随机数生成器使用了bitset来跟踪这些数
在随机数生成器下一次工作周期重新开始之前
所有在允许范围内的数都将无重复地按照随机顺序返回
//: C05:Urand.h {-bor}
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Unique randomizer.
#ifndef URAND_H
#define URAND_H
#include <bitset>
#include <cstddef>
#include <cstdlib>
#include <ctime>
using std::size_t;
using std::bitset;
template<size_t UpperBound> class Urand {
bitset<UpperBound> used;
public:
Urand() { srand(time(0)); } // Randomize
size_t operator()(); // The "generator" function
};
template<size_t UpperBound>
inline size_t Urand<UpperBound>::operator()() {
if(used.count() == UpperBound)
used.reset(); // Start over (clear bitset)
size_t newval;
while(used[newval = rand() % UpperBound])
; // Until unique value is found
used[newval] = true;
return newval;
}
#endif // URAND_H ///:~
Urand生成的数全是独一无二的
因为bitset used跟踪了随机空间所有可能产生的数
并且设置相应的状态位来记录每一个使用过的数
//: C05:UrandTest.cpp {-bor}
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
#include <iostream>
#include "Urand.h"
using namespace std;
int main() {
Urand<10> u;
for(int i = 0; i < 20; ++i)
cout << u() << ' ';
getchar();
} ///:~
输出
2 4 8 5 9 3 0 6 7 1 5 7 0 1 6 2 8 3 4 9
无类型模板参数在优化数值计算方面重要