Summary - Collections

C# course · Beginner · Lesson 70/116

Combine List<T> and Dictionary<K,V> to write a phone book.

Summary - Collections

In this section you learned the List<T>, Dictionary<K,V> and HashSet<T> collections. They share the Add and Count members; they differ in item management. List is ordered, dictionary is keyed, set is unique.

Now we will combine them into a small phone book. We will use List for names, Dictionary for numbers and TryGetValue for safe lookup.

Exercise Add two people to the phone book

Declare Dictionary<string, string> phone = new Dictionary<string, string>();. Use Add to put "555-1000" under the key "Ayşe" and "555-2000" under "Mehmet". Then print phone.Count with Console.WriteLine.

Show example solution

Call phone.Add("Ayşe", "555-1000"); and phone.Add("Mehmet", "555-2000");, then print with Console.WriteLine(phone.Count);.
Example solution:
using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        // Declare phone dictionary, add two people and print the count
        Dictionary<string, string> phone = new Dictionary<string, string>();
        phone.Add("Ayşe", "555-1000");
        phone.Add("Mehmet", "555-2000");
        Console.WriteLine(phone.Count);
    }
}

Exercise Hold names with a list

Declare List<string> names = new List<string>();. Add the values "Ayşe" and "Mehmet" with Add. Then print names.Count with Console.WriteLine.

Show example solution

Declare with List<string> names = new List<string>();, call names.Add("..."); twice, then print with Console.WriteLine(names.Count);.
Example solution:
using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        // Declare names list, add two names and print the count
        List<string> names = new List<string>();
        names.Add("Ayşe");
        names.Add("Mehmet");
        Console.WriteLine(names.Count);
    }
}

Test yourself

What does phone.Count print after one Add?

Dictionary<string, string> phone = new Dictionary<string, string>();
phone.Add("Ayşe", "555-1000");
Console.WriteLine(phone.Count);

0
1
2
555-1000

Show answer

1 — Correct. One pair was added; Count returns 1.

TryGetValue call

Dictionary<string, string> phone = new Dictionary<string, string>();
phone.Add("Ayşe", "555-1000");
string num;
bool ok = ____;
Console.WriteLine(ok);

Show answer

Dictionary<string, string> phone = new Dictionary<string, string>();
phone.Add("Ayşe", "555-1000");
string num;
bool ok = phone.TryGetValue("Ayşe", out num);
Console.WriteLine(ok);

Match the collections

List<T>

Ordered, dynamic
Dictionary<K,V>

Key-value
HashSet<T>

Unique
TryGetValue

Safe access

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Summary - Collections