int* ptr = lower_bound(a, a + n, k);

returns the pointer to the first element in a that is larger than or equal to k

int* ptr = upper_bound(a, a + n, k);

returns the pointer to the first element in a that is larger than k

You can use the following to get the index. This also means "how many elements in a are smaller than k"

int index = lower_bound(a, a + n, k) - a;

According to cplusplus.com lower_bound(a,a+n,k) returns pointer to the first element, where k can be inserted. upper_bound(a,a+n,k) returns pointer to the last element, where k can be inserted :).