How to get the index of an element into a vector most efficiently?

[abdalimran]

I found binary_search() in STL which will return true if an element equivalent to val is found, and false otherwise. But it doesn't return index. As I knew binary search is one of the most efficient ways to find a number into an array/vector. But the STL binary search function is not giving me the index.

I found a code like following:

vector<int>::iterator indx=lower_bound(v.begin(),v.end(), num);

int pos=indx-v.begin();
if(indx == v.end() || *indx!=num)
    cout<<num<<" not found";
else
    cout<<num<<" found at "<<pos+1;

How much is the code efficient?

Is there any better way to find out the index which will take lest running time?

[MiiNiPaa]

Do you really need index? Why not use iterators?
Also in your code pos != element index. It is off by one.

Your code is mostly fine. Here is example of how you could use iterator/index to change found value:

auto it = std::lower_bound(v.begin(), v.end(), num);
if(it == v.end() || *it != num) {
    std::cout << num << " not found\n"
} else {
    std::size_t ind = std::distance(v.begin(), it)
    std::cout << num << " was found at " << ind << '\n';
    std::cout << "Replacing with 0";
    *it = 0; // v[ind] = 0
}