Wu Yudong's Blog

Counting fractions in a range

Consider the fraction, n/d, where n and d are positive integers. If n < d and HCF(n,d)=1, it is called a reduced proper fraction.

If we list the set of reduced proper fractions for d ≤ 8 in ascending order of size, we get:

How many fractions lie between 1/3 and 1/2 in the sorted set of reduced proper fractions for d ≤ 12,000?

分数有范围计数

考虑形如n/d的分数，其中n和d均为正整数。如果n < d且其最大公约数为1，则该分数称为最简真分数。

如果我们将d ≤ 8的最简真分数构成的集合按大小升序列出，我们得到：

将d ≤ 12,000的最简真分数构成的集合排序后，在1/3和1/2之间有多少个分数？

//（Problem 73）Counting fractions in a range
// Completed on Wed, 19 Feb 2014, 16:34
// Language: C11
//
// 版权所有（C）wuyudong
// 博客地址：http://www.wuyudong.com

#include<stdio.h>
#define N 12000

int gcd(int a, int b)  //求最大公约数函数
{
    int r;
    while(b) {
        r = a % b;
        a = b;
        b = r;
    }
    return a;
}
 
void solve()
{
    int a, b, i, j, ans;
    ans = 0;
    for(i = 5; i <= N; i++) {
        a = i / 3;  b = i / 2;
        for(j = a + 1; j < b + 1; j++) {
            if(gcd(i, j) == 1)
                ans++;
        }
    }
    printf("%d\n", ans);
}
 
int main()
{
    solve();
    return 0;
}