1. Multi-Dictionaries, Nested Dictionaries, Sets
Advanced Dictionaries
SoftUni Team
Technical Trainers
Software University

2. Table of Contents Multi-Dictionaries Nested Dictionaries Sets
A Dictionary Holding List of Values
Nested Dictionaries
A Dictionary Holding Another Dictionary
Sets
HashSet<T>
SortedSet<T>

3. sli.do #extended-softuni
Questions?
sli.do #extended-softuni

4. Dictionaries Holding a List of Values
Multi-Dictionaries
Dictionaries Holding a List of Values
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5. Value  List<int>
Multi-Dictionaries
A dictionary could hold a set of values by given key
Example: students may have multiple grades:
Peter  [5, 5, 6]
Kiril  [6, 6, 3, 4, 6]
Dictionary<string, List<int>> grades;
Key  String
Value  List<int>

6. Example: Student Grades
Each student has a list of grades
We can access the student’s grades by name:
Value  List<int>
var grades = new Dictionary<string, List<int>>();
grades["Peter"] = new List<int>();
grades["Peter"].Add(5);
grades["Peter"].Add(6);
var kirilGrades = new List<int>() { 6, 6, 3, 4, 6 };
grades["Kiril"] = kirilGrades;
Console.WriteLine(string.Join(" ", grades["Kiril"]); //
Initialize the list
Accessing list by key (Peter)
Assigning list to key

7. Problem: Average Student Grades
Write a program to read student names + grades
Print the grades + average grade for each student as shown below 7
Ivancho 5.20
Mariika 5.50
Ivancho 3.20
Mariika 2.50
Stamat 2.00
Mariika 3.46
Stamat 3.00
Ivancho -> (avg: 4.20)
Mariika -> (avg: 3.82)
Stamat -> (avg: 2.50)

8. Solution: Average Student Grades (1)
var grades = new Dictionary<string, List<double>>();
var n = int.Parse(Console.ReadLine());
for (int i = 0; i < n; i++) {
var tokens = Console.ReadLine().Split();
var name = tokens[0];
var grade = double.Parse(tokens[1]);
if (!grades.ContainsKey(name))
grades[name] = new List<double>();
grades[name].Add(grade); }
// continued on next slide ...
Make sure the list is initialized
Add grade into list

9. Solution: Average Student Grades (2)
KeyValuePair<string, List<double>
foreach (var pair in grades) {
var name = pair.Key;
var studentGrades = pair.Value;
var average = studentGrades.Average();
Console.Write($"{name} -> ");
foreach (var grade in studentGrades)
Console.Write($"{grade:f2} ");
Console.WriteLine($"(avg: {average:f2})"); }
Key: string
Value: List<string>
Average value of the list

10. Dictionary Holding Dictionary Inside
BG 
UK 
USA 
Sofia  1,211,000
Plovdiv  338,657
London  8,674,000
Manchester  2,550,000
New York City, NY  8,406,000
Washington, DC  658,893
Nested Dictionaries
Dictionary Holding Dictionary Inside
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11. Value: Dictionary <string, int>
Nested Dictionaries
Dictionaries may hold another Dictionary<K,V> as value
Example: population by country and city
Value: Dictionary <string, int>
BG 
UK 
USA 
Sofia  1,211,000
Plovdiv  338,657
London  8,674,000
Manchester  2,550,000
New York City, NY  8,406,000
Washington, DC  658,893
Key: string

12. Problem: Cities by Continent and Country
Write a program to read continents, countries and their cities, put them in a nested dictionary and print them 9
Europe Bulgaria Sofia
Asia China Beijing
Asia Japan Tokyo
Europe Poland Warsaw
Europe Germany Berlin
Europe Poland Poznan
Europe Bulgaria Plovdiv
Africa Nigeria Abuja
Asia China Shanghai
Europe:
Bulgaria -> Sofia, Plovdiv
Poland -> Warsaw, Poznan
Germany -> Berlin
Asia:
China -> Beijing, Shanghai
Africa:
Nigeria -> Abuja

13. Solution: Cities by Continent and Country (1)
var continentsData = new Dictionary<string, Dictionary<string, List<string>>>();
var n = int.Parse(Console.ReadLine());
for (int i = 0; i < n; i++) {
var tokens = Console.ReadLine().Split();
var continent = tokens[0];
var country = tokens[1];
var city = tokens[2];
// continued on next slide
Initialize the dictionary

14. Solution: Cities by Continent and Country (2)
Continent doesn’t exist  create it
if (!continentsData.ContainsKey(continent))
continentsData[continent] =
new Dictionary<string, List<string>>();
if (!continentsData[continent].ContainsKey(country))
continentsData[continent][country] = new List<string>();
continentsData[continent][country].Add(city); }
// continued on next slide...
Country doesn’t exist
Initialize cities
Append a city to the country

15. Solution: Cities by Continent and Country (3)
foreach (var continentCountries in continentsData) {
var continentName = continentCountries.Key;
var countries = continentCountries.Value;
Console.WriteLine($"{continentName}:");
foreach (var countryCities in countries)
var countryName = countryCities.Key;
var cities = countryCities.Value;
Console.WriteLine(" {0} -> {1}",
countryName, string.Join(", ", cities)); }
Continent name
Countries in the continent
Country name
Cities in the country

16. Multi and Nested Dictionaries
Exercises in Class
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17. HashSet<T> and SortedSet<T>
Sets
HashSet<T> and SortedSet<T>

18. Sets in C# A set keeps unique elements HashSet<T>
Allows add / remove / search elements
Very fast performance
HashSet<T>
Keeps a set of elements in a hash-table
Elements are in no particular order
Similar to List<T>, but a different implementation
SortedSet<T>
The elements are ordered incrementally

19. List<T> vs HashSet<T>
Fast "add", slow "search" and "remove" (pass through each element)
Duplicates are allowed
Insertion order is guaranteed
HashSet<T>
Fast "add", "search" and "remove" (hash-table behind)
Does not allow duplicates
Does not guarantee the insertion order

20. HashSet<T> – Example
HashSet<string> set = new HashSet<string>();
set.Add("Pesho");
set.Add("Pesho"); // Not added again
set.Add("Gosho");
set.Add("Alice");
Console.WriteLine(string.Join(", ", set));
// Pesho, Gosho, Alice
Console.WriteLine(set.Contains("Georgi")); // false
Console.WriteLine(set.Contains("Pesho")); // true
set.Remove("Pesho");
Console.WriteLine(set.Count); // 2

21. Problem: Record Unique Names
Read a sequence of names and print only the unique ones 8
Ivan
Pesho
Stamat
Alice
Peter 7
Lyle
Bruce
Alice
Easton
Shawn
Peter 6
Roki
Roki Roki

22. Solution: Record Unique Names
HashSet stores unique values
var names = new HashSet<string>();
var n = int.Parse(Console.ReadLine());
for (int i = 0; i < n; i++) {
var name = Console.ReadLine();
names.Add(name); }
foreach (var name in names)
Console.WriteLine(name);
Add non-existing names only

23. SortedSet<T> – Example
SortedSet<string> set = new SortedSet<string>();
set.Add("Pesho");
set.Add("Gosho");
set.Add("Maria");
set.Add("Alice");
Console.WriteLine(string.Join(", ", set));
// Alice, Gosho, Maria, Pesho
Ordered alphabeticaly

24. Problem: Group Continents, Countries and Cities
Write a program to read continents, countries and city names
Keep them in alphabetical order, no duplicates allowed
For each continent print all its countries with their towns 9
Europe Bulgaria Sofia
Asia China Beijing
Europe Poland Warsaw
Europe Germany Berlin
Europe Poland Poznan
Asia China Shanghai
Asia:
China -> Beijing, Shanghai
Europe:
Bulgaria -> Sofia
Germany -> Berlin
Poland -> Poznan, Warsaw

25. Solution: Cities by Continent and Country (1)
var continentsData = new SortedDictionary<string, SortedDictionary<string, SortedSet<string>>>();
var n = int.Parse(Console.ReadLine());
for (int i = 0; i < n; i++) {
var tokens = Console.ReadLine().Split();
var continent = tokens[0];
var country = tokens[1];
var city = tokens[2];
// continued on next slide
Initialize the sorted dictionary

26. Solution: Cities by Continent and Country (2)
Continent doesn’t exist  create it
if (!continentsData.ContainsKey(continent))
continentsData[continent] =
new SortedDictionary<string, SortedSet<string>>();
if (!continentsData[continent].ContainsKey(country))
continentsData[continent][country] =
new SortedSet<string>();
continentsData[continent][country].Add(city); }
// continued on next slide...
Country doesn’t exist  create it
Add the city into country

27. Solution: Cities by Continent and Country (3)
foreach (var continentCountries in continentsData) {
var continentName = continentCountries.Key;
var countries = continentCountries.Value;
Console.WriteLine($"{continentName}:");
foreach (var countryCities in countries)
var countryName = countryCities.Key;
var cities = countryCities.Value;
Console.WriteLine(" {0} -> {1}",
countryName, string.Join(", ", cities)); }
Continent name
Countries in the continent
Country name
Cities in country

28. Sets Exercises in Class
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29. Summary
Multi-dictionaries allow keeping a collection as a dictionary value
E.g. Dictionary<string, List<int>>
Nested dictionaries allow keeping a dictionary as dictionary value
E.g. Dictionary<string, Dictionary<string, int>>
Sets allow keeping unique values in unspecified order
No duplicates, fast add / search / remove
SortedDictionary<K,V> / SortedSet<T> keep keys sorted

30. Advanced Dictionaries
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31. License
This course (slides, examples, demos, videos, homework, etc.) is licensed under the "Creative Commons Attribution- NonCommercial-ShareAlike 4.0 International" license
Attribution: this work may contain portions from
"Fundamentals of Computer Programming with C#" book by Svetlin Nakov & Co. under CC-BY-SA license
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