Understanding Dictionaries, Lambda, and LINQ: Associative Arrays, Expressions, and Queries, Study Guides, Projects, Research of Programming Languages

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Dictionaries, Lambda and

LINQ

Collections and Queries

• Associative Arrays
  • Dictionary <key, value>
  • SortedDictionary <key, value>
• Lambda
• LINQ
  • Filtering
  • Mapping
  • Ordering Table of Contents 2

• Associative arrays are arrays indexed by keys
  • Not by the numbers 0, 1, 2, … (like arrays)
• Hold a set of pairs {key → value} Associative Arrays (Maps, Dictionaries) 4 John Smith +1- 555 - 8976 Lisa Smith +1- 555 - 1234 Sam Doe +1- 555 - 5030 Key Value

• Dictionary<K, V> - collection of key and value pairs
• Keys are unique
• Keeps the keys in their order of addition
• Uses a hash-table + list Dictionary var fruits = new SortedDictionary<string, double>(); fruits["kiwi"] = 4.50; fruits["orange"] = 2.50; fruits["banana"] = 2.20;

▪ Add ( key , value ) method ▪ Remove(key ) method Built-In Methods var airplanes = new Dictionary<string, int>(); airplanes.Add("Boeing 737", 130); airplanes.Add("Airbus A320", 150); var airplanes = new Dictionary<string, int>(); airplanes.Add("Boeing 737", 130); airplanes.Remove("Boeing 737");

▪ ContainsKey(key) ▪ ContainsValue(value) Built-In Methods (2) var dictionary = new Dictionary<string, int>(); dictionary.Add("Airbus A320", 150); if (dictionary.ContainsKey("Airbus A320")) Console.WriteLine($"Airbus A320 key exists"); var dictionary = new Dictionary<string, int>(); dictionary.Add("Airbus A320", 150); Console.WriteLine(airplanes.ContainsValue(150)); //true Console.WriteLine(airplanes.ContainsValue(100)); //false

Traditional Dictionary: Add() 10 Dictionary<string, string> Key Value Pesho 0881 - 123 - 987 Gosho 0881 - 123 - 789 Alice 0881 - 123 - (^978) Hash Function

Dictionary: Remove() 11 Dictionary<string, string> Key Value Hash Function Pesho Pesho 0881 - 123 - 987 Gosho 0881 - 123 - 789 Alice 0881 - 123 - 978

• Using foreach loop
• Iterate through objects of type KeyValuePair<K, V>
• Cannot modify the dictionary (read-only) Iterating through Dictionary var fruits = new Dictionary<string, double>(); fruits.Add("banana", 2.20); fruits.Add("kiwi", 4.50); foreach (var fruit in fruits) Console.WriteLine($"{fruit.Key} - > {fruit.Value}"); fruit.Key - > fruit name fruit.Value - > fruit price

• Read 2 * N lines of pairs word and synonym
• Each word may have many synonyms Problem: Word Synonyms 14 3 cute adorable cute charming smart clever cute - adorable, charming smart - clever

LAMBDA EXPRESSIONS

Anonymous Functions

• A lambda expression is an anonymous function containing

expressions and statements

• Lambda expressions
• Use the lambda operator =>
  • Read as "goes to"
• The left side specifies the input parameters
• The right side holds the expression or statement Lambda Functions a => a > 5; 17

• Min() – finds the smallest element in a collection
• Max() – finds the largest element in a collection
• Sum() – finds the sum of all elements in a collection
• Average() – finds the average of all elements in a collection Processing Sequences with LINQ 19 new List() { 1, 2, 3, 4, - 1, - 5, 0, 50 }.Min() //- 5 new int[] { 1, 2, 3, 40, - 1, - 5, 0, 5 }.Max() // 40 new long[] {1, 2, 3, 4, - 1, - 5, 0, 50}.Sum() // 54 new int[] {1, 2, 3, 4, - 1, - 5, 0, 50}.Average() //6.

• Select() manipulates elements in a collection Manipulating Collections 20 string[] words = { "abc", "def" } ; var result = words.Select(w => w + "x"); // words - > abcx, defx var nums = Console.ReadLine() .Split() .Select(int.Parse);