Finding a Student

Lucy has come to complain: the teacher added up her points wrong. She should have 35, not 27. The program needs a function that fixes a student's score by name. But first, it has to find the student in names, which means finding their index.

Searching is a pattern from the lesson on loops: the loop goes through the elements, and when it finds the right one, it remembers its index, and break ends the loop, because there's no need to search any further. The variable for the index starts at -1. No vector has such an index. If the loop finds nothing, the variable stays -1, which means that what you're looking for isn't in the vector:

#include <iostream>
#include <string>
#include <vector>

int findSubject(const std::vector<std::string>& subjects, std::string wanted) {
    int index = -1;
    for (int i = 0; i < subjects.size(); i++) {
        if (subjects[i] == wanted) {
            index = i;
            break;
        }
    }
    return index;
}

int main() {
    std::vector<std::string> timetable = {"Maths", "Physics", "History"};
    std::cout << findSubject(timetable, "Physics") << std::endl;
    std::cout << findSubject(timetable, "Chemistry") << std::endl;
    return 0;
}

1
-1

You compare strings with ==, and the comparison is exact: "lucy" is not the same as "Lucy".

The function fixScore() gets both vectors. It only reads names, so it gets them as a const reference. It changes scores, so it gets them as a reference without const.

The order of the calls in main() matters. Lucy scored 35 points in the test, and she gets the 3 points for the broken question like everyone else. So the fix goes before the call of addPoints(scores, 3). After it, the fix would overwrite Lucy's bonus, and she'd have just 35 points.

And what if the teacher types a name that isn't in the vector? Max, for example, is in the class next door. findStudent() returns -1, and scores[-1] is outside the vector. You know from the lesson on vectors that an index out of range is a bug: the program can crash, or quietly overwrite memory that doesn't belong to the vector. That's why fixScore() checks the index first.