Problem
Write a method to generate a random number between 1 and 7, given a method that generates a random number between 1 and 5 (i. e. , implement rand7() using rand5()).
Solution
How to generate number uniformly?
#include <iostream>
#include <stdlib.h>
#include <time.h>
using namespace std;
int rand7()
{
return rand() % 7 + 1;
}
int rand5()
{
return rand() % 5 + 1;
}
int rand7_use_rand5()
{
while(true){
int n = (rand5() - 1) + (rand5()-1) * 5;
if(n < 21){
return n % 7 + 1;
}
}
}
int main(int argc, char* argv[])
{
srand((unsigned)time( NULL ));
int count[7];
for(int i = 0; i < 7; i ++){
count[i] = 0;
}
for(int i = 0; i < 21000000; i ++){
int n = rand7_use_rand5();
count[n-1] ++;
}
cout << endl;
for(int i = 0; i < 7; i ++){
cout << "Generate " << i + 1 << " times " << count[i] << endl;
}
return 0;
}
Output
Generate 1 times 3003430
Generate 2 times 2999505
Generate 3 times 2999024
Generate 4 times 3000855
Generate 5 times 2998199
Generate 6 times 3001455
Generate 7 times 2997532