How do i apply lower bound to a range of unsorted vector elements

Question

I have a vector of integer scores = 10, 23, 29, 77, 8, 43, 56, 3, a number predict = 42, and a start index = 2 as well as a number predict = 42.

I'm looking for two values that are (greater than and less than) 42 in the range of start index = 2 and finish index = 7.

scores = 10, 23, 29, 77, 8, 43, 56, 3 (search only inside i=2 to i=7)

As a result, 29 is little less than predict=42.

Also, 43 is a touch higher than the forecasted 42.

How did I come up with these figures?

Sample Code:

#include <algorithm>
#include <iostream>
#include <vector>

using namespace std;

int main() {
    vector<int> scores = {10, 23, 29, 77, 8, 43, 56, 3}; //Unsorted
    int predict = 42;
    int startFromIndex = 2;
    
    int littleLessThanPredict; // = 29
    int littleMoreThanPredict; // = 43
    
    //lower_bound
    //upper_bound
    
    return 0;
}

In consideration of this scenario,

On an unsorted range, how do I utilise the std::lower bound and std::upper bound functions?

How can I temporarily sort a portion of a vector and apply my own function?

Is there a method to get the desired result by combining the std::min element and std::lower bound functions?

Answer 1

What's the point of sorting your array? The complexity of the algorithm will be O(N.ln(N)). You can create an O(N) method by combining upper and lower bounds research in an unsorted array:

#include <algorithm>
#include <iostream>
#include <vector>
#include <climits>

using namespace std;

pair<int, int> nearestNumbers(const vector<int> &scores, int predict, int start) {
    int nearestLower = INT_MIN;
    int nearestUpper = INT_MAX;

    for (size_t i = start; i < scores.size(); i++) {
        int value = scores[i];
        if (value <= predict && value > nearestLower)
            nearestLower = value;
        else if (value >= predict && value < nearestUpper)
            nearestUpper = value;
    }

    return pair<int, int>(nearestLower, nearestUpper);
}

int main() {
    vector<int> scores = {10, 23, 29, 77, 8, 43, 56, 3};
    int predict = 42;
    int start = 2;

    pair<int, int> results = nearestNumbers(scores, predict, start);
    cout << results.first << " " << results.second;

    return 0;
}