大数减法

#include <iostream>
#include <cstring>
#include <cstdio>
using namespace std;
int max(int m,int n)
{
    if(m>n)
    return m;
    return n;
}
int compare(char *str_a,char *str_b)
{
    int len_a, len_b;
    len_a = strlen(str_a);          //分别获取大数的位数进行比较
    len_b = strlen(str_b);

    if ( strcmp(str_a, str_b) == 0 )    //返回比较结果
        return 0;
    if ( len_a > len_b )
        return 1;
    else if( len_a == len_b )
        return strcmp(str_a, str_b);
    else
        return -1;
}
int main()
{
    int f, n;
    int i, k;
    char str_a[1000], str_b[1000];
    int num_a[1000] ;          //初始化大数数组，各位全清0
    int num_b[1000] ;
    int num_c[1000];

    while (scanf("%s%s",str_a,str_b)!= EOF) //可进行多组测试
    {
        memset(num_a,0,sizeof(num_a));
        memset(num_b,0,sizeof(num_b));
       int  len_a = strlen(str_a);         //分别获得两个大数的位数
       int  len_b = strlen(str_b);

        k = max(len_a,len_b)  ;                 //获得最大的位数;
        num_c[0] = 0;
        f = 0;
        n = compare(str_a,str_b);
        for (i=0;i<len_a;i++)                   //颠倒存储
            num_a[i] = str_a[len_a-i-1] - '0';
        for (i=0;i<len_b;i++)
            num_b[i] = str_b[len_b-i-1] - '0';

        for (i=0;i<k;i++)         //逐位进行减法
        {
            if (n>=0)
            {
                if (num_a[i] >= num_b[i])
                    num_c[i] = num_a[i] - num_b[i];
                else
                {
                    num_c[i] = num_a[i] - num_b[i] + 10;
                    num_a[i+1]--;
                }
            }
            else
            {
                if ( num_b[i] >= num_a[i])
                    num_c[i] = num_b[i] - num_a[i];
                else
                {
                    num_c[i] = num_b[i] - num_a[i] + 10;
                    num_b[i+1]--;
                }
            }

        }
        if (n<0)            //按要求打印
            printf("-");
        for (i=k-1; i>=0; i--)
        {
            if (num_c[i])
                f = 1;
            if (f || i == 0 )
                printf("%d",num_c[i]);
        }
        printf("\n");
    }
    return 0;
}