Enumerating Dictionary Keys

In this article, we’ll explore how to enumerate dictionary keys while working with a collection of collections. To make it engaging, let's take a new example where we manage a collection of universities categorized by countries. Each country is a key in a dictionary, and the value is a list of universities in that country.

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Key Concepts

1. Dictionary Keys Enumeration:

   • A dictionary allows us to store data as key-value pairs. The Keys property is used to enumerate all the keys in the dictionary.

2. Checking Key Existence:

   • The ContainsKey method ensures that we don’t access a key that doesn’t exist in the dictionary.

3. Accessing and Processing Values:

   • The value corresponding to a key is accessed using square bracket syntax, which in this example, is a list of universities.

4. LINQ for Filtering Results:

   • LINQ provides methods like Take to process collections effectively.

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Full Code Example

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main()
    {
        // Dictionary of countries and their respective universities
        var universities = new Dictionary<string, List<University>>
        {
            {
                "USA",
                new List<University>
                {
                    new University("MIT", "Massachusetts"),
                    new University("Stanford University", "California"),
                    new University("Harvard University", "Massachusetts"),
                    // Add more universities if needed
                }
            },
            {
                "UK",
                new List<University>
                {
                    new University("University of Oxford", "Oxford"),
                    new University("University of Cambridge", "Cambridge"),
                    new University("Imperial College London", "London"),
                    // Add more universities if needed
                }
            }
        };

        // Display available countries
        Console.WriteLine("Available countries:");
        foreach (var country in universities.Keys)
        {
            Console.WriteLine(country);
        }

        // Ask the user to select a country
        Console.WriteLine("\nEnter a country:");
        string selectedCountry = Console.ReadLine();

        // Check if the selected country exists in the dictionary
        if (universities.ContainsKey(selectedCountry))
        {
            Console.WriteLine($"\nTop universities in {selectedCountry}:");

            // Retrieve and display universities in the selected country
            foreach (var university in universities[selectedCountry].Take(5))
            {
                Console.WriteLine($"- {university.Name}, located in {university.Location}");
            }
        }
        else
        {
            Console.WriteLine("Country not found.");
        }
    }
}

// University class definition
class University
{
    public string Name { get; set; }
    public string Location { get; set; }

    public University(string name, string location)
    {
        Name = name;
        Location = location;
    }
}

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Explanation of the Code

1. Defining the Dictionary:

   • A dictionary named universities maps country names (string) to a list of University objects.

2. Enumerating Keys:

   • The Keys property is used to list all available countries. This makes it easy for the user to select a country.

3. User Input Validation:

   • The ContainsKey method ensures that the program doesn’t crash when an invalid country is entered.

4. Displaying Results:

   • For the selected country, the top universities (up to 5) are displayed using the Take method from LINQ.

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Sample Output

Input Scenario 1: Valid Country

Available countries:
USA
UK

Enter a country:
USA

Top universities in USA:
- MIT, located in Massachusetts
- Stanford University, located in California
- Harvard University, located in Massachusetts

Input Scenario 2: Invalid Country

Available countries:
USA
UK

Enter a country:
Canada
Country not found.

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Key Takeaways

• Dictionary Keys Enumeration: Use the Keys property to access all keys in a dictionary.
• Key Validation: Always validate user input with ContainsKey.
• LINQ for Filtering: Methods like Take make it easy to limit and process collections.