C++ Problem Solving Tutorial: Vector Pair Sorting, Quick Pair Creation, and Lambda Expressions for Efficient Problem Solving

Efficient C++ Techniques for Problem Solving: A Deep Dive with Vectors, Pairs, and Modern C++ Practices

In the world of competitive programming and software development, problem-solving is often about making the most of your language’s strengths. C++ offers a powerful array of tools and techniques that, when used effectively, can make code both expressive and efficient. Whether you're tackling algorithm challenges or building real-world applications, knowing how to leverage these tools can make a big difference in your coding journey.

In this article, we'll explore a real-world inspired problem that puts C++'s vectors, pairs, lambda functions, and smart sorting techniques to the test. But we won’t just be solving the problem — we'll dive deep into the best practices and modern C++ idioms that make the solution not only correct but also elegant and efficient. Here’s a quick look at the techniques we’ll be focusing on:

• Vectors and how they simplify dynamic array management and element access
• Pairs and the { } syntax for quick, clean pair creation
• Passing by reference (using &) to avoid unnecessary copying and speed up function calls
• Lambda functions ([]()) for on-the-fly function creation, especially for custom sorting
• The concise range-based for loop (for (auto &element : container)) that makes iterating over collections a breeze

By breaking down this problem using these techniques, you'll see how C++ can handle complex logic with clarity and conciseness, empowering you to write cleaner, faster, and more maintainable code. Let's dive into the problem setup and see how each of these tools comes into play!

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Problem Statement

Tyler Durden is a salesperson assigned the task of traveling to different countries to sell soaps. Each country has S states. Tyler's manager has provided an integer array A of length N, where:

• Array A represents a rating list that shows past sales ratings in each state.
• Tyler needs to follow certain rules to improve the efficiency of his travels.

Rules for Travel

1. Tyler should begin his travel from the state with the lowest rating.
2. When traveling within a country, he must visit all states in that country before moving to a different country.

Your task is to help Tyler plan his travel route and determine:

• The country and rating of the state where Tyler will be during month M based on the rating list.

Additional Notes

• The rating list contains data for all states combined in a sequence. The first S ratings are for Country 1, the next S ratings are for Country 2, and so on.
• Month count starts from 1.
• If multiple countries have the same lowest rating, Tyler should start with the country that has the second-lowest rating as a tie-breaker.

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Input Specifications

1. input1: An integer N, representing the length of the rating list.
2. input2: An integer S, representing the number of states in each country.
3. input3: An integer M, representing the month number.
4. input4: An integer array A, representing the rating list.

Output Specifications

1. Output1: An integer representing the country where Tyler will be in month M.
2. Output2: An integer representing the state rating in that country where Tyler will be in month M.

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Example

Input

• input1: 6
• input2: 3
• input3: 6
• input4: (2, 1, 9, 3, 1, 4)

Output

• Output1: 2
• Output2: 4

Explanation

There are 6 states for Tyler to visit, with each country containing 3 states. Based on the rating list, Tyler's travel route will be organized as follows:

1. Country Order: Since both Country 1 and Country 2 have the same lowest rating, Tyler will start with Country 1 based on the second-lowest rating.

2. Travel Route:

   • Country 1: States in order of rating: [1, 2, 9]
   • Country 2: States in order of rating: [1, 3, 4]

Following this travel route, by the 6th month, Tyler will be in:

• Country 2, with the state rating of 4.

Thus, the outputs are:

• Output1: 2
• Output2: 4

Sure, here's the revised, simplified guide that removes const and refines the comparison logic without the more advanced syntax to keep things approachable.

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Step-by-Step Solution for Tyler's Travel Problem in C++

Let’s break down the solution into manageable steps. We’ll gradually walk through how to structure and sort the data, flatten it into a travel route, and find Tyler's exact location in month M.

Problem Breakdown

Given:

• An integer list A of ratings for states in multiple countries.
• N total ratings, where each country has S states.
• Tyler's task is to start with the country having the lowest rating and visit all states within each country before moving to the next. In case two countries have the same lowest rating, Tyler should prioritize based on all ratings until one country is found to have a smaller rating.

Our goal is to find the country and state rating Tyler will be visiting in month M.

Approach

1. Group states by country and store each country's states for sorting.
2. Sort the countries based on their lowest ratings. In case two countries have the same lowest ratings, compare all ratings sequentially to break the tie.
3. Flatten the sorted list of states to create a travel route.
4. Find the target country and state for the M-th month.

Step 1: Organize States by Country

Each group of S elements in A represents one country. Let’s start by grouping states by country and sorting each group individually.

Code

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

Now, let’s define a function that will group states by country and sort each group based on its ratings.

pair<int, int> findCountryAndState(int N, int S, int M, vector<int> &A) {
    int numCountries = N / S;
    vector<pair<int, vector<int>>> countries;

    // Group states by country and sort each country's states
    for (int i = 0; i < numCountries; ++i) {
        vector<int> states(A.begin() + i * S, A.begin() + (i + 1) * S); // Extract states for this country
        sort(states.begin(), states.end()); // Sort states within the country
        countries.push_back({i + 1, states}); // Store with country number
    }

Explanation:

• numCountries is calculated by dividing N by S.
• Each country’s states are extracted using slicing (A.begin() + i * S to A.begin() + (i + 1) * S) and stored with a country index.
• Sorting within each country allows for easy comparison of ratings.

Step 2: Sort Countries by Ratings

We need to sort the countries based on their lowest rating. If two countries have the same lowest rating, we’ll compare their states sequentially until a difference is found.

    // Sort countries by lowest and subsequent ratings if needed
    sort(countries.begin(), countries.end(), [](pair<int, vector<int>> &a, pair<int, vector<int>> &b) {
        if (a.second[0] != b.second[0]) 
            return a.second[0] < b.second[0];

        // Traverse both vectors to determine which country should come first in case of ties
        for (int i = 1; i < a.second.size(); i++) {
            if (a.second[i] != b.second[i])
                return a.second[i] < b.second[i];
        }
        return false;
    });

Explanation:

• We start by comparing the lowest rating (a.second[0] < b.second[0]).
• If the lowest ratings are identical, we iterate through each rating until we find a difference. This ensures that the country with the "smaller" set of ratings appears first.

Step 3: Flatten the Travel Route

After sorting, we can flatten the travel route by appending each state’s rating to a travelRoute vector, which represents Tyler's travel order.

    vector<pair<int, int>> travelRoute;
    for (int i = 0; i < countries.size(); ++i) {
        for (int j = 0; j < countries[i].second.size(); ++j) {
            travelRoute.push_back({countries[i].first, countries[i].second[j]}); // Add each state with its country number
        }
    }

Explanation:

• We use two nested loops here: the outer loop iterates through sorted countries, while the inner loop iterates through each country's states.
• Each state, along with its country, is appended to travelRoute, creating the entire sequence of Tyler’s travel.

Step 4: Determine the State in Month M

Since month M corresponds to the M-th index in travelRoute, we can directly retrieve it.

    int targetIndex = M - 1; // Adjust M from 1-based to 0-based index
    int targetCountry = travelRoute[targetIndex].first;
    int targetState = travelRoute[targetIndex].second;

    return {targetCountry, targetState}; // Return country and state rating
}

Explanation:

• targetIndex is calculated by subtracting 1 from M (since C++ arrays are 0-based).
• We retrieve the country and state rating from travelRoute at targetIndex.

Complete Code with main Function

Finally, here’s the complete program with a main function to test our solution:

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

pair<int, int> findCountryAndState(int N, int S, int M, vector<int> &A) {
    int numCountries = N / S;
    vector<pair<int, vector<int>>> countries;

    // Step 1: Group states by country and sort each country's states
    for (int i = 0; i < numCountries; ++i) {
        vector<int> states(A.begin() + i * S, A.begin() + (i + 1) * S);
        sort(states.begin(), states.end());
        countries.push_back({i + 1, states});
    }

    // Step 2: Sort countries by lowest and subsequent ratings
    sort(countries.begin(), countries.end(), [](pair<int, vector<int>> &a, pair<int, vector<int>> &b) {
        if (a.second[0] != b.second[0]) 
            return a.second[0] < b.second[0];
        for (int i = 1; i < a.second.size(); i++) {
            if (a.second[i] != b.second[i])
                return a.second[i] < b.second[i];
        }
        return false;
    });

    // Step 3: Flatten travel route by countries and states
    vector<pair<int, int>> travelRoute;
    for (int i = 0; i < countries.size(); ++i) {
        for (int j = 0; j < countries[i].second.size(); ++j) {
            travelRoute.push_back({countries[i].first, countries[i].second[j]});
        }
    }

    // Step 4: Find the M-th month result
    int targetIndex = M - 1;
    int targetCountry = travelRoute[targetIndex].first;
    int targetState = travelRoute[targetIndex].second;

    return {targetCountry, targetState};
}

int main() {
    int N = 6;
    int S = 3;
    int M = 6;
    vector<int> A = {2, 1, 9, 3, 1, 4};

    pair<int, int> result = findCountryAndState(N, S, M, A);
    cout << "Output1: " << result.first << endl; // Country
    cout << "Output2: " << result.second << endl; // State Rating

    return 0;
}

Explanation of main

We test with:

• N = 6, S = 3, M = 6, and ratings {2, 1, 9, 3, 1, 4}.
• Tyler ends up in Country 2, in the state with a rating of 4, after 6 months.

Summary

This solution leverages basic vector manipulation, lambda functions for custom sorting, and logical data structuring to efficiently solve the problem. By breaking down each step, you can understand how the code manages data and applies custom sorting logic for a real-world style problem.