UVa 748 Exponentiation

　　浮点数求幂，先把浮点数转化成整数，再按大数乘法计算，最后再考虑加上小数点，对小数点的位置和关于0的处理要考虑清楚。

　　代码如下：

#include <cstdio>
#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;

const int maxn = 1000;

struct bign
{
    int d[maxn];
    int len;

    bign()
    {
        memset(d, 0, sizeof(d));
        len = 1;
    }

    bign(int num)
    {
        *this = num;
    }

    bign(char * num)
    {
        *this = num;
    }

    bign operator = (const char * num)
    {
        len = strlen(num);
        for(int i = 0; i < len; i++)
            d[i] = num[len-1-i] - '0';
        return *this;
    }

    bign operator = (int num)
    {
        char s[maxn];
        sprintf(s, "%d", num);
        *this = s;
        return *this;
    }

    bign operator + (const bign& b) const
    {
        bign c;
        c.len = 0;
        int g = 0;
        for(int i = 0; i < max(len, b.len) || g; i++)
        {
            int x = g;
            if(i < len)   x += d[i];
            if(i < b.len)   x += b.d[i];
            c.d[c.len++] = x % 10;
            g = x / 10;
        }
        return c;
    }

    bign operator += (const bign& b)
    {
        *this = *this + b;
        return *this;
    }

    void clean()
    {
        while(len > 1 && !d[len-1])
            len--;
    }
    
    bign operator - (const bign& b)
    {
        bign c;
        c.len = 0;
        int g = 0;
        for(int i = 0; i < len; i++)
        {
            int x = d[i] - g;
            if(i < b.len)   x -= b.d[i];
            if(x >= 0)   g = 0;
            else 
            {
                g = 1;
                x += 10;
            }
            c.d[c.len++] = x;
        }
        c.clean();
        return c;
    }

    bign operator * (const bign& b)
    {
        bign c;
        c.len = len + b.len;
        for(int i = 0; i < len; i++)
            for(int j = 0; j < b.len; j++)
                c.d[i+j] += d[i] * b.d[j];
        for(int i = 0; i < c.len-1; i++)
        {
            c.d[i+1] += c.d[i] / 10;
            c.d[i] %= 10;
        }
        c.clean();
        return c;
    }

    bool operator < (const bign& b) const
    {
        if(len != b.len)   return len < b.len;
        for(int i = len-1; i >= 0; i--)
            if(d[i] != b.d[i])   return d[i] < b.d[i];
        return false;
    }

    bool operator > (const bign& b) const
    {
        return b < *this;
    }

    bool operator <= (const bign& b) const
    {
        return !(b < *this);
    }

    bool operator >= (const bign& b) const
    {
        return !(*this < b);
    }

    bool operator != (const bign& b) const
    {
        return b < *this || *this < b;
    }

    bool operator == (const bign& b) const
    {
        return !(b < *this) && !(b > *this);
    }

    string str() const
    {
        string res = "";
        for(int i = 0; i < len; i++)
            res = (char)(d[i]+'0') + res;
        if(res == "")   res = "0";
        return res;
    }
};

istream& operator >> (istream &in, bign& x)
{
    string s;
    in >> s;
    x = s.c_str();
    return in;
}

ostream& operator << (ostream & out, const bign& x)
{
    out << x.str();
    return out;
}

int main()
{
#ifdef LOCAL
    freopen("in", "r", stdin);
#endif
    double x;
    int y;
    while(scanf("%lf%d", &x, &y) != EOF)
    {
        char num[10];
        sprintf(num, "%lf", x);
        int t = 0, dot = 0, frac = 0;
        for(int i = 0; num[i] != '\0'; i++)
        {
            if(num[i] == '.')   dot = 1;
            else
            {
                t = t * 10 + num[i] - '0';
                if(dot)   frac++;
            }
        }
        bign a = t, ans = 1;
        for(int i = 0; i < y; i++)
            ans = ans * a;
        frac *= y;
        if(frac < ans.len)
        {
            for(int i = ans.len-1; i >= frac; i--)   printf("%d", ans.d[i]);
            printf(".");
            int tail;
            for(tail = 0; tail < frac; tail++)
                if(ans.d[tail])   break;
            for(int i = frac-1; i >= tail; i--)   printf("%d", ans.d[i]);
            printf("\n");
        }
        else
        {
            printf(".");
            for(int i = 0; i < frac-ans.len; i++)   printf("0");
            int tail;
            for(tail = 0; tail < ans.len; tail++)
            if(ans.d[tail])   break;
            for(int i = ans.len-1; i >= tail; i--)   printf("%d", ans.d[i]);
            printf("\n");
        }

    }
    return 0;
}