都知道int,float,double的精度和可使用的范围都是有限的，对于大数相减，总是会产生溢出的情况。
所以对于大数相减，认为可以用数组来做。比如，定义一个数组a[100]a[1]用来保存个位数，a[2]用来保存十位数，等等。
如果a[i]的值大于或等于10，就让a[i+1]++，a[i]-10就行了。
在打印的时候就用一个for语句，再调一下输出格式就可以了。
这是个老问题了，解决方法就是，自己模拟计算机的位运算，下面有个代码例子，你看一下吧，摘自百度 输出的时候效率太低，还可以改进Code:
/*****************************************************************
大数运算库头文件：BigInt.h
版本：1.2 (2003.5.13)
说明：适用于MFC，1024位RSA运算
*****************************************************************/
//允许生成1120位（二进制）的中间结果
#ifndef BI_MAXLEN
#define BI_MAXLEN 35
#define DEC 10
#define HEX 16
class CBigInt
{
public:
//大数在0x100000000进制下的长度
unsigned m_nLength
//用数组记录大数在0x100000000进制下每一位的值
unsigned long m_ulValue[BI_MAXLEN]
CBigInt()
~CBigInt()
/*****************************************************************
基本操作与运算
Mov，赋值运算，可赋值为大数或普通整数，可重载为运算符“=”
Cmp，比较运算，可重载为运算符“==”、“!=”、“>=”、“<=”等
Add，加，求大数与大数或大数与普通整数的和，可重载为运算符“+”
Sub，减，求大数与大数或大数与普通整数的差，可重载为运算符“-”
Mul，乘，求大数与大数或大数与普通整数的积，可重载为运算符“*”
Div，除，求大数与大数或大数与普通整数的商，可重载为运算符“/”
Mod，模，求大数与大数或大数与普通整数的模，可重载为运算符“%”
*****************************************************************/
void Mov(unsigned __int64 A)
void Mov(CBigInt&A)
CBigInt Add(CBigInt&A)
CBigInt Sub(CBigInt&A)
CBigInt Mul(CBigInt&A)
CBigInt Div(CBigInt&A)
CBigInt Mod(CBigInt&A)
CBigInt Add(unsigned long A)
CBigInt Sub(unsigned long A)
CBigInt Mul(unsigned long A)
CBigInt Div(unsigned long A)
unsigned long Mod(unsigned long A)
int Cmp(CBigInt&A)
/*****************************************************************
输入输出
Get，从字符串按10进制或16进制格式输入到大数
Put，将大数按10进制或16进制格式输出到字符串
*****************************************************************/
void Get(CString&str, unsigned int system=HEX)
void Put(CString&str, unsigned int system=HEX)
/*****************************************************************
RSA相关运算
Rab，拉宾米勒算法进行素数测试
Euc，欧几里德算法求解同余方程
RsaTrans，反复平方算法进行幂模运算
GetPrime，产生指定长度的随机大素数
*****************************************************************/
int Rab()
CBigInt Euc(CBigInt&A)
CBigInt RsaTrans(CBigInt&A, CBigInt&B)
void GetPrime(int bits)
}
#endif
/*****************************************************************
大数运算库源文件：BigInt.cpp
版本：1.2 (2003.5.13)
说明：适用于MFC，1024位RSA运算
*****************************************************************/
#include "stdafx.h"
#include "BigInt.h"
//小素数表
const static int PrimeTable[550]=
{ 3, 5, 7, 11, 13, 17, 19, 23, 29, 31,
37, 41, 43, 47, 53, 59, 61, 67, 71, 73,
79, 83, 89, 97, 101, 103, 107, 109, 113, 127,
131, 137, 139, 149, 151, 157, 163, 167, 173, 179,
181, 191, 193, 197, 199, 211, 223, 227, 229, 233,
239, 241, 251, 257, 263, 269, 271, 277, 281, 283,
293, 307, 311, 313, 317, 331, 337, 347, 349, 353,
359, 367, 373, 379, 383, 389, 397, 401, 409, 419,
421, 431, 433, 439, 443, 449, 457, 461, 463, 467,
479, 487, 491, 499, 503, 509, 521, 523, 541, 547,
557, 563, 569, 571, 577, 587, 593, 599, 601, 607,
613, 617, 619, 631, 641, 643, 647, 653, 659, 661,
673, 677, 683, 691, 701, 709, 719, 727, 733, 739,
743, 751, 757, 761, 769, 773, 787, 797, 809, 811,
821, 823, 827, 829, 839, 853, 857, 859, 863, 877,
881, 883, 887, 907, 911, 919, 929, 937, 941, 947,
953, 967, 971, 977, 983, 991, 997, 1009, 1013, 1019,
1021, 1031, 1033, 1039, 1049, 1051, 1061, 1063, 1069, 1087,
1091, 1093, 1097, 1103, 1109, 1117, 1123, 1129, 1151, 1153,
1163, 1171, 1181, 1187, 1193, 1201, 1213, 1217, 1223, 1229,
1231, 1237, 1249, 1259, 1277, 1279, 1283, 1289, 1291, 1297,
1301, 1303, 1307, 1319, 1321, 1327, 1361, 1367, 1373, 1381,
1399, 1409, 1423, 1427, 1429, 1433, 1439, 1447, 1451, 1453,
1459, 1471, 1481, 1483, 1487, 1489, 1493, 1499, 1511, 1523,
1531, 1543, 1549, 1553, 1559, 1567, 1571, 1579, 1583, 1597,
1601, 1607, 1609, 1613, 1619, 1621, 1627, 1637, 1657, 1663,
1667, 1669, 1693, 1697, 1699, 1709, 1721, 1723, 1733, 1741,
1747, 1753, 1759, 1777, 1783, 1787, 1789, 1801, 1811, 1823,
1831, 1847, 1861, 1867, 1871, 1873, 1877, 1879, 1889, 1901,
1907, 1913, 1931, 1933, 1949, 1951, 1973, 1979, 1987, 1993,
1997, 1999, 2003, 2011, 2017, 2027, 2029, 2039, 2053, 2063,
2069, 2081, 2083, 2087, 2089, 2099, 2111, 2113, 2129, 2131,
2137, 2141, 2143, 2153, 2161, 2179, 2203, 2207, 2213, 2221,
2237, 2239, 2243, 2251, 2267, 2269, 2273, 2281, 2287, 2293,
2297, 2309, 2311, 2333, 2339, 2341, 2347, 2351, 2357, 2371,
2377, 2381, 2383, 2389, 2393, 2399, 2411, 2417, 2423, 2437,
2441, 2447, 2459, 2467, 2473, 2477, 2503, 2521, 2531, 2539,
2543, 2549, 2551, 2557, 2579, 2591, 2593, 2609, 2617, 2621,
2633, 2647, 2657, 2659, 2663, 2671, 2677, 2683, 2687, 2689,
2693, 2699, 2707, 2711, 2713, 2719, 2729, 2731, 2741, 2749,
2753, 2767, 2777, 2789, 2791, 2797, 2801, 2803, 2819, 2833,
2837, 2843, 2851, 2857, 2861, 2879, 2887, 2897, 2903, 2909,
2917, 2927, 2939, 2953, 2957, 2963, 2969, 2971, 2999, 3001,
3011, 3019, 3023, 3037, 3041, 3049, 3061, 3067, 3079, 3083,
3089, 3109, 3119, 3121, 3137, 3163, 3167, 3169, 3181, 3187,
3191, 3203, 3209, 3217, 3221, 3229, 3251, 3253, 3257, 3259,
3271, 3299, 3301, 3307, 3313, 3319, 3323, 3329, 3331, 3343,
3347, 3359, 3361, 3371, 3373, 3389, 3391, 3407, 3413, 3433,
3449, 3457, 3461, 3463, 3467, 3469, 3491, 3499, 3511, 3517,
3527, 3529, 3533, 3539, 3541, 3547, 3557, 3559, 3571, 3581,
3583, 3593, 3607, 3613, 3617, 3623, 3631, 3637, 3643, 3659,
3671, 3673, 3677, 3691, 3697, 3701, 3709, 3719, 3727, 3733,
3739, 3761, 3767, 3769, 3779, 3793, 3797, 3803, 3821, 3823,
3833, 3847, 3851, 3853, 3863, 3877, 3881, 3889, 3907, 3911,
3917, 3919, 3923, 3929, 3931, 3943, 3947, 3967, 3989, 4001
}
//构造大数对象并初始化为零
CBigInt::CBigInt()
{
m_nLength=1
for(int i=0i<BI_MAXLENi++)m_ulValue[i]=0
}
//解构大数对象
CBigInt::~CBigInt()
{
}
/****************************************************************************************
大数比较
调用方式：N.Cmp(A)
返回值：若N<A返回-1；若N=A返回0；若N>A返回1
****************************************************************************************/
int CBigInt::Cmp(CBigInt&A)
{
if(m_nLength>A.m_nLength)return 1
if(m_nLength<A.m_nLength)return -1
for(int i=m_nLength-1i>=0i--)
{
if(m_ulValue[i]>A.m_ulValue[i])return 1
if(m_ulValue[i]<A.m_ulValue[i])return -1
}
return 0
}
/****************************************************************************************
大数赋值
调用方式：N.Mov(A)
返回值：无，N被赋值为A
****************************************************************************************/
void CBigInt::Mov(CBigInt&A)
{
m_nLength=A.m_nLength
for(int i=0i<BI_MAXLENi++)m_ulValue[i]=A.m_ulValue[i]
}
void CBigInt::Mov(unsigned __int64 A)
{
if(A>0xffffffff)
{
m_nLength=2
m_ulValue[1]=(unsigned long)(A>>32)
m_ulValue[0]=(unsigned long)A
}
else
{
m_nLength=1
m_ulValue[0]=(unsigned long)A
}
for(int i=m_nLengthi<BI_MAXLENi++)m_ulValue[i]=0
}
/****************************************************************************************
大数相加
调用形式：N.Add(A)
返回值：N+A
****************************************************************************************/
CBigInt CBigInt::Add(CBigInt&A)
{
CBigInt X
X.Mov(*this)
unsigned carry=0
unsigned __int64 sum=0
if(X.m_nLength<A.m_nLength)X.m_nLength=A.m_nLength
for(unsigned i=0i<X.m_nLengthi++)
{
sum=A.m_ulValue[i]
sum=sum+X.m_ulValue[i]+carry
X.m_ulValue[i]=(unsigned long)sum
carry=(unsigned)(sum>>32)
}
X.m_ulValue[X.m_nLength]=carry
X.m_nLength+=carry
return X
}
CBigInt CBigInt::Add(unsigned long A)
{
CBigInt X
X.Mov(*this)
unsigned __int64 sum
sum=X.m_ulValue[0]
sum+=A
X.m_ulValue[0]=(unsigned long)sum
if(sum>0xffffffff)
{
unsigned i=1
while(X.m_ulValue[i]==0xffffffff){X.m_ulValue[i]=0i++}
X.m_ulValue[i]++
if(m_nLength==i)m_nLength++
}
return X
}
