --j, ++k){
                            temp = x.value[i] * y.value[j];
                            if (total.size() <= (i + multi + 1))
                                                     total.resize(i + multi + 1 + 1);
                                                total[i + multi] += (temp % 10);
                                                temp = (temp - total[i]) / 10; 
                                                total[i + multi + 1] += temp;
                                                     }
                                                     multi++;
                                                    }

                                        reverse(total.begin(), total.end());

                return newTotal;
                }

I mainly seem to be stuck on the overloaded multiplication part. The rest I think i got.

Thanks,
Noah

Answer 1

One likely the problem is here:

total[i+multi]+=(8%10);

You use the total vector, but haven’t allocated memory for it. This can be done through the resize function:

total.resize(MAX_SIZE);

A good way to find unexpected crashes is to use the debugger. If you run the program in a debugger, it will halt when there is a problem so you can examine variables and see both where the crash is and what may have caused it.

Edit:

If you don’t know the size of the total vector beforehand, you have to resize it dynamically:

if (total.size() <= (i + multi + 1))
    total.resize(i + multi + 1 + 1);  // An extra +1 because vectors are zero-indexed

total[i+multi]+=(8%10);
// ...