【C++】大数的+-*/四则运算

所谓大数，则指数值特别大的数，可能会有99位，100位，远远超过了long long表示的范围。

这样的数作四则运算，需要用到字符串。用字符串通过每一位的字符的四则运算来模拟。

废话少说，上代码：

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

class BigNum{
    vector<char> m_vec;
    //构造函数，析构函数，size()函数，友元重载>>,<<，重载[] + - * / =。
public:
    BigNum();
    ~BigNum();

public:
    int size() const {
        return m_vec.size();
    }
    void clr(){
        m_vec.clear();
    }

public:
    friend ostream& operator<<(ostream &os,const BigNum &data);
    friend istream& operator>>(istream &is,BigNum &data);

public:
    char operator[](int nIndex) const;

    BigNum operator+(const BigNum &data);
    BigNum operator-(BigNum &data);
    BigNum operator*(const BigNum &data);
    BigNum operator/(BigNum &data);
    BigNum& operator=(const BigNum &data);
    bool operator==(const BigNum &data);
    bool operator>(const BigNum &data);
    bool operator>=(const BigNum &data);
};

BigNum::BigNum(){}

BigNum::~BigNum(){}

ostream& operator<<(ostream &os,const BigNum &data){
    vector<char>::const_iterator itVec = data.m_vec.begin();
    for(; itVec != data.m_vec.end(); itVec++){
        os << *itVec;
    }
    os << endl;
    return os;
}

istream& operator>>(istream &is,BigNum &data){
    //必须清空is的缓冲区，不然is.get()得到的是上一次回车的
。并且不能用is.ignore()，因为ignore()会把第一次输入的第一个字符删除掉。
    is.sync();
    while(true){
        char c = is.get();
        if(' ' == c || '
' == c || '
' == c)
            break;
        if( c < '0' || c > '9'){
            data.m_vec.clear();
            cerr << "not accept" << endl;
            break;
        }
        data.m_vec.push_back(c);
    }
    
    return is;
}

char BigNum::operator[](int nIndex) const
{
    if(nIndex < 0 || nIndex >= m_vec.size())
    {
        cerr << "error occur in []";
        return ' ';
    }
    return m_vec[nIndex];
}

bool BigNum::operator==(const BigNum &data){
    if(m_vec.size() != data.m_vec.size())
        return false;
    for(int i = 0; i < m_vec.size(); i++){
        if(m_vec[i] != data.m_vec[i])
            return false;
    }
    return true;
}

bool BigNum::operator>(const BigNum &data){
    int a = m_vec.size();
    int b = data.m_vec.size();
    if(a > b){
        return true;
    }
    else if(a < b){
        return false;
    }
    else if(a == b)
    {
        for(int i = 0; i < a; i++){            
            if(m_vec[i] > data.m_vec[i]){
                return true;
            }
            else if(m_vec[i] < data.m_vec[i]){
                return false;
            }
        }
        return false;
    }
}

bool BigNum::operator>=(const BigNum &data){
    if(*this > data || *this == data)
        return true;
    return false;
}

BigNum BigNum::operator+ (const BigNum &data){
        int nCarry = 0;
        BigNum numTemp;
        vector<char>& vecTemp = numTemp.m_vec; //必须是引用
        
        int i = data.size() - 1;
        int j = m_vec.size() -1;
        for(; i >= 0 || j >= 0; i--,j--){
            char a = j>=0? m_vec[j] - '0': 0;
            char b = i>=0? data[i] - '0' :0;
            char c = a + b + nCarry;
            nCarry = c / 10;
            vecTemp.push_back(c%10 + '0');
        }

        if(nCarry != 0)
        {
            vecTemp.push_back(nCarry + '0');
        }

        //reverse vecTemp
        for(i = 0, j = vecTemp.size() - 1; i < j; i++,j--)
        {
            char cTemp = vecTemp[i];
            vecTemp[i] = vecTemp[j];
            vecTemp[j] = cTemp;
        }
        return numTemp;
}

BigNum& BigNum::operator=(const BigNum &data)
{
    m_vec.clear();
    vector<char>::const_iterator itVec = data.m_vec.begin();
    for(; itVec != data.m_vec.end(); itVec++)
    {
        m_vec.push_back(*itVec);
    }
    return *this;
}

BigNum BigNum::operator*(const BigNum &data){
    int nCarry = 0;
    BigNum result;
    BigNum numTemp;
    vector<char>& vecTemp = numTemp.m_vec;

    int i = data.size()-1;
    for(; i >= 0; i--){
        char a = data[i] - '0';
        int j = m_vec.size() - 1;
        for(; j >= 0; j--){
            char b = m_vec[j] - '0';
            char c = b * a + nCarry;
            nCarry = c/10;
            vecTemp.push_back(c % 10 + '0');
        }
        if(nCarry != 0){
            vecTemp.push_back(nCarry + '0');
            nCarry = 0;
        }

        //reverse vecTemp
        int n = 0;
        int m = vecTemp.size() - 1;
        for(; n < m; n++,m--){
            char cTemp = vecTemp[n];
            vecTemp[n] = vecTemp[m];
            vecTemp[m] = cTemp;
        }

        for(int t = data.size() - 1; t > i; t--)
        {
            vecTemp.push_back('0');
        }

        result = result + numTemp;
        vecTemp.clear();
    }
    return result;
}

BigNum BigNum::operator-(BigNum &data){
    //m_vec > data.m_vec
    int nCarry = 0;
    BigNum numTemp;
    vector<char>& vecTemp = numTemp.m_vec;

    int i = data.size() - 1;
    int j = m_vec.size() -1;
    for(; i >= 0 || j >= 0; i--,j--){
        char a = j>=0? m_vec[j] - '0': 0;
        char b = i>=0? data[i] - '0' :0;
        char c = a - nCarry;
        if(c < b){
            //需要借位
            c = c + 10;
            nCarry = 1;
            vecTemp.push_back( (c-b) + '0');
        }
        else{
            nCarry = 0;
            vecTemp.push_back( (c-b) + '0');
        }
    }

    //记录前面产生几个0
    int zero = 0;
    vector<char>::const_iterator itVec = vecTemp.end()-1;
    for(; ; itVec--){
        if( *itVec == '0'){
            zero++;
        }
        else
            break;
        if(itVec == vecTemp.begin())
            break;        
    }    

    //pop掉0
    for(int k = zero; k > 0; k--)
        vecTemp.pop_back();

    //reverse
    for(i = 0, j = vecTemp.size() - 1; i < j; i++,j--)
    {
        char cTemp = vecTemp[i];
        vecTemp[i] = vecTemp[j];
        vecTemp[j] = cTemp;
    }

    return numTemp;
}

BigNum BigNum::operator/(BigNum &data){
    //m_vec > data.m_vec
    BigNum numTemp;
    vector<char>& vecTemp = numTemp.m_vec;
    BigNum &numThis = *this;

    int lenA = m_vec.size();
    int lenB = data.m_vec.size();
    for (int i = lenB; i < lenA; i++){
        data.m_vec.push_back('0');
    }
    for(int j = lenB; j <= lenA; j++){
        int a = 0;
        while(numThis >= data){
            numThis = numThis - data;
            a++;
        }
        vecTemp.push_back( a + '0');
        data.m_vec.pop_back();
    }
    return numTemp;
}

int main()
{
    BigNum numA;
    BigNum numB;
    BigNum numC;
    char opt;
    while(1){
        cin >> numA;
        cin >> opt;
        cin >> numB;
        
        switch(opt){
        case '+':
            numC = numA + numB;
            cout << numC;
            break;
        case '-':
            if(numA == numB){
                cout << 0 << endl;
            }
            else if(numA > numB){
                numC = numA - numB;
                cout << numC;
            }
            else{
                numC = numB - numA;
                cout << '-';
                cout << numC;
            }
            break;
        case '*':
            numC = numA * numB;
            cout << numC;
            break;
        case '/':
            if(numA == numB){
                cout << 1 << endl;
            }
            else if(numA > numB){
                numC = numA / numB;
                cout << numC;
            }
            else{
                cout << 0 << endl;
            }
            break;
        }
        //清空numA,numB。
        numA.clr();
        numB.clr();
        numC.clr();
    }
    return 0;
}