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# C++で二分探索で配列の中の条件を満たす要素の数を求める

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# １点

## xより大きい要素の数

``````return distance(upper_bound(begin, end, x), end);
``````

## xと等しい要素の数

``````auto er = equal_range(begin, end, r);
return distance(er.first, er.second);
``````

## xより小さい

``````return distance(begin, lower_bound(begin, end, x));
``````

# 区間

## xより大きくyより小さい

``````if(x > y) return 0;
if(begin >= end) return 0;
return distance(upper_bound(begin, end, left), lower_bound(begin, end, right));

``````

## ライブラリ化

``````
struct RangeCount {
int upper, r_equal, between, l_equal, lower;

friend std::ostream &operator<<(std::ostream &out, const RangeCount &o) {
cout << endl;
cout << "upper" << ' ' << o.upper << endl;
cout << "r_equal" << ' ' << o.r_equal << endl;
cout << "between" << ' ' << o.between << endl;
cout << "l_equal" << ' ' << o.l_equal << endl;
cout << "lower" << ' ' << o.lower << endl;
return out;
}
};

// startは含む
// endは含まない
RangeCount range_count(vector<int>::iterator begin, vector<int>::iterator end, int l, int r) {
if (l > r) return RangeCount{0, 0, 0, 0, 0};
if (begin >= end) return RangeCount{0, 0, 0, 0, 0};

if (l == r) {
RangeCount rc;
auto it = std::equal_range(begin, end, r);

rc.upper = distance(it.second, end);
rc.r_equal = distance(it.first, it.second);
rc.between = 0;
rc.l_equal = rc.r_equal;
rc.lower = distance(begin, it.first);

return rc;
}

RangeCount rc;
auto it_r = std::equal_range(begin, end, r);
auto it_l = std::equal_range(begin, end, l);

auto it_ru = it_r.second;
auto it_rl = it_r.first;
rc.upper = distance(it_ru, end);
rc.r_equal = distance(it_rl, it_ru);
auto it_lu = it_l.second;
auto it_ll = it_l.first;
rc.between = distance(it_lu, it_rl);
rc.l_equal = distance(it_ll, it_lu);
rc.lower = distance(begin, it_ll);

return rc;

}

``````

### 使い方

betweenにl_equalとr_equalを足すかどうかで選べます。

``````
// left <= x < rightだったら
RangeCount rc = range_count(left, right);
return  rc.l_equal + rc.between;
``````

rc.l_equal + rc.between + rc.r_equal と書きたくなりますが、
left == rightだったらバグるので、その分岐が必要です。

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