Seek item within vector - c++

Question

I have a vector of real values, sorted by increasing order.
These values can be within the range [0, 1]. Then I pick a value x within this range and I need to find which is the index of the smaller value greater or equal to x.
I can solve this problem by iterating over the whole array:
vector<double> values;
double x;
for (auto val : values)
{
if (x <= values)
{
// found
break;
}
}
Is there a faster way to get the same result? I was thinking about a binary search, but how to implement it?

Answer 1

Use std::lower_bound:
#include <iterator>
#include <algorithm>
std::distance(begin(values)
, std::lower_bound(begin(values), end(values), x));
If the item doesn't exist, it'll give you an index one greater than that of the last element.
DEMO

Answer 2

The function lower_bound might meet your demand,and you can use it like below:
iter =lower_bound(values.begin(),values.end(),x);

Answer 3

You can use operator[] to directly access the items in a vector just like an array, instead of using an iterator to start from the begin. I assume you already know Binary Search. Implement it in an array is something you can find anywhere, so I won't explain it to you here. Just treat the vector as an array.

Answer 4

You know SO is not a site that you request someone to write you the code, so take this example of std::binary_search and make your way:
// binary_search example
#include <iostream> // std::cout
#include <algorithm> // std::binary_search, std::sort
#include <vector> // std::vector
bool myfunction (int i,int j) { return (i<j); }
int main () {
int myints[] = {1,2,3,4,5,4,3,2,1};
std::vector<int> v(myints,myints+9); // 1 2 3 4 5 4 3 2 1
// using default comparison:
std::sort (v.begin(), v.end());
std::cout << "looking for a 3... ";
if (std::binary_search (v.begin(), v.end(), 3))
std::cout << "found!\n"; else std::cout << "not found.\n";
// using myfunction as comp:
std::sort (v.begin(), v.end(), myfunction);
std::cout << "looking for a 6... ";
if (std::binary_search (v.begin(), v.end(), 6, myfunction))
std::cout << "found!\n"; else std::cout << "not found.\n";
return 0;
}
As Piotr said, this will not give you the index, but a yes/no answer. However, is should be the simplest approach, thus the fastest.