CC150 17.11

17_11.cpp

/* CC150 17.11 */
// implement a method rand7() given rand5()

#include <iostream>
#include <cstdlib>
#include <map>
#define N 10000

using namespace std;

int rand5() { return rand() % 5; }

// use 5 * rand5()
int rand7()
{
	while (1) {
		int n = 5 * rand5() + rand5(); // evenly between [0, 24]
		if (n < 21)
			return n % 7;
	}
}

// use 2 * rand5()
int rand7_2()
{
	while (1) {
		int n1 = 2 * rand5(); // {0, 2, 4, 6, 8]}
		int n2 = rand5();
		if (n2 < 4) { // disgard one extra value
			int n = n1 + n2 % 2; // evenly between [0, 9]
			if (n < 7)
				return n;
		}
	}
}

// use 4 * rand5()
int rand7_4()
{
	while (1) {
		int n1 = 4 * rand5(); // {0, 4, 8, 12, 16}
		int n2 = rand5();
		if (n2 < 4) {
			int n = n1 + n2; // evenly between [0, 19]
			if (n < 14)
				return n % 7;
		}
	}
}

int main()
{
	map<int, int> prob;
	for (int i = 0; i != 7; ++i)
		prob[i] = 0;
	for (int i = 0; i != N; ++i)
		++prob[rand7()];
	int count = 0;
	for (int i = 0; i != 7; ++i) {
		count += prob[i];
		cout << prob[i] << endl;
	}
	cout << count << endl;
	return 0;
}

output

5X:
1353
1463
1447
1427
1474
1451
1385
10000

2X:
1376
1459
1421
1417
1418
1421
1488
10000

4X:
1480
1441
1466
1380
1456
1381
1396
10000