I have a large List in memory, from a class that has about 20 properties.
I'd like to filter this list based on just one property, for a particular task I only need a list of that property. So my query is something like:
data.Select(x => x.field).Where(x => x == "desired value").ToList()
Which one gives me a better performance, using Select first, or using Where?
data.Where(x => x.field == "desired value").Select(x => x.field).ToList()
Please let me know if this is related to the data type I'm keeping the data in memory, or field's type. Please note that I need these objects for other tasks too, so I can't filter them in the first place and before loading them into memory.
Which one gives me a better performance, using Select first, or using Where.
Where first approach is more performant, since it filters your collection first, and then executes Select for filtered values only.
Mathematically speaking, Where-first approach takes N + N' operations, where N' is the number of collection items which fall under your Where condition.
So, it takes N + 0 = N operations at minimum (if no items pass this Where condition) and N + N = 2 * N operations at maximum (if all items pass the condition).
At the same time, Select first approach will always take exactly 2 * N operations, since it iterates through all objects to acquire the property, and then iterates through all objects to filter them.
Benchmark proof
I have completed the benchmark to prove my answer.
Results:
Condition value: 50
Where -> Select: 88 ms, 10500319 hits
Select -> Where: 137 ms, 20000000 hits
Condition value: 500
Where -> Select: 187 ms, 14999212 hits
Select -> Where: 238 ms, 20000000 hits
Condition value: 950
Where -> Select: 186 ms, 19500126 hits
Select -> Where: 402 ms, 20000000 hits
If you run the benchmark many times, then you will see that Where -> Select approach hits change from time to time, while Select -> Where approach always takes 2N operations.
IDEOne demonstration:
https://ideone.com/jwZJLt
Code:
class Point
{
public int X { get; set; }
public int Y { get; set; }
}
class Program
{
static void Main()
{
var random = new Random();
List<Point> points = Enumerable.Range(0, 10000000).Select(x => new Point { X = random.Next(1000), Y = random.Next(1000) }).ToList();
int conditionValue = 250;
Console.WriteLine($"Condition value: {conditionValue}");
Stopwatch sw = new Stopwatch();
sw.Start();
int hitCount1 = 0;
var points1 = points.Where(x =>
{
hitCount1++;
return x.X < conditionValue;
}).Select(x =>
{
hitCount1++;
return x.Y;
}).ToArray();
sw.Stop();
Console.WriteLine($"Where -> Select: {sw.ElapsedMilliseconds} ms, {hitCount1} hits");
sw.Restart();
int hitCount2 = 0;
var points2 = points.Select(x =>
{
hitCount2++;
return x.Y;
}).Where(x =>
{
hitCount2++;
return x < conditionValue;
}).ToArray();
sw.Stop();
Console.WriteLine($"Select -> Where: {sw.ElapsedMilliseconds} ms, {hitCount2} hits");
Console.ReadLine();
}
}
Related questions
These questions can also be interesting to you. They are not related to Select and Where, but they are about LINQ order performance:
Does the order of LINQ functions matter?
Order of LINQ extension methods does not affect performance?
The answer will depend on the state of your collection.
If most entities will pass the Where test, apply Select first;
If fewer entities will pass the Where test, apply Where first.
Update:
#YeldarKurmangaliyev wrote the answer with a concrete example and benchmarking. I ran similar code to verify his claim and our results are exactly opposite and that is because I ran the same test as his but with an object not as simple as the Point type he used to run his tests.
The code very much looks like his code, except that I changed the name of class from Point to EnumerableClass.
Given below the classes I used to constitute the EnumerableClass class:
public class EnumerableClass
{
public int X { get; set; }
public int Y { get; set; }
public String A { get; set; }
public String B { get; set; }
public String C { get; set; }
public String D { get; set; }
public String E { get; set; }
public Frame F { get; set; }
public Gatorade Gatorade { get; set; }
public Home Home { get; set; }
}
public class Home
{
private Home(int rooms, double bathrooms, Stove stove, InternetConnection internetConnection)
{
Rooms = rooms;
Bathrooms = (decimal) bathrooms;
StoveType = stove;
Internet = internetConnection;
}
public int Rooms { get; set; }
public decimal Bathrooms { get; set; }
public Stove StoveType { get; set; }
public InternetConnection Internet { get; set; }
public static Home GetUnitOfHome()
{
return new Home(5, 2.5, Stove.Gas, InternetConnection.Att);
}
}
public enum InternetConnection
{
Comcast = 0,
Verizon = 1,
Att = 2,
Google = 3
}
public enum Stove
{
Gas = 0,
Electric = 1,
Induction = 2
}
public class Gatorade
{
private Gatorade(int volume, Color liquidColor, int bottleSize)
{
Volume = volume;
LiquidColor = liquidColor;
BottleSize = bottleSize;
}
public int Volume { get; set; }
public Color LiquidColor { get; set; }
public int BottleSize { get; set; }
public static Gatorade GetGatoradeBottle()
{
return new Gatorade(100, Color.Orange, 150);
}
}
public class Frame
{
public int X { get; set; }
public int Y { get; set; }
private Frame(int x, int y)
{
X = x;
Y = y;
}
public static Frame GetFrame()
{
return new Frame(5, 10);
}
}
The classes Frame, Gatorade and Home have a static method each to return an instance of their type.
Below is the main program:
public static class Program
{
const string Chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
private static readonly Random Random = new Random();
private static string RandomString(int length)
{
return new string(Enumerable.Repeat(Chars, length)
.Select(s => s[Random.Next(s.Length)]).ToArray());
}
private static void Main()
{
var random = new Random();
var largeCollection =
Enumerable.Range(0, 1000000)
.Select(
x =>
new EnumerableClass
{
A = RandomString(500),
B = RandomString(1000),
C = RandomString(100),
D = RandomString(256),
E = RandomString(1024),
F = Frame.GetFrame(),
Gatorade = Gatorade.GetGatoradeBottle(),
Home = Home.GetUnitOfHome(),
X = random.Next(1000),
Y = random.Next(1000)
})
.ToList();
const int conditionValue = 250;
Console.WriteLine(#"Condition value: {0}", conditionValue);
var sw = new Stopwatch();
sw.Start();
var firstWhere = largeCollection
.Where(x => x.Y < conditionValue)
.Select(x => x.Y)
.ToArray();
sw.Stop();
Console.WriteLine(#"Where -> Select: {0} ms", sw.ElapsedMilliseconds);
sw.Restart();
var firstSelect = largeCollection
.Select(x => x.Y)
.Where(y => y < conditionValue)
.ToArray();
sw.Stop();
Console.WriteLine(#"Select -> Where: {0} ms", sw.ElapsedMilliseconds);
Console.ReadLine();
Console.WriteLine();
Console.WriteLine(#"First Where's first item: {0}", firstWhere.FirstOrDefault());
Console.WriteLine(#"First Select's first item: {0}", firstSelect.FirstOrDefault());
Console.WriteLine();
Console.ReadLine();
}
}
Results:
I ran the tests multiple times and found that
.Select().Where() performed better than .Where().Select().
when collection size is 1000000.
Here is the first test result where I forced every EnumerableClass object's Y value to be 5, so every item passed Where:
Condition value: 250
Where -> Select: 149 ms
Select -> Where: 115 ms
First Where's first item: 5
First Select's first item: 5
Here is the second test result where I forced every EnumerableClass object's Y value to be 251, so no item passed Where:
Condition value: 250
Where -> Select: 110 ms
Select -> Where: 100 ms
First Where's first item: 0
First Select's first item: 0
Clearly, the result is so dependent on the state of the collection that:
In #YeldarKurmangaliyev's tests .Where().Select() performed better; and,
In my tests .Select().Where() performed better.
The state of the collection, which I am mentioning over and over includes:
the size of each item;
the total number of items in the collection; and,
the number of items likely to pass the Where clause.
Response to comments on the answer:
Further, #Enigmativity said that knowing ahead of time the result of Where in order to know whether to put Where first or Select first is a Catch-22. Ideally and theoretically, he is correct and not surprisingly, this situation is seen in another domain of Computer Science - Scheduling.
The best scheduling algorithm is Shortest Job First where we schedule that job first that will execute for the least time. But, how would anyone know how much time will a particular job take to complete? Well, the answer is that:
Shortest job next is used in specialized environments where accurate estimates of running time are available.
Therefore, as I said right at the top (which was also the first, shorter version of my answer), the correct answer to this question will depend on the current state of the collection.
In general,
if your objects are within a reasonable size range; and,
you are Selecting a very small chunk out of each object; and,
your collection size is also not just in thousands,
then the guideline mentioned right at the top of this answer will be useful for you.
Related
I need sequence numbering for my records display. 50 record for each page.
public List<string[]> InstructionsData(IEnumerable<Assets> InstructionsEntry, int currentUserId)
{
return InstructionsEntry.Select((entry,index) => new string[]
{
(index + 1).ToString(),
entry.state_Id,
entry.batchRef_Id,
entry.assetCategory_Id,
GetAge(entry.age),
entry.assetStatus_Id,
GetStatusTag(entry.recordStatus ??false),
entry.availbaleQty.ToString(),
entry.createdBy,
}).ToList();
The above code which is used for displaying index is working fine. My problem is when I move to next page the index again start from first. Please help me to continue the index number on next page as well.
Simple :
public List<string[]> InstructionsData(IEnumerable<Assets> InstructionsEntry, int currentUserId, int startIndex)
{
return InstructionsEntry.Select((entry,index) => new string[]
{
(startIndex + index + 1).ToString(),
entry.state_Id,
entry.batchRef_Id,
entry.assetCategory_Id,
GetAge(entry.age),
entry.assetStatus_Id,
GetStatusTag(entry.recordStatus ??false),
entry.availbaleQty.ToString(),
entry.createdBy,
}).ToList();
I will not argue about your design choices like
why do you need to return an array of strings instead of structured object?
or why do you need the index when you must have a unique identifier to rely on?
you might have your very peculiar specific reasons behind the scenes, but I'll try to pin point what i might end up doing to retrieve a paginated indexed result set.
First thing first, get a paginated result set of assets (or whatever your use case scenario requires), having some metadata about the given page (current page number, page size, total records)
var paged = data.Page(page: 2, size: 5);
Then on the frontend, or wherever the result is to be displayed, attach the index to returned records (I'll show the C# equivalent, but you will easily achieve the same results in your frontend of choice tech stack)
var indexed = paged.Items
.Select((current, index) =>
{
var skipped = (paged.Page - 1) * paged.Size;
return new
{
Index = skipped + index,
Item = current,
};
});
where the paged result would look like this
public class Paged<T>
{
public IEnumerable<T> Items { get; set; }
public int Page { get; set; }
public int Size { get; set; }
public int Total { get; set; }
}
public class Asset
{
public int Id { get; set; }
public string Name { get; set; }
}
while the actual paging mechanism simply applies Skip and Take
public static class Extensions
{
public static Paged<T> Page<T>(this IEnumerable<T> instance, int? page = 1, int? size = 10)
{
var items = instance
.Skip((page.Value - 1) * size.Value)
.Take(size.Value)
.ToList();
return new Paged<T>()
{
Page = page.Value,
Size = size.Value,
Total = instance.Count() - 1,
Items = items,
};
}
}
check gist for an overview code sample
Why I'm doing this:
So I'm trying to make an application for a game called clash royale, after winning the games there you get a "random" chest, which is actually not random... When you create your account you get a digit assigned to you from 0 to 239, and after that it follows a pattern for the chest drops. The applciation I'm making would take a user's entries and compare it to the pattern, thus being able to predict how soon the next chests of a higher quality would drop.
The help I need with the code:
Is it possible to make an array kind of... loop within itself.. So for example when going through the array in a loop, if "i" is 239, then adding +1 would take it back to the beginning, or #0 (239 not necessarily being the limit).
The class (and it's container that I want to loop):
class Chest
{
public int ID { get; set; }
public string Type { get; set; }
public Chest()
{
}
public Chest(int id, string type)
{
ID = id;
Type = type;
}
}
class ChestContainer
{
private Chest[] ChestList = new Chest[240];
public int Count { get; set; }
public ChestContainer(int size)
{
ChestList = new Chest[size];
}
public void Add(Chest chest)
{
ChestList[Count++] = chest;
}
public Chest Get(int index)
{
return ChestList[index];
}
}
Also wouldn't mind any tips to improve my class / container class, at the moment this is what I've been doing for pretty much my entire "career" as this is what we were thought in uni (minus the string override for the class).
You could use Modulo % in order to get a loop kind of thing.
If you replace the Container.Add method with the one below, the index will be "reset" (for lack of better words).
public void Add(Chest chest)
{
ChestList[Count++%(ChestList.Length)] = chest;
}
After updating the method, if you want an example, you can try the code below:
var container = new ChestContainer(240);
for (int i = 0; i < 1000; i++)
container.Add(new Chest(i, $"{i}"));
Edit In order to have the Get method working as well, modifying it as mentioned below will ensure your container works as expected:
public Chest Get(int index)
{
return ChestList[index%(ChestList.Length)];
}
To test it out, you can use the code below:
var container = new ChestContainer(240);
for (int i = 0; i < 1000; i++)
{
container.Add(new Chest(i, $"{i}"));
var value = container.Get(i);
}
You can overload the [] operator to define it's behaviour.
Something like this:
public static Chest operator [] (int index) {
return ChestList[index%240];
}
public Chest Get(int index)
{
return ChestList[index%240]; //put your limit here
}
How it works: % is the modulo operator.
It returns the remainder of a devision.
Example:
5/2 = 2, remaining 1
=> 5%2 = 1
In your case, when numbers higher than 239 are entered, with modulo it just wraps around.
i have a List which get's and set's data from/to my class "Type" after a specific condition is fulfilled i want to clear all the variables value's that have been gathered here's my code
List<Type> Win = new List<Type>();
void Check(int a)
{
if (a>10)
{
Win.Add(new Type() { Power = 10 + a * 100, Current = 1 });
}
if(a<10)
{
Win.Add(new Type() { Power = 10 + a * 100, Current = 1 });
}
}
My class:
public class Type
{
public int Power { get; set; }
public int Current { get; set; }
}
And like this it will always enter the 2 if's at least once because I'm giving him a parameter "a" and a will change let's say 5 times and it will get a lot of different values.How can i clear the entire "Type.Power" and "Type.Current" variables or just clear the entire list "Win" ?
To empty the entire list, you can use .Clear().
Win.Clear();
To reset all instances of `Type.Power' and 'Type.Current' per instance in your list, you would need to iterate the list and update the members to their default value 0.
Win.ForEach(x =>
{
x.Power = 0;
x.Current = 0;
});
I'm writing a WP 8 that will show list returned from the WCF service. I want to select all teachers from the database and then from the created list return another one that will show 5 teachers whose maximum income (maxInc) is closest to mine (MyMaxInc). For example if MyMaxInc = 200 and I have user with 225, user2 with 240 and user3 with 210 the returned list will show teachers in the following order: user3, user, user2.
My service implementation:
public IEnumerable<mTeachers> GetStuffList(int MyMaxInc, int MyMinInc)
{
List<mTeachers> stuffList = new List<mTeachers>();
DataClasses1DataContext data = new DataClasses1DataContext();
int inc = 0;
List<mTeachers> finalList = new List<mTeachers>();
foreach (var d in data.Stuffs)
{
if (d.stuffJob == "teacher")
{
stuffList.Add(new mTeachers(d.stuffName, (int)d.maxInc, (int)d.minInc, "teacher", inc)); inc++;
}
}
if (inc > 0)
{
foreach (mTeachers element in stuffList)
if () {/*didn't finish this bit because
don't know how to create second list*/ }
return finalList;
}
else return null;
}
Service interface:
[ServiceContract]
public interface IService1
{
[OperationContract]
IEnumerable<mTeachers> GetStuffList(int MyMaxInc, int MyMinInc);
}
[DataContract]
public class mTeachers
{
[DataMember]
public string Name;
[DataMember]
public int maxInc;
[DataMember]
public int minInc;
[DataMember]
public string Job;
[DataMember]
public int Number;
public mTeachers(string Name, int maxInc, int minInc, string Job, int Number)
{
this.maxInc = maxInc;
this.minInc = minInc;
this.Name = Name;
this.Job = Job;
this.Number = Number;
}
}
Also I created Number member of mTeachers class to group finalList later. For example I can assign some number when I'm creating a list in the loop and the group it with private static List<Group<T>> GetItemGroups<T>(IEnumerable<T> itemList, Func<T, string> getKeyFunc) method.
Replace
foreach (mTeachers element in stuffList)
if () {/*didn't finish this bit because
don't know how to create second list*/ }
with
finalList.AddRange(
stuffList.OrderBy(x => Math.Abs(x.maxInc - MyMaxInc)).Take(5));
Math.Abs(...) converts negative differences to positive to be able to compare smaller and larger values at the same time.
OrderBy combined with Take(5) takes the 5 first elements sorted by the smallest difference.
I want correctly return some variables (arrays)
kazkas.Ads[n]; (n = how many ads are)
kazkas.Ads[n].id;
kazkas.Ads[n].Days[m].Stats.Clicks; // every day have his own clicks
kazkas.Ads[n].Days[m].Stats.Impresons; // every day have his own impresions
from this method and use these variables in other class.
public static void GetAdsStats(string Ticket, DateTime start, DateTime end, int CamId)
{
var client = new CampaignStatsServiceClient();
var id = new CampaignIdFilter();
id.CampaignId = CamId;
var statsdata = new GetAdStatsData();
var kazkas = new Campaign();
kazkas = client.GetAdStats(Ticket, new GetAdStatsData
{
IdFilter = id,
StartDate = start,
EndDate = end
});
long AllClicks = 0;
long AllImpresions = 0;
int reklamos = kazkas.Ads.Length;
long[] statistikaClikai = new long[reklamos];
long[] statistikaImpresions = new long[reklamos];
for (int i = 0; i < reklamos; i++)
{
int dienos = kazkas.Ads[i].Days.Length;
for (int lop = 0; lop < dienos; lop++)
{
AllClicks = AllClicks + kazkas.Ads[i].Days[lop].Stats.Clicks;
AllImpresions = AllImpresions + kazkas.Ads[i].Days[lop].Stats.Impressions;
}
statistikaClikai[i] = AllClicks;
statistikaImpresions[i] = AllImpresions;
}
}
I know that void type can't return anything, but this how I know that my method works ( from debugging). Like you see I was trying do that with for loop. Here i have 9 Ads and every ad have one day.
Like I says I want return every Ads id[in array], and every days.stats.impresions and days.stats.click
how can I do that ? Ore how return more variables/arrays from method to other class, I am using webservises, so i cant use database ore something like that.
As can be seen by the downvotes of the question, you need to design the return value and then code against it.
Your query almost does it (now):
kazkas.Ads[n]; (n = how many ads are)
kazkas.Ads[n].id;
kazkas.Ads[n].Days[m].Stats.Clicks; // every day have his own clicks
kazkas.Ads[n].Days[m].Stats.Impressions; // every day have his own impressions
Your existing code show this should be expanded to include:
kazkas.Ads[n].Total.Clicks;
kazkas.Ads[n].Total.Impressions;
So now you're ready to design. First you want a Stat Class that just contains CLicks and Impressions:
public class Stat
{
public long Impressions { get; set; }
public long Clicks { get; set; }
}
An optimisation here may be to use a struct, but I won't go into that.
As you currently have defined it each Day has just a Stats property:
public class DayStat
{
public Stat Stats { get; set; }
}
Now finally we can define the top level AdStat:
public class AdStat
{
public int id { get; set; }
public DayStat Day[];
public Stat Total { get; set; }
}
Etc... There's further issues here, such as ensuring arrays are created and Stat instances are never null (which is why making some of these classes structs is an option). But I'm really a VB programmer so I'll stop here before I get caught typing crap into the SO IDE :-)
Create a class or struct with members you need
public class Stat
{
public int Id { get; set; }
public long Clicks { get; set; }
...
}
Change the signature of your method from void GetAdsStats to IEnumberable<Stat> GetAdsStats and either return a collection of stats or use yield keyword to return the stat object.
Also if you do not want your method to return anything (return type void) do not use a name starting with Get.
Example:
public static IEnumerable<Stat> GetAdsStats(...)
{
...
var statList = new List<Stat>();
for (int i = 0; i < reklamos; i++)
{
var stat = new Stat();
statList.Add(stat);
int dienos = kazkas.Ads[i].Days.Length;
for (int lop = 0; lop < dienos; lop++)
{
AllClicks = AllClicks + kazkas.Ads[i].Days[lop].Stats.Clicks;
AllImpresions = AllImpresions + kazkas.Ads[i].Days[lop].Stats.Impressions;
}
stat.Clicks = AllClicks;
stat.Impression = AllImpresions;
}
return statList;
}
Change your void to the type you want to return, say Campaign, and return the appropriate variable. The variables you define in your method, only live in your method and are not accessible from another method or class.