Practice

Each example is a separate complete program: first the problem, then the code and the explanation.

Example 1. Caesar cipher

Read one ASCII line and shift uppercase and lowercase Latin letters by 3. Keep spaces, digits and punctuation. This is a teaching transformation, not cryptographic protection. Cyrillic UTF-8 must be rejected.

#include <string>
#include <iostream>
#include <print>

int main()
{
    std::string text;
    if (!std::getline(std::cin, text)) return 1;
    for (unsigned char c : text) if (c > 127) return 1;
    for (char& c : text)
    {
        if (c >= 'a' && c <= 'z')
            c = static_cast<char>('a' + (c - 'a' + 3) % 26);
        else if (c >= 'A' && c <= 'Z')
            c = static_cast<char>('A' + (c - 'A' + 3) % 26);
    }
    std::println("{}", text);
}

Abc xyz! turns into Def abc!. The end of the alphabet wraps around to the beginning thanks to %26. Separate ranges for uppercase and lowercase letters preserve the case.

Example 2. Seat booking

The hall has 3 rows of 4 seats. Enter row/seat pairs starting from 1, and 0 0 ends the input. Reject a repeated booking.

#include <array>
#include <iostream>
#include <print>

int main()
{
    std::array<std::array<bool, 4>, 3> booked{};
    int row{}, seat{};
    while (std::cin >> row >> seat)
    {
        if (row == 0 && seat == 0) break;
        if (row < 1 || row > 3 || seat < 1 || seat > 4)
        {
            std::println("Invalid seat");
            continue;
        }
        auto& occupied = booked[row - 1][seat - 1];
        if (occupied) std::println("Already booked");
        else occupied = true;
    }
    for (const auto& seats : booked)
    {
        for (bool occupied : seats)
            std::print("{}", occupied ? 'X' : '.');
        std::println();
    }
}

For 1 2 1 2 4 1 0 0 messages appear about a repeated and an invalid seat, and the first row of the map is .X... The conversion from numbering from 1 to indexing from 0 happens only after the check.

Example 3. Shopping list

Commands are read as whole lines: add name, remove name, find name, list, quit. Names may contain spaces but must not be empty. The first match is removed; duplicates are allowed.

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

int main()
{
    std::vector<std::string> items;
    std::string command;
    while (std::getline(std::cin, command) && command != "quit")
    {
        if (command == "list")
        {
            for (const auto& item : items) std::println("{}", item);
            continue;
        }
        const auto space = command.find(' ');
        if (space == command.npos || space + 1 == command.size())
        {
            std::println("Invalid command");
            continue;
        }
        const auto action = command.substr(0, space);
        const auto name = command.substr(space + 1);
        if (action == "add") items.push_back(name);
        else if (action == "find" || action == "remove")
        {
            std::size_t i{};
            while (i < items.size() && items[i] != name) ++i;
            if (i == items.size()) std::println("Not found");
            else if (action == "remove")
                items.erase(items.begin() + i);
            else std::println("Position: {}", i + 1);
        }
        else std::println("Unknown action");
    }
}

After add green tea, find green tea the result is Position: 1. After remove green tea and a repeated search, it is Not found. The lines are stored as owning string objects, so the next getline does not destroy the names in the collection. Replacing them with a string_view of the current command without changing the architecture would be an error.