C++ - lesson 3 | HackIndia Skills

C++ - lesson 3 | HackIndia Skills | HackIndia

Example

Expense summary example

Here is a small pattern you can use in a project. It stores spends by category, sorts them, and prints the biggest categories first.

This is useful for a hostel expense tracker, event budget tool, or any project with grouped totals.

#include <algorithm>
#include <iostream>
#include <string>
#include <unordered_map>
#include <vector>
using namespace std;

int main() {
    vector<pair<string, int>> expenses = {
        {"food", 120}, {"travel", 40}, {"food", 80},
        {"printing", 30}, {"travel", 60}
    };

    unordered_map<string, int> total;
    for (const auto& item : expenses) {
        const string& category = item.first;
        int amount = item.second;
        total[category] += amount;
    }

    vector<pair<string, int>> report(total.begin(), total.end());

    sort(report.begin(), report.end(), [](const auto& a, const auto& b) {
        return a.second > b.second;
    });

    for (const auto& row : report) {
        cout << row.first << ": Rs " << row.second << '\n';
    }
}

Tip

Use algorithms first

Before writing a loop, ask if an STL algorithm already does it. This makes your intent clear to a reviewer reading your GitHub repo.

Common picks: sort for ordering, find for search, count_if for filtering count, accumulate for totals, and lower_bound for binary search on sorted data.

#include <algorithm>
#include <numeric>
#include <vector>
using namespace std;

vector<int> marks = {55, 82, 41, 90, 76};

sort(marks.begin(), marks.end());

int passed = count_if(marks.begin(), marks.end(), [](int x) {
    return x >= 50;
});

int total = accumulate(marks.begin(), marks.end(), 0);

Solve faster with STL

Main idea

What STL means

Choose containers

Expense summary example

Use algorithms first

Common mistakes

Do this now

Quick check