STL Map Set

STL Map & Set

Folder: 05-stl-map-set

What it is

The same roster problem from the previous lab, solved with associative containers. rosterMap.cpp keys a std::map by a custom Student type so each student maps to a list of course names. rosterSet.cpp puts students in a std::set to track who is enrolled. A third file, set.cpp, is a short demo of set and multiset behavior. The lesson is that an ordered associative container needs a way to compare keys, and a single operator< on the key type is enough.

How this code does it

Both roster programs define a Student holding a first and last name. The key line is the friend operator<, which orders by first name and then last name. Because std::map and std::set are ordered, they call that operator to place and find elements, so no separate comparator object is needed.

In rosterMap.cpp the container is map<Student, list<string>>. Reading a course file does roster[student].push_back(courseName), which inserts the student on first sight and appends the course. Dropouts are read into a vector and removed with map::erase. Printing walks the map in sorted order for free.

In rosterSet.cpp the container is set<Student>. Each course file inserts students, duplicates collapse automatically because a set holds one of each key, and dropouts are erased. The set version answers a simpler question (who is enrolled) and shows that the same comparator drives both containers.

set.cpp contrasts set and multiset: it inserts random characters into each, shows that the multiset keeps duplicates, then erases and searches to show count, erase, and find.

Structure

classDiagram
    class Student {
        -string firstName_
        -string lastName_
        +print() string
        +operator<(Student, Student) bool
        +operator==(Student, Student) bool
    }
    class StudentMap["map&lt;Student, list&lt;string&gt;&gt;"] {
        +operator[](Student) list&
        +erase(Student) size_t
    }
    class StudentSet["set&lt;Student&gt;"] {
        +insert(Student) void
        +erase(Student) size_t
        +find(Student) iterator
    }
    StudentMap ..> Student : orders keys with operator<
    StudentSet ..> Student : orders elements with operator<

Loading

Build and run

g++ -std=c++17 05-stl-map-set/rosterMap.cpp -o rosterMap
./rosterMap cs1.txt cs2.txt cs3.txt cs4.txt dropouts.txt

g++ -std=c++17 05-stl-map-set/rosterSet.cpp -o rosterSet
./rosterSet cs1.txt cs2.txt cs3.txt cs4.txt dropouts.txt

Notes

set.cpp is an instructor demo. The two roster programs are my work; solving the same problem with both a map and a set side by side is what makes the container trade-off concrete.