first_occurrence.cpp

#include <iostream>
#include <vector>

using namespace std;


int first_occurrence(double x, const vector<double>& v){
    int esq = 0;
    int dre = v.size()-1;
    while (esq < dre){
        if (v[esq] > x or x > v[dre]) return -1;
        if (v[esq] == x) return esq;
        if (v[dre] == x) return dre;
        ++esq;
        --dre;
    }
    return -1;
}

int main(){
    vector<double> v(8);
    for (int i=0; i<8; ++i) v[i] = i;
    double x;
    cin >> x;
    cout << first_occurrence(x,v) << endl;
    return 0;
}