给定能随机生成整数1到5的函数，写出能随机生成整数1到7的函数

#include <iostream>
#include <cstdio>
#include <ctime>
#include <cstdlib>
#include <cmath>
using namespace std;

int rand5()
{
//随机生成[1-5]之间的随机数
return (rand() % 5 + 1);
}

int rand7_1()
{
//随机生成[1-7]之间的随机数
int a;
while((a = rand5() * 5 + rand5()) > 26);
return (a - 3)/3;
}

int rand7_2()
{
//随机生成[1-7]之间的随机数
int num = rand5() + rand5() * 10 + rand5() * 100 + rand5() * 1000;
return (num % 7 + 1);
}

int rand7_3()
{
return (rand() % 7 + 1);
}

double variance(int *a, int average, int len)
{
//求方差：sqrt(sum((a[i]-平均数)^2))
double s = 0;
for(int i = 1; i < len; i++)
{
s += (a[i] - average) * (a[i] - average) ;
}
return sqrt(s);
}

int main()
{
srand((unsigned)time(NULL));
int cnt = 1000;
int a[8] = {0};
int b[8] = {0};
int c[8] = {0};
int a1, a2, a3;
int ave = cnt / 7;
while(cnt--)
{
a1 = rand7_1();
a2 = rand7_2();
a3 = rand7_3();
a[a1]++;
b[a2]++;
c[a3]++;
}
cout << "The variance of rand7_1(): " << variance(a, ave, 8) << endl;
cout << "The variance of rand7_2(): " << variance(b, ave, 8) << endl;
cout << "The variance of rand7_2(): " << variance(c, ave, 8) << endl;
}