C#•9mo ago

❔ Help me understand this lambda expression

var myKey = availableProducts.FirstOrDefault(x => x.Value == input).Key; Console.WriteLine("Quantity: left " + myKey); Im trying to understand how this code works (how it gets the key for the input value in a dictionary). Been searching the web but not really getting more clear.. mostly I dont understand the use of "x". Many thanks for any help!

80 Replies

cap5lut•9mo ago

the lamba is basically just a function that takes a KeyValuePair<TKey, TValue> and returns a bool x is the parameter name chosen, u could use also any other valid name for that. if u would write the lambda as function manually (lets assume that both the TKey and the TValue are ints for simplicity), it would look like this:

int input; // capured input value

bool MyLambda(KeyValuePair<int, int> x /* the parameter name x */)
{
  return x.Value == input; // the lambda method body
}

int input; // capured input value

bool MyLambda(KeyValuePair<int, int> x /* the parameter name x */)
{
  return x.Value == input; // the lambda method body
}

FirstOrDefault() is a LINQ extension method on IEnumerable<T> and takes a Func<T, bool> as parameter Dictionary<TKey, TValue> implements this interface as IEnumerable<KeyValuePair<TKey, TVAlue>> so it results in the Func<KeyValuePair<int, int>, bool> from the example

Emelie•9mo ago

Sorry for a probably stupid question, but what is a LINQ extension method?

cap5lut•9mo ago

do u know what extension methods are in general?

Angius•9mo ago

An extension method is a method that's added to the class from outside of the class

Emelie•9mo ago

not really :/

Angius•9mo ago

Syntactic sugar over static methods, really LINQ is a collection of such methods that extend the IEnumerable

Emelie•9mo ago

why is it added from outside the class?

Angius•9mo ago

Because you can't always just modify the class itself You can't modify int to add new methods, can you? But you can write an extension method for int

cap5lut•9mo ago

yeah u can basically "add methods" to another class

public static void PrintMeUpperCase(this string value)
{
  Console.WriteLine(value.ToUpper());
}

"hello world".PrintMeUpperCase();

public static void PrintMeUpperCase(this string value)
{
  Console.WriteLine(value.ToUpper());
}

"hello world".PrintMeUpperCase();

this would print HELLO WORLD and it looks like the method is part of the string class

Emelie•9mo ago

what is meant by "modifying int" , in this context? I think Im getting it

Angius•9mo ago

Well, int is a struct That has value, properties, and what not You can't just edit the source code of .NET standard library to add new methods to that int struct

Emelie•9mo ago

I also have a really hard time understanding exactly what a struct is, which probably makes it all even harder

Angius•9mo ago

Funky class For all intents and purposes

cap5lut•9mo ago

LINQ is a powerful collection of such extension methods to simplify handling a lot of data the same way

Emelie•9mo ago

so, FirstOrDefault is an extented method

cap5lut•9mo ago

yes

Emelie•9mo ago

on the enumerable interface?

cap5lut•9mo ago

yep

Emelie•9mo ago

which the dictionary implements?

cap5lut•9mo ago

correct

Emelie•9mo ago

but why is it called a lambda function?

Angius•9mo ago

The name "lambda" comes from math, IIRC You can call it an "anonymous function" Both names are correct

Azrael•9mo ago

I prefer Anonymous functions.

Emelie•9mo ago

okay! but is there any point in making it anonymous? could it also be written non-anonymous?

Angius•9mo ago

Sure

Azrael•9mo ago

Yes. I prefer not using anonymous functions, I focus on readability.

cap5lut•9mo ago

its actually lambda "expression", a lambda on its own isnt a complete function, it always needs a delegate type to be assigned to. no matter if thats a method parameter like in the FirstOrDefault() case or if its a local variable

Angius•9mo ago

You can use not-anon functions, but it will just uselessly bloat your code

Emelie•9mo ago

but if the lambda uses parameters (keyValuePair<TKey, TValue) and returns a bool, is it not a function?

Angius•9mo ago

Why write

public static bool IsCountMoreThanFour(Thing x)
{
    return x.Count > 4;
}

public static bool IsCountMoreThanFour(Thing x)
{
    return x.Count > 4;
}

and

stuff.Where(IsCountMoreThanFour)

stuff.Where(IsCountMoreThanFour)

when you can write

stuff.Where(x => x.Count > 4)

stuff.Where(x => x.Count > 4)

Why would it not be?

cap5lut•9mo ago

the type here defines what the lambda is mapped to

Func<int, int, int> addMethod = (a, b) => a + b;

Func<int, int, int> addMethod = (a, b) => a + b;

its a Func delegate, that takes two ints (first two generic parameters) and returns an int (last one)

Emelie•9mo ago

@ZZZZZZZZZZZZZZZZZZZZZZZZZ what @cap5lut wrote a bit earlier! that it isnt completely a function "its actually lambda "expression", a lambda on its own isnt a complete function, it always needs a delegate type to be assigned to. no matter if thats a method parameter like in the FirstOrDefault() case or if its a local variable"

Azrael•9mo ago

It’s still a function.

cap5lut•9mo ago

its not a function on its own, it only makes sense in combination with a delegate type

Emelie•9mo ago

and what is a delegate type 😅

Angius•9mo ago

A type that describes the function

cap5lut•9mo ago

it basically defines a method signature/method type, return type and its parameter list

Angius•9mo ago

Func<int, string, string> for example would be a function that takes two strings and returns an int

Emelie•9mo ago

like the actual function name? would that be the delegate?

Angius•9mo ago

I think we're getting way deeper into it than necessary lol

cap5lut•9mo ago

delegate int LengthGetter(string value) would be a delegate declaration which can then be later used: LengthGetter lengthGetter = str => str.Length;

Emelie•9mo ago

ok, I think I need to read up on a few things, haha but this is what Im getting: FirstOrDefault is an extended method from the enumerable interface which the dictionary implements

Angius•9mo ago

*extension *IEnumerable But yes

Emelie•9mo ago

and as a PARAMETER, a lambda expression is used (also called an anonymus function)

Angius•9mo ago

Yep

Emelie•9mo ago

stupid question again, but why must a lambda expression or anonymous function be used?

cap5lut•9mo ago

its simply better this way, because the user of the method can write the actual check for it

Emelie•9mo ago

its the x I dont understand :/

Angius•9mo ago

It's "each element of the collection" Think of it as a foreach loop With the check inside of it

cap5lut•9mo ago

maybe better to write the code out a bit:

public delegate TResult Func<TParameter, TResult>(TParameter parameter);

public static T FirstOrDefault<T>(this IEnumerable<T> elements, Func<T, bool> predicate)
{
  foreach (var element in elements)
  {
    if (predicate(element)) {
      return element;
    }
  }
  return default(T);
}

public delegate TResult Func<TParameter, TResult>(TParameter parameter);

public static T FirstOrDefault<T>(this IEnumerable<T> elements, Func<T, bool> predicate)
{
  foreach (var element in elements)
  {
    if (predicate(element)) {
      return element;
    }
  }
  return default(T);
}

this would be an example implementation

Emelie•9mo ago

Thank you! Im getting closer now 😅 buuut, what about it makes it a foreach loop?

cap5lut•9mo ago

predicate(element) is basically calling the delegate/ur lambda like any other method as well

Emelie•9mo ago

is it the lambda that works as a loop?

cap5lut•9mo ago

and as u can see im passing the element to it

Angius•9mo ago

No, the inside of the method

Emelie•9mo ago

ah now I see!!

Angius•9mo ago

The method itself loops over all elements of the collection, and returns whichever element matches your lambda

Emelie•9mo ago

the FirstOrDefault already contains an actual ForEach and x is = var ?

cap5lut•9mo ago

x is the parameter of the lambda/delegate public delegate TResult Func<TParameter, TResult>(TParameter parameter); <--- that parameter

MODiX•9mo ago

Angius

REPL Result: Success

string? GetMatching(string[] strings, Func<string, bool> func)
{
    foreach (var str in strings)
    {
        if (func(str))
        {
            return str;
        }
    }
    return null;
}

var arr = new[]{
    "cat",
    "parrot",
    "ox",
    "sandworm"
};
GetMatching(arr, str => str.Length == 3)

string? GetMatching(string[] strings, Func<string, bool> func)
{
    foreach (var str in strings)
    {
        if (func(str))
        {
            return str;
        }
    }
    return null;
}

var arr = new[]{
    "cat",
    "parrot",
    "ox",
    "sandworm"
};
GetMatching(arr, str => str.Length == 3)

Result: string

cat

cat

Compile: 663.692ms | Execution: 83.204ms | React with ❌ to remove this embed.

Angius•9mo ago

Here's a way simplified example

Emelie•9mo ago

soo x is the parameter used in the anonymous function/expression.

cap5lut•9mo ago

correct

Emelie•9mo ago

and the anonymous function doesnt really do anything else then define the x as parameter. The only thing that does anything is the FirstOrDefault function So you could say that the anonymus function is a parameter to the FirstOrDefault function

Angius•9mo ago

It most certainly is

Emelie•9mo ago

Ok, thank you all very much 😄 I have some things to consider now, but I really appreciate you taking the time 😍

Angius•9mo ago

When in doubt, use the debugger and go through the code step-by-step to see what it does and when

Emelie•9mo ago

yeah I try to, but it gets difficult to follow sometimes 😅

cap5lut•9mo ago

a delegate is a data type to pass functions around as variables and to call them. the crucial point here is that they merely define the parameter list (count and type) and the return type itself lambdas are syntax sugar to write anonymous functions, so u do not have to write a whole new method and pass that to the delegate parameter/variable in ZZZZZZZZZ's example, without lambdas it would become quite more verbose instead of just calling GetMatching(arr, str => str.Length == 3) u would have to write a new method and pass that:

bool HasLengthOfThree(string str)
{
  return str.Length == 3;
}
GetMatching(arr, HasLengthOfThree);

bool HasLengthOfThree(string str)
{
  return str.Length == 3;
}
GetMatching(arr, HasLengthOfThree);

this will quickly become quite bloaty if u would have too write a lot of these methods (imagine u want to use GetMatching() to get the first element with a length of 3, then the first with the length of 4 nd the first with the length of 5, this would mean 2 more functions really similar to HasLengthOfThree()) this is a lot simplier to read:

var str3 = GetMatching(arr, str => str.Length == 3);
var str4 = GetMatching(arr, str => str.Length == 4);
var str5 = GetMatching(arr, str => str.Length == 5);

var str3 = GetMatching(arr, str => str.Length == 3);
var str4 = GetMatching(arr, str => str.Length == 4);
var str5 = GetMatching(arr, str => str.Length == 5);

Emelie•9mo ago

Im copying all of your replies into my notepad! very useful and important topics to cover, I am understanding so delegates are methods used as variables

cap5lut•9mo ago

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/lambda-expressions covers all of this also and is probably better phrased

Lambda expressions - Lambda expressions and anonymous functions - C#

C# lambda expressions that are used to create anonymous functions and expression bodied members.

Emelie•9mo ago

therefore, in this context, the lambda IS a delegate?

cap5lut•9mo ago

a delegate is a method type, just like int is a number type a lambda is just a short hand version to specify the value of such a type

Emelie•9mo ago

so lambda is an example of a delegate?

cap5lut•9mo ago

yep (well in detail its a bit complexer, but this would confuse right now more than it would help)

Emelie•9mo ago

ok, thanks again!! Im going to read through everything again in further detail, but I think its getting clearer

cap5lut•9mo ago

oh and since c# 10.0 lambdas have a natural type now, so what i earlier said:

its actually lambda "expression", a lambda on its own isnt a complete function, it always needs a delegate type to be assigned to

is not true anymore

Emelie•9mo ago

okay! thanks for clarifying, don´t think its super important for me at the moment, still trying to understand the very basics

cap5lut•9mo ago

this will help u understand more about it and some LINQ as well. https://learn.microsoft.com/en-us/dotnet/csharp/linq/write-linq-queries they describe method syntax and query syntax, ignore the latter for now, i dont think its used that often anymore

Write LINQ queries in C# - C#

Learn how to write LINQ queries in C#.

Accord•9mo ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Help me understand this lambda expression