Chapter 22. Lambda Expressions and LINQ

Dictionaries, Lambda and LINQ
Collections and Queries
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2
 1. Associative Arrays
 Dictionary <key, value>
 2. Lambda Functions and LINQ
 Filtering, Mapping, Ordering
Table of Contents

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Associative Arrays
Dictionary<Key, Value>
ivan
gosho
pesho
0845-346-356
2350-452-167
1255-377-131

 Associative arrays are arrays indexed by keys
 Not by the numbers 0, 1, 2, … (like traditional arrays)
 Hold a set of pairs {key  value}
Associative Arrays (Maps, Dictionaries)
Associative array
John Smith +1-555-8976
Lisa Smith +1-555-1234
Sam Doe +1-555-5030
key value
Traditional array
0 1 2 3 4
8 -3 12 408 33
key
value
5

Dictionary Example – Phonebook
var phonebook = new Dictionary<string, string>();
phonebook["John Smith"] = "+1-555-8976";
phonebook["Lisa Smith"] = "+1-555-1234";
phonebook["Sam Doe"] = "+1-555-5030";
phonebook["Nakov"] = "+359-899-555-592";
phonebook["Nakov"] = "+359-2-981-9819"; // Replace
phonebook.Remove("John Smith");
foreach (var pair in phonebook)
Console.WriteLine("{0} --> {1}",
pair.Key, pair.Value);
6

7
 Traditional dictionaries
 Uses a hash-table + list
 Dictionary<K, V>
 Keep the keys in their order of addition
 Sorted dictionaries
 Uses a balanced search tree
 SortedDictionary<K, V>
 Keep the keys sorted in their natural order
Dictionary<K, V> vs. SortedDictionary<K, V>
var dict =
new Dictionary<string, int>();
var sortedDict = new
SortedDictionary<int,int>();

 Count – holds the number of key-value pairs
 Keys – a set of unique keys
 Values – a collection of all values
 Basic operations: Add() / indexer [], Remove(), Clear()
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var dict = new Dictionary<string, int>();
foreach(var key in dict.Keys)
Console.WriteLine(key);
Console.WriteLine(String.Join(", ", dict.Values));
Dictionaries: Functionality

 Find key / value:
 ContainsKey() – checks if a key is present in the dictionary
(fast operation)
 ContainsValue() – checks if a value is present in the
dictionary (slow operation)
 TryGetValue() – check if a key is present in the dictionary
and ouputs the value (or returns the default value)
9
Dictionaries: Functionality (2)

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

Dictionary: Remove()
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Key Value
Hash Function
Pesho Pesho 0881-123-987
Gosho 0881-123-789
Alice 0881-123-978

Pesho 0881-123-987
SortedDictionary<K, V> – Example
12
SortedDictionary
<string, string>
Key Value
Alice +359-899-55-592
Comparator
Function

13
Iterating through Dictionaries
Gosho 0881-456-987
Pesho 0881-123-987
Alice +359-899-55-592
KeyValuePair<string, string> keyValuePair in
foreach loop
.Key .Value
Alice +359-899-55-592
Pesho 0881-123-987
0881-456-987Gosho

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 Write a program to extract from given sequence of words all
elements that present in it odd number of times (case-insensitive)
 Words are given in a single line, space separated
 Print the result elements in lowercase, in their order of appearance
Problem: Odd Occurrences
Java C# PHP PHP JAVA C java java, c#, c
3 5 5 hi pi HO Hi 5 ho 3 hi pi 5, hi
Check your solution here: https://judge.softuni.bg/Contests/Practice/Index/174#1
a a A SQL xx a xx a A a XX c a, sql, xx, c

15
Solution: Odd Occurrences
string input = Console.ReadLine().ToLower();
string[] words = input.Split(' ');
var counts = new Dictionary<string, int>();
foreach (var word in words)
if (counts.ContainsKey(word))
counts[word]++;
else counts[word] = 1;
var results = new List<string>();
foreach (var pair in counts)
// TODO: add pair.Key to results if pair.Value is odd
Console.WriteLine(string.Join(", ", results));
counts[word]
holds how many
times word occurs
in words

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SortedDictionary Example – Events
var events = new SortedDictionary<DateTime, string>();
events[new DateTime(1998, 9, 4)] = "Google's birth date";
events[new DateTime(2013, 11, 5)] = "SoftUni's birth date";
events[new DateTime(1975, 4, 4)] = "Microsoft's birth date";
events[new DateTime(2004, 2, 4)] = "Facebook's birth date";
events[new DateTime(2013, 11, 5)] = "SoftUni was founded";
foreach (var entry in events)
{
Console.WriteLine("{0:dd-MMM-yyyy}: {1}",
entry.Key, entry.Value);
}

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 Read a list of real numbers and print them in ascending order
along with their number of occurrences
Problem: Count Real Numbers
8 2.5 2.5 8 2.5
2.5 -> 3 times
8 -> 2 times
1.5 5 1.5 3
1.5 -> 2 times
3 -> 1 times
5 -> 1 times
-2 0.33 0.33 2
-2 -> 1 times
0.33 -> 2 times
2 -> 1 times

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Solution: Count Real Numbers
double[] nums = Console.ReadLine().Split(' ')
.Select(double.Parse).ToArray();
var counts = new SortedDictionary<double, int>();
foreach (var num in nums)
if (counts.ContainsKey(num))
counts[num]++;
else
counts[num] = 1;
foreach (var num in counts.Keys)
Console.WriteLine($"{num} -> {counts[num]}");
counts[num] will
hold how many times
num occurs in nums

Associative Arrays
Live Exercises in Class

Lambda Functions and LINQ
LINQ in Action: Filtering, Mapping, Ordering

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 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
new List<int>() { 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()  6.75

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 Write a program to read n integers and print their sum, min,
max and average values:
Problem: Sum, Min, Max, Average
5
12
20
-5
37
8
Sum = 72
Min = -5
Max = 37
Average = 14.4
4
50
20
25
40
Sum = 135
Min = 20
Max = 50
Average = 33.75

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Solution: Sum, Min, Max, Average
using System.Linq;
…
int n = int.Parse(Console.ReadLine());
int[] nums = new int[n];
for (int i = 0; i < n; i++)
nums[i] = int.Parse(Console.ReadLine());
Console.WriteLine("Sum = {0}", nums.Sum());
Console.WriteLine("Min = {0}", nums.Min());
// TODO: print also max and average values
Use System.Linq to enable LINQ
functions like .Max() and .Sum()

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Reading Collections on a Single Line
 Using Select() to read collections:
var nums = Console.ReadLine()
.Split()
.Select(number => double.Parse(number));
// .Select(double.Parse); // short version
var nums = Console.ReadLine()
.Split()
.Select(int.Parse);
// .Select(number => int.Parse(number)); // long version

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Converting Collections
 Using ToArray(), ToList() to convert collections:
int[] nums = Console.ReadLine()
.Split()
.Select(number => int.Parse(number))
.ToArray();
List<double> nums = Console.ReadLine()
.Split()
.Select(double.Parse)
.ToList();

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Sorting Collections
 Using OrderBy() to sort collections:
 Using OrderByDescending() to sort collections:
List<int> nums = { 1, 5, 2, 4, 3 };
nums = nums
.OrderBy(num => num)
.ToList();
List<int> nums = { 1, 5, 2, 4, 3 };
nums = nums.OrderByDescending(num => num).ToList();
Console.WriteLine(String.Join(", ", nums));

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Sorting Collections by Multiple Criteria
 Using ThenBy() to sort collections by multiple criteria:
Dictionary<int, string> products =
new Dictionary<int, string>();
Dictionary<int, string> sortedDict = products
.OrderBy(pair => pair.Value)
.ThenBy(pair => pair.Key)
.ToDictionary(pair => pair.Key, pair => pair.Value);

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Take / Skip Elements from Collection
 Using Take(), Skip():
var nums = new List<int>() { 10, 20, 30, 40, 50, 60}
.Take(3)
.ToArray();
// nums = [10, 20, 30]
var nums = new List<int>() { 10, 20, 30, 40, 50, 60}
.Skip(3).Take(2)
.ToArray();
// nums = [40, 30]

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Problem: Largest 3 Numbers
 Read a list of real numbers and print largest 3 of them
10 30 15 20 50 5 50 30 20
20 30 30 20
0 -5 -1 -3 -2 0 -1 -2

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Solution: Largest 3 Numbers
List<int> nums = Console.ReadLine().Split()
.Select(int.Parse)
.ToList();
var sortedNums = nums.OrderByDescending(x => x);
var largest3Nums = sortedNums.Take(3);
Console.WriteLine(string.Join(" ", largest3Nums));

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 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 Expressions
var lambda = (a => a > 5);

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 Lambda functions are inline methods (functions) that take input
parameters and return values:
Lambda Functions
x => x / 2 static int Func(int x) { return x / 2; }
static bool Func(int x) { return x != 0; }x => x != 0
() => 42 static int Func() { return 42; }
(x, y) => x+y static int Func(int x, int y)
{ return x+y; }

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Filter Collections
 Using Where(), Count():
int[] nums = { 1, 2, 3, 4, 5, 6};
nums = nums
.Where(num => num % 2 == 0)
.ToArray();
// nums = [2, 4, 6]
int[] nums = { 1, 2, 3, 4, 5, 6};
int count = nums.Count(num => num % 2 == 0);
// count = 3

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Filtering and Sorting with Lambda Functions
int[] nums = { 11, 99, 33, 55, 77, 44, 66, 22, 88 };
nums.OrderBy(x => x).Take(3);
// 11 22 33
nums.Where(x => x < 50);
// 11 33 44 22
nums.Count(x => x % 2 == 1);
// 5
nums.Select(x => x * 2).Take(5);
// 22 198 66 110 154

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Getting Unique Elements from Collection
 Distinct() takes the unique elements from a collection:
int[] nums = { 1, 2, 2, 3, 4, 5, 6, -2, 2, 0,
15, 3, 1, 0, 6 };
nums = nums
.Distinct()
.ToArray();
// nums = [1, 2, 3, 4, 5, 6, -2, 0, 15]

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 Read a text, extract its words, find all short words (less than 5
characters) and print them alphabetically, in lower case
 Use the following separators: . , : ; ( ) [ ] " ' / ! ? (space)
 Use case-insensitive matching; remove duplicated words
Problem: Short Words Sorted
In SoftUni you can study Java, C#, PHP and JavaScript.
JAVA and c# developers graduate in 2-3 years. Go in!
2-3, and, c#, can, go, in, java, php, you

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Solution: Short Words Sorted
char[] separators = new char[]
{'.',',',':',';','(',')','[',']','','"',''','/','!','?',' '};
string sentence = Console.ReadLine().ToLower();
string[] words = sentence.Split(separators);
var result = words
.Where(w => w != "")
// TODO: filter by word length < 5
.OrderBy(w => w).Distinct();
Console.WriteLine(string.Join(", ", result));

38
Take Single Element from Collection
 Using First(), Last() , Single():
int[] nums = { 1, 2, 3, 4, 5, 6 };
int firstNum = nums.First(x => x % 2 == 0); // 1
int lastNum = nums.Last(x => x % 2 == 1); // 6
int singleNum = nums.Single(x => x == 4); // 4

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Other Operations over Collections
 Using Reverse()
 Using Concat():
int[] nums = { 1, 2, 3, 4, 5, 6};
nums = nums.Reverse();
// nums = 6, 5, 4, 3, 2, 1
int[] nums = { 1, 2, 3, 4, 5, 6 };
int[] otherNums = { 7, 8, 9, 0 };
nums = nums.Concat(otherNums);
// nums = 1, 2, 3, 4, 5, 6, 7, 8, 9, 0

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 Read an array of 4*k integers, fold it like shown below, and print
the sum of the upper and lower rows (2*k integers):
Problem: Fold and Sum
1 2 3 4 5 6 7 8
2 1 8 7
3 4 5 6
5 5 13 13
4 3 -1 2 5 0 1 9 8 6 7 -2
-1 3 4 -2 7 6
2 5 0 1 9 8
1 8 4 -1 16 14
5 2 3 6
5 6
2 3
7 9
3 4 5 6
3 4 5 6

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Solution: Fold and Sum
int[] arr = Console.ReadLine()
.Split(' ').Select(int.Parse).ToArray();
int k = arr.Length / 4;
int[] row1left = arr.Take(k).Reverse().ToArray();
int[] row1right = arr.Reverse().Take(k).ToArray();
int[] row1 = row1left.Concat(row1right).ToArray();
int[] row2 = arr.Skip(k).Take(2 * k).ToArray();
var sumArr =
row1.Select((x, index) => x + row2[index]);
Console.WriteLine(string.Join(" ", sumArr));

Lambda Expressions and LINQ
Live Exercises in Class (Lab)

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 Dictionaries hold {key  value} pairs
 .Keys holds a set of unique keys
 .Values holds a collection of values
 Iterating over dictionary takes the entries as
KeyValuePair<K, V>
 Dictionary<K, V> vs.
SortedDictionary<K, V>
 Lambda and LINQ dramatically simplifies
collection processing
Summary

?
Programming Fundamentals – Dictionaries
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is licensed under the "Creative Commons Attribution-
NonCommercial-ShareAlike 4.0 International" license
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