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.