2.1.1 Basic types: Lists and Linq - caveat, C#

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//2.1.1 Basic types: Lists and Linq - caveat

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            //Linq uses deffered execution. That means that unless you have "materialized" result of linq query (by calling .ToList() or other .ToMethod())
            //the result is actually the query itself and not the actual result of it. Therefore whenever you use the query it will be executed again and again.
            //ToList() turns the query into result produced by that query.
            //Here is an example.
            
            var dic = new Dictionary<int, string>();
            for(int i=0; i<20000; i++)
            {
                dic.Add(i, i.ToString());
            }
            
            var list = dic.Where(f => f.Value.StartsWith("1")).Select(f => f.Key);//.ToList(); //uncomment for fast results 
            //list is actually the query when there is no .ToList() above
            Console.WriteLine(list.GetType());
            //which means that whenever we call list.Contains() below we will evaluate this query again and again turning this algorithm into O(n^2)
            //with ToList() uncommented the algorithm is O(n)
            var list2 = dic.Where(f => list.Contains(f.Key)).ToList();
            
            Console.WriteLine(list2.Count());
        }
    }
}

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Compilation time: 0.16 sec, absolute running time: 5.15 sec, cpu time: 5.12 sec, average memory usage: 24 Mb, average nr of threads: 6 (cached)