Tracking orders – C++ Exercise with Solution

Move orders through a series of states.

What to build

A small state machine using a map and an enum class.

Requirements

The states are New, Paid and Shipped.
add ID creates an order as New, or prints exists.
pay ID moves it from New to Paid. ship ID moves it from Paid to Shipped.
A command on a missing order prints missing. A move not allowed from the current state prints not allowed.
report prints each order in increasing ID order, as ID state, with the state in lower case.

Input

The first line has n. Each of the next n lines is a command.

Output

Only what the commands above print.

Constraints

n is from 1 to 200.

Example

Input:

4
add 2
add 1
pay 2
report

Output:

1 new
2 paid

Why it works

Naming every state makes the rules read like the description, and the map keeps the orders sorted for the report.

Examples

Example 1 · Input

4
add 2
add 1
pay 2
report

Expected output

1 new
2 paid

Example 2 · Input

2
pay 9
add 1

Expected output

missing

Hints

Hint 1

Use map<int, Status>, with an enum class for the status.

Hint 2

Use count to check whether an order exists.

Hint 3

Only a New order can be paid, and only a Paid order can be shipped.

Solution

Show one correct solution (try it yourself first)

#include <iostream>
#include <map>
#include <string>
using namespace std;

enum class Status { New, Paid, Shipped };

string text_of(Status s) {
    if (s == Status::New) return "new";
    if (s == Status::Paid) return "paid";
    return "shipped";
}

int main() {
    int n;
    cin >> n;
    map<int, Status> orders;
    for (int i = 0; i < n; i++) {
        string command;
        cin >> command;
        if (command == "report") {
            for (const auto& order : orders) {
                cout << order.first << " " << text_of(order.second) << endl;
            }
            continue;
        }
        int id;
        cin >> id;
        if (command == "add") {
            if (orders.count(id)) {
                cout << "exists" << endl;
            } else {
                orders[id] = Status::New;
            }
        } else if (!orders.count(id)) {
            cout << "missing" << endl;
        } else if (command == "pay" && orders[id] == Status::New) {
            orders[id] = Status::Paid;
        } else if (command == "ship" && orders[id] == Status::Paid) {
            orders[id] = Status::Shipped;
        } else {
            cout << "not allowed" << endl;
        }
    }
    return 0;
}

Lesson: Modern C++ →