Lære Introduction to Dictionaries | Dictionaries and Key-Value Collections

A dictionary in C# is a collection designed to store data as pairs of keys and values. Each key in a dictionary must be unique, meaning you cannot have two entries with the same key. This makes dictionaries ideal for situations where you need to quickly look up a value using a unique identifier, such as a user's ID or a product code.

In C#, a dictionary is represented by the generic type Dictionary<TKey, TValue>, where TKey is the type of the key and TValue is the type of the value. You must specify both types when creating a dictionary. The basic syntax to declare and initialize a dictionary is:

Dictionary<string, int> ages = new Dictionary<string, int>();

Here, string is the type of the key (for example, a person's name), and int is the type of the value (for example, their age). Remember, keys must be unique within the dictionary, but values can be duplicated.

Program.cs


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using System;
using System.Collections.Generic;

namespace ConsoleApp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            // Create a dictionary with country codes as keys and country names as values
            Dictionary<string, string> countryCodes = new Dictionary<string, string>();

            // Add entries to the dictionary
            countryCodes.Add("US", "United States");
            countryCodes.Add("CA", "Canada");
            countryCodes.Add("MX", "Mexico");

            // Retrieve and display a value using its key
            string code = "CA";
            if (countryCodes.ContainsKey(code))
            {
                Console.WriteLine("The country with code " + code + " is " + countryCodes[code]);
            }
            else
            {
                Console.WriteLine("Country code not found.");
            }
        }
    }
}

To work effectively with dictionaries, you need to know how to add, access, and update entries. When you want to add a new key-value pair, use the Add method, specifying both the key and the value. Accessing a value is straightforward: use the key inside square brackets, like dictionary[key]. If the key exists, you get the corresponding value; if not, you get an error—so it is a good idea to check if the key exists first with ContainsKey.

If you want to update the value for an existing key, simply assign a new value using the key: dictionary[key] = newValue;. This replaces the old value for that key. If the key does not exist, this syntax will add a new entry.

Program.cs


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using System;
using System.Collections.Generic;

namespace ConsoleApp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            Dictionary<string, string> countryCodes = new Dictionary<string, string>();
            countryCodes.Add("US", "United States");
            countryCodes.Add("CA", "Canada");
            countryCodes.Add("MX", "Mexico");

            // Check if a key exists before accessing or removing
            string keyToCheck = "MX";
            if (countryCodes.ContainsKey(keyToCheck))
            {
                Console.WriteLine("Found: " + keyToCheck + " - " + countryCodes[keyToCheck]);
            }

            // Remove a key-value pair
            countryCodes.Remove("MX");

            // Try to access the removed key
            if (!countryCodes.ContainsKey("MX"))
            {
                Console.WriteLine("Key 'MX' has been removed from the dictionary.");
            }
        }
    }
}

1. What is the main feature of a dictionary compared to a list?

2. Which method checks if a key exists in a dictionary?

3. Fill in the blanks to add a new key-value pair to a `Dictionary<string, int>` named `ages` with key "Alice" and value `30`:

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Sektion 2. Kapitel 1

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Suggested prompts:

Can you show me an example of adding and accessing values in a C# dictionary?

How do I check if a key exists before accessing its value?

What happens if I try to add a duplicate key to a dictionary?

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Dictionary<string, int> ages = new Dictionary<string, int>();

Program.cs


              12345678910111213141516171819202122232425262728293031
            
using System;
using System.Collections.Generic;

namespace ConsoleApp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            // Create a dictionary with country codes as keys and country names as values
            Dictionary<string, string> countryCodes = new Dictionary<string, string>();

            // Add entries to the dictionary
            countryCodes.Add("US", "United States");
            countryCodes.Add("CA", "Canada");
            countryCodes.Add("MX", "Mexico");

            // Retrieve and display a value using its key
            string code = "CA";
            if (countryCodes.ContainsKey(code))
            {
                Console.WriteLine("The country with code " + code + " is " + countryCodes[code]);
            }
            else
            {
                Console.WriteLine("Country code not found.");
            }
        }
    }
}

Program.cs


              123456789101112131415161718192021222324252627282930313233
            
using System;
using System.Collections.Generic;

namespace ConsoleApp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            Dictionary<string, string> countryCodes = new Dictionary<string, string>();
            countryCodes.Add("US", "United States");
            countryCodes.Add("CA", "Canada");
            countryCodes.Add("MX", "Mexico");

            // Check if a key exists before accessing or removing
            string keyToCheck = "MX";
            if (countryCodes.ContainsKey(keyToCheck))
            {
                Console.WriteLine("Found: " + keyToCheck + " - " + countryCodes[keyToCheck]);
            }

            // Remove a key-value pair
            countryCodes.Remove("MX");

            // Try to access the removed key
            if (!countryCodes.ContainsKey("MX"))
            {
                Console.WriteLine("Key 'MX' has been removed from the dictionary.");
            }
        }
    }
}

1. What is the main feature of a dictionary compared to a list?

2. Which method checks if a key exists in a dictionary?

3. Fill in the blanks to add a new key-value pair to a `Dictionary<string, int>` named `ages` with key "Alice" and value `30`:

Var alt klart?

Tak for dine kommentarer!

Sektion 2. Kapitel 1

Introduction to Dictionaries

1. What is the main feature of a dictionary compared to a list?

2. Which method checks if a key exists in a dictionary?

3. Fill in the blanks to add a new key-value pair to a Dictionary<string, int> named ages with key "Alice" and value 30:

Introduction to Dictionaries

1. What is the main feature of a dictionary compared to a list?

2. Which method checks if a key exists in a dictionary?

3. Fill in the blanks to add a new key-value pair to a Dictionary<string, int> named ages with key "Alice" and value 30:

3. Fill in the blanks to add a new key-value pair to a `Dictionary<string, int>` named `ages` with key "Alice" and value `30`:

3. Fill in the blanks to add a new key-value pair to a `Dictionary<string, int>` named `ages` with key "Alice" and value `30`: