上下限不符合我的预期，无法理解为什么 [英] Upper/lower bounds don't work as I expect, can not understand why

问题描述

这是代码.结果，我得到"4 4".不明白为什么它不是"2 4"(根据上下限的定义).

Here is the code. As a result I get "4 4". Don't understand why it is not "2 4" (according to lower and upper bounds' defenitions).

#include <bits/stdc++.h>
using namespace std;
int main()
{
    vector<int> v = {1, 2, 4, 5};
    vector<int>::iterator s , f;
    s = lower_bound(v.begin(), v.end(), 3);
    f = upper_bound(v.begin(), v.end(), 3);
    cout << (*s) << " " << (*f);
    return 0;
}

推荐答案

来自 std::lower_bound :

返回指向范围内第一个元素的迭代器 [first，last)的总和不小于 val .

Returns an iterator pointing to the first element in the range [first,last) which does not compare less than val.

不小于3的第一个元素(从向量的开头)为4，因此lower_bound返回4.

The first element (from the beginning of the vector) which is not less than 3 is 4 and hence lower_bound returns 4.

来自 std::upper_bound :

返回指向范围 [first，last)中第一个元素的迭代器，该元素的比较值大于 val .

Returns an iterator pointing to the first element in the range [first,last) which compares greater than val.

(从向量的开头开始)大于3的第一个元素是4，因此upper_bound返回4.

The first element (from the beginning of the vector) which is greater than 3 is 4 and hence upper_bound returns 4.

造成这种混乱的原因是，由于upper_bound返回的第一个元素大于给定值，因此，对称起见，我们期望lower_bound返回的最后一个元素(从向量的开头)小于给定值.但可惜，std函数不遵循这种预期的"对称性.

The reason for this confusion is because upper_bound returns the first element that is greater than the given value, so by symmetry we expect lower_bound to return the last element (from the beginning of the vector) that is less than the given value. But alas, the std function doesn't obey this "expected" symmetry.

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