c++ - weird things using std::vector -

this question has answer here:

• binary_search in c++ unexpected behaviour 4 answers

i wrote simple code insert 2,4,8,16,32,3,9,27,5,6,7 vector object. after insert these numbers, check std::binary_search 8, weirdly returns 0.

here code. not know why. me? lot!

#include <iostream> #include <math.h> #include <vector> #include <algorithm>  using namespace std;  void printvector(vector<int>const & p) {     (int = 0; < p.size(); i++)          cout << p[i] << ' ';     cout << endl; }         int main() {     const int max = 100;     int num;     vector<int> base;      (int = 2; <= 7; i++) {         int expo = log(max) / log(i);         num = 1;         (int iexp = 1; iexp < expo; iexp++) {             num *= i;             if (!binary_search(base.begin(), base.end(), num)) { // if number not in vector                 base.push_back(num);    // insert number                  printvector(base);      // reprint vector                 cout << endl;             }                }            }            cout << binary_search(base.begin(), base.end(), 8) << endl;     printvector(base);      return 0; }   

the sequence must sorted std::binary_search. behavior undefined if sequence not sorted.

you can use std::sort sort first, or, depending on kind of performance need, can use std::find linear search.