First occurrence P84219

Write an efficient function

    int first_occurrence(double x, const vector<double>& v);

that returns the position of the first occurrence of @x@ in the vector @v@. If @x@ does not belong to @v@, return a -1.

Precondition

The vector @v@ is sorted in nondecreasing order.

Observation

You only need to submit the required procedure; your main program will be ignored.