Once we’ve computed the cluster index, we can use it to subscript our vector and fetch the counter we want to increment

// count the number of grades by clusters of ten: 0--9, 10--19, . .. 90--99, 100
vector<unsigned> scores(11, 0); // 11 buckets, all initially 0
unsigned grade;
while (cin >> grade) { // read the grades
if (grade <= 100) // handle only valid grades
++scores[grade/10]; // increment the counter for the current cluster
}