Lesson # 13. Lists. LINQ Extension Methods, Lambda expressions

Lesson # 13. Theory

List<T> Class Represents a strongly typed list of objects that can be accessed by index. Provides methods to search, sort, and manipulate lists.

List<int> numList = new List<int> { 1,9,2,6,3 }; // of integer type
var nameList = new List<string> { "Doug", "Sally", "Sue" }; // of string type

To output the list:

foreach (var k in numList)
     Console.WriteLine(k);

List generators:

// Generate a list from 1 to 10
var L = new List<int>();
L.AddRange(Enumerable.Range(1, 10));
foreach (var k in L)
    Console.WriteLine(k);

Delegate that is assigned a Lambda expression:

 class Program
    {
        delegate double doubleIt(double val);
        static void Main(string[] args)
        {
            doubleIt dblIt = x => x * 2;
            Console.WriteLine($"5 * 2 = {dblIt(5)}");
        }

Problem: find all the even numbers in the list:

List<int> numList = new List { 1,9,2,6,3 };
 
var evenList = numList.Where(a => a % 2 == 0).ToList();
 
foreach (var j in evenList)
    Console.WriteLine(j);

Problem: Add values in a range to a list

List<int> numList = new List<int> { 1, 9, 2, 6, 3 };
var rangeList = numList.Where(x => (x > 2) || (x < 9)).ToList();

foreach (var k in rangeList)
   Console.WriteLine(k);

Problem: Find all names starting with s

var nameList = new List<string> { "Doug", "Sally", "Sue" };

var sNameList = nameList.Where(x => x.StartsWith("S"));

foreach (var m in sNameList)
    Console.WriteLine(m); // Sally Sue

Select allows us to execute a function on each item in a list.

var lst= new List<int>();
lst.AddRange(Enumerable.Range(1, 10));
 
var squares = lst.Select(x => x * x);
 
foreach (var l in squares)
    Console.Write(l+" "); // 1 4 9 16 25 36 49 64 81 100

Zip applies a function to two lists
Problem: Add values in 2 lists together

var listOne = new List(new int[] { 1, 3, 4});
var listTwo = new List(new int[] { 4, 6, 8});
 
var sumList = listOne.Zip(listTwo, (x, y) => x + y).ToList();
 
foreach (var n in sumList)
   Console.WriteLine(n); // 5 9 12

Aggregate performs an operation on each item in a list and carries the results forward
Problem: Sum values in a list

var numList2 = new List<int>() { 1, 2, 3, 4, 5 };
Console.WriteLine("Sum : {0}",
   numList2.Aggregate((a, b) => a + b)); // Sum : 15

AsQueryable allows you to manipulate the collection with the Average function
Problem: Get the average of a list of values

var numList3 = new List<int>() { 1, 2, 3, 4, 5 };
 
Console.WriteLine("AVG : {0}", 
     numList3.AsQueryable().Average()); // AVG : 3

Determines if all items in a list meet a condition

var numList4 = new List() { 1, 2, 3, 4, 5 };

Console.WriteLine("All > 3 : {0}",
   numList4.All(x => x > 3)); // All > 3 : False

Determines if any items in a list meet a condition

var numList5 = new List<int>() { 1, 2, 3, 4, 5 };
 
Console.WriteLine("Any > 3 : {0}",
   numList5.Any(x => x > 3)); // Any > 3 : True

Labs and Tasks

// 1.
-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10

// 2.
3 + 1 = 4

// 3.
The list:
-5 1 9 -2 6 -3 
result 1:
-5 -2 -3
result 2:
9 6 -3

// 4.
The list:
Ivan John Boris Bony
result 1:
Ivan John
result 1:
Boris Bony

// 5.
-5 -4 -3 -2 -1 0 1 2 3 4 5
result 1:
-125 -64 -27 -8 -1 0 1 8 27 64 125
result 2:
-8 -7 -6 -5 -4 -3 -2 -1 0 1 2

// 6.
-77  15  39

// 7.
2  4  8  9  1  5
Product : 2880

// 8.
3  12  18  9
All are divisable by 3 : True

while (br.PeekChar() != -1)
            {
                L1.Add(br.ReadInt32()); // L1 is a name of list
            }

// Positive elements:
7 4 2 9 8 4 9 9 1 3
// The greatest from odd numbers:
9