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I have been learning about the lambda expression, I got happy when I finally can read/understand the => operator, it kind of means "where" to me
List<int> a = new List<int>(){0,1,2,1,3,4,5,6,7,8,9};
IEnumerable<int> b = a.FindAll(x => x>=5);
foreach (int x in b)
Console.WriteLine(x);
Reading the above line, personally makes sense to read it as "Find all x's from this list WHERE x is greater than or equal 5", very good.
But then I come across a different use of the lambda expression with the Select method.
List<int> a = new List<int>(){0,1,2,1,3,4,5,6,7,8,9};
IEnumerable<int> b1 = a.Select(x => x*2);
foreach (int x in b)
Console.WriteLine(x);
With this one, the previous way of reading this operator doesn't make sense, as to me this code does "For each x return x*2", which is very different "function" to what the same operator does in the previous case.
I understand that the difference is between .FindAll and .Select, different way of dealing with input and output parameters, but I am talking about the use of the operator => in the lambda expression.
There's no question in this question, so let's make one up.
Characterizing the lambda operator as "where" works when the lambda returns a bool and is used as a predicate to test a value. Is there a more general characterization of the lambda operator that makes sense in other contexts, such as projection?
Yes. Read the lambda operator as "goes to".
a.Select(x => x * 2);
"each x goes to x times two"
You can use that for predicates as well:
a.Where(x => x > 2);
"each x goes to 'is x greater than two?'"
But that's awkward. As you note, it is easier to think of this as "where" or "such that"
"each x such that x is greater than two"
Similarly
a.OrderBy(x => x.LastName)
"order each x by x goes to last name" is awkward. Just say "order each x by last name".
In short: the English language interpretation of the operator depends on the context. The formal interpretation of the operator is always the same: it simply describes a function.
The => operator has exactly the same meaning in both cases: it creates a function whose parameter is the thing on the left, and whose return value is the thing on the right.
You wrote in a comment that the x in the first case is not a parameter as you understand it. That's not correct; it is a parameter in both cases here.
Here's your first example, again:
List<int> a = new List<int>(){0,1,2,1,3,4,5,6,7,8,9};
IEnumerable<int> b = a.FindAll(x => x>=5);
foreach (int x in b)
Console.WriteLine(x);
If you wanted to write this without using lambda notation, you would define a function somewhere, like this...
static bool MyCondition(int x)
{
return x >= 5;
}
...and then use that function as the argument to FindAll:
List<int> a = new List<int>(){0,1,2,1,3,4,5,6,7,8,9};
IEnumerable<int> b = a.FindAll(MyCondition);
foreach (int x in b)
Console.WriteLine(x);
The lambda notation is a shorter notation which allows you to define the function right where you use it.
Likewise, if you wanted to write your second example without using lambda notation, you'd define a function elsewhere, like this...
static int MyOperation(int x)
{
return x * 2;
}
...and pass your function as the argument to Select, like this:
List<int> a = new List<int>(){0,1,2,1,3,4,5,6,7,8,9};
IEnumerable<int> b1 = a.Select(MyOperation);
foreach (int x in b)
Console.WriteLine(x);
Think of it this way:
Mathematics: f(x) = x + x
This is a mathematical function f that takes a number x and spits out its double.
Lambda: f = x => x + x C#'s way of defining the same function f.
Another example:
Mathematics: g(x, y) = x > y
g is a function that takes two numbers x and y and returns wether the former is greater than the latter.
Lambda: g = (x, y) => x > y C#'s way of defining the same function g.
Clearer now?
P.D: I've omitted talking about type inference and the type of the lambda's themselves; its an unnecessary distraction considering the context of this question.
I found a little script that I understand fully. I've got a string with "1 -2 5 40" for example. It reads the input string, splits it into a temporary array. Then this array is parsed and each element is transformed into an integer. The whole thing is order to give the nearest integer to zero.
But what I don't understand is the notation Select(int.Parse). There is no lambda expression here and the method int.Parse isn't called with brackets. Same with the OrderBy(Math.Abs)
Thank you in advance =)
var temps = Console.ReadLine().Split(new []{' '}, StringSplitOptions.RemoveEmptyEntries);
var result = temps.Select(int.Parse)
.OrderBy(Math.Abs)
.ThenByDescending(x => x)
.FirstOrDefault();
int.Parse is a method group - what you're seeing is a method group conversion to a delegate. To see it without LINQ:
Func<string, int> parser = int.Parse;
int x = parser("10"); // x=10
It's mostly equivalent to:
Func<string, int> parser = text => int.Parse(text);
... although there are plenty of differences if you want to go into the details :)
Select(int.Parse) is nearly equivalent to Select(x => int.Parse(x)).
The Select demands an Func<T, R>, which in this case is also the signature of int.Parse (it has a single parameter with a return value). It convers the method group to the matching delegate.
In this case Func<T, R> will map to Func<string, int>, so it matches the int Parse(string) signature.
int.Parse is a method with signature string -> int (or actually, a method group, with different signatures. But the compiler can infer you need this one, because it is the only one that fits.
You could use this method as a parameter wherever you would supply a delegate parameter with the same signature.
The parameter for .Select() is Func<T1, T2>() where T1 is the input parameter (the individual values of temps), and T2 is the return type.
Typically, this is written as a lambda function: x => return x + 1, etc. However, any method that fits the generic definitions can be used without having to be written as a lambda since the method name is the same as assigning the lambda to a variable.
So Func<string, int> parseInt = s => Convert.ToInt32(s); is syntactically equivalent to calling the method int.Parse(s).
The language creates the shortcut of automatically passing the Func parameter to the inside method to create more readable code.
Select LINQ IEnumerable<> extension method signature looks like that:
public static IEnumerable<TResult> Select<TSource, TResult>(
this IEnumerable<TSource> source,
Func<TSource, TResult> selector
)
Look at the selector argument. In your case you pass to Select .Net standard function int.Parse which has signature:
public static int Parse(
string s
)
.Net compiler can convert delegates to Func<...> or Action<...>. In case of int.Parse it can be converted to Func and therefore can be passed as argument to Select method.
Exactly the same with OrderBy. Look at its signature too.
This question already has answers here:
Delegates: Predicate vs. Action vs. Func
(10 answers)
Closed 8 years ago.
With real examples and their use, can someone please help me understand:
When do we need a Func<T, ..> delegate?
When do we need an Action<T> delegate?
When do we need a Predicate<T> delegate?
The difference between Func and Action is simply whether you want the delegate to return a value (use Func) or not (use Action).
Func is probably most commonly used in LINQ - for example in projections:
list.Select(x => x.SomeProperty)
or filtering:
list.Where(x => x.SomeValue == someOtherValue)
or key selection:
list.Join(otherList, x => x.FirstKey, y => y.SecondKey, ...)
Action is more commonly used for things like List<T>.ForEach: execute the given action for each item in the list. I use this less often than Func, although I do sometimes use the parameterless version for things like Control.BeginInvoke and Dispatcher.BeginInvoke.
Predicate is just a special cased Func<T, bool> really, introduced before all of the Func and most of the Action delegates came along. I suspect that if we'd already had Func and Action in their various guises, Predicate wouldn't have been introduced... although it does impart a certain meaning to the use of the delegate, whereas Func and Action are used for widely disparate purposes.
Predicate is mostly used in List<T> for methods like FindAll and RemoveAll.
Action is a delegate (pointer) to a method, that takes zero, one or more input parameters, but does not return anything.
Func is a delegate (pointer) to a method, that takes zero, one or more input parameters, and returns a value (or reference).
Predicate is a special kind of Func often used for comparisons (takes a generic parameter and returns bool).
Though widely used with Linq, Action and Func are concepts logically independent of Linq. C++ already contained the basic concept in form of typed function pointers.
Here is a small example for Action and Func without using Linq:
class Program
{
static void Main(string[] args)
{
Action<int> myAction = new Action<int>(DoSomething);
myAction(123); // Prints out "123"
// can be also called as myAction.Invoke(123);
Func<int, double> myFunc = new Func<int, double>(CalculateSomething);
Console.WriteLine(myFunc(5)); // Prints out "2.5"
}
static void DoSomething(int i)
{
Console.WriteLine(i);
}
static double CalculateSomething(int i)
{
return (double)i/2;
}
}
Func - When you want a delegate for a function that may or may not take parameters and returns a value. The most common example would be Select from LINQ:
var result = someCollection.Select( x => new { x.Name, x.Address });
Action - When you want a delegate for a function that may or may not take parameters and does not return a value. I use these often for anonymous event handlers:
button1.Click += (sender, e) => { /* Do Some Work */ }
Predicate - When you want a specialized version of a Func that evaluates a value against a set of criteria and returns a boolean result (true for a match, false otherwise). Again, these are used in LINQ quite frequently for things like Where:
var filteredResults =
someCollection.Where(x => x.someCriteriaHolder == someCriteria);
I just double checked and it turns out that LINQ doesn't use Predicates. Not sure why they made that decision...but theoretically it is still a situation where a Predicate would fit.
In c# we can create delegates via a variety of means (e.g. Action<>, Func<>, delegate, lambdas, etc). But when you invoke those methods, you have to provide the parameter values for the delegate you are invoking:
delegate int del(int i);
del myDelegate = x => x * x;
int j = myDelegate(5);
Is there a way in c# to encapsulate a method delegate WITH parameter values? Essentially delay invocation of a fully parametrized method? So you don't have to supply parameter values at invocation time?
For example something like this invalid code:
delegate int del(int i);
del myDelegate(5) = x => x * x;
int j = myDelegate;
I'm aware the use case isn't immediately obvious. In the case I'm currently looking at, I have a non-deterministic method that I would like the caller to be able to invoke without having to contain or be aware of the parameters the method needs. One way to achieve this would be via creating a class that encapsulates both the parameter values and the method delegate and have that referenced and invoked by the caller. But I'm just curious if there is an alternate, more succinct way.
This is called currying.
For example:
Action curried = () => myFunc(5);
Or,
Func<int, int, int> multiplier = (x, y) => x * y;
Func<int, int> doubler = x => multiplier(x, 2);
int eight = doubler(4);
Func<int, int> myDelegate = x => x * x;
Func<int> myDelegate5 = () => myDelegate(5);
int j = myDelegate5();
You could always wrap one delegate in another. As SLaks mentioned, this is called currying:
Func<int, int> square = i => i * i;
Func<int> squareFive = () => square(5);
int j = squareFive();
This can be done reasonably nicely without lambdas, with the aid of some generic classes and helper functions. I used such an approach in some vb.net/vs2005 code. If the goal is to yield a MethodInvoker which calls a function with three arguments of types T, U, and V, then create a class ParamInvoker<T,U,V> which holds fields param1, param2, and param3 (as types T, U, and V), and Action (of type Action<T,U,V>) and has a method DoIt(void) which calls Action(param1, param2, param3). The generic classes and helper functions get repetitive, but the syntax is pretty nice. For example (vb syntax, from memory, and C# syntax, guessing):
TheMethodInvoker = MakeParamInvoker(AddressOf MyFunction, 5, "Hello")
or
TheMethodInvoker = MakeParamInvoker(MyFunction, 5, "Hello")
assuming MyFunction takes an Integer and a String.
I read This article and i found it interesting.
To sum it up for those who don't want to read the entire post. The author implements a higher order function named Curry like this (refactored by me without his internal class):
public static Func<T1, Func<T2, TResult>>
Curry<T1, T2, TResult>(this Func<T1, T2, TResult> fn)
{
Func<Func<T1, T2, TResult>, Func<T1, Func<T2, TResult>>> curry =
f => x => y => f(x, y);
return curry(fn);
}
That gives us the ability to take an expression like F(x, y)
eg.
Func<int, int, int> add = (x, y) => x + y;
and call it in the F.Curry()(x)(y) manner;
This part i understood and i find it cool in a geeky way. What i fail to wrap my head around is the practical usecases for this approach. When and where this technique is necessary and what can be gained from it?
Thanks in advance.
Edited:
After the initial 3 responses i understand that the gain would be that in some cases when we create a new function from the curried some parameters are not re evalued.
I made this little test in C# (keep in mind that i'm only interested in the C# implementation and not the curry theory in general):
public static void Main(string[] args)
{
Func<Int, Int, string> concat = (a, b) => a.ToString() + b.ToString();
Func<Int, Func<Int, string>> concatCurry = concat.Curry();
Func<Int, string> curryConcatWith100 = (a) => concatCurry(100)(a);
Console.WriteLine(curryConcatWith100(509));
Console.WriteLine(curryConcatWith100(609));
}
public struct Int
{
public int Value {get; set;}
public override string ToString()
{
return Value.ToString();
}
public static implicit operator Int(int value)
{
return new Int { Value = value };
}
}
On the 2 consecutive calls to curryConcatWith100 the ToString() evaluation for the value 100 is called twice (once for each call) so i dont see any gain in evaluation here. Am i missing something ?
Currying is used to transform a function with x parameters to a function with y parameters, so it can be passed to another function that needs a function with y parameters.
For example, Enumerable.Select(this IEnumerable<T> source, Func<TSource, bool> selector) takes a function with 1 parameter. Math.Round(double, int) is a function that has 2 parameters.
You could use currying to "store" the Round function as data, and then pass that curried function to the Select like so
Func<double, int, double> roundFunc = (n, p) => Math.Round(n, p);
Func<double, double> roundToTwoPlaces = roundFunc.Curry()(2);
var roundedResults = numberList.Select(roundToTwoPlaces);
The problem here is that there's also anonymous delegates, which make currying redundant. In fact anonymous delegates are a form of currying.
Func<double, double> roundToTwoPlaces = n => Math.Round(n, 2);
var roundedResults = numberList.Select(roundToTwoPlaces);
Or even just
var roundedResults = numberList.Select(n => Math.Round(n, 2));
Currying was a way of solving a particular problem given the syntax of certain functional languages. With anonymous delegates and the lambda operator the syntax in .NET is alot simpler.
Its easier to first consider fn(x,y,z). This could by curried using fn(x,y) giving you a function that only takes one parameter, the z. Whatever needs to be done with x and y alone can be done and stored by a closure that the returned function holds on to.
Now you call the returned function several times with various values for z without having to recompute the part the required x and y.
Edit:
There are effectively two reasons to curry.
Parameter reduction
As Cameron says to convert a function that takes say 2 parameters into a function that only takes 1. The result of calling this curried function with a parameter is the same as calling the original with the 2 parameters.
With Lambdas present in C# this has limited value since these can provide this effect anyway. Although it you are use C# 2 then the Curry function in your question has much greater value.
Staging computation
The other reason to curry is as I stated earlier. To allow complex/expensive operations to be staged and re-used several times when the final parameter(s) are supplied to the curried function.
This type of currying isn't truely possible in C#, it really takes a functional language that can natively curry any of its functions to acheive.
Conclusion
Parameter reduction via the Curry you mention is useful in C# 2 but is considerably de-valued in C# 3 due to Lambdas.
In a sense, curring is a technique to
enable automatic partial application.
More formally, currying is a technique
to turn a function into a function
that accepts one and only one
argument.
In turn, when called, that function
returns another function that accepts
one and only one argument . . . and so
on until the 'original' function is
able to be executed.
from a thread in codingforums
I particularly like the explanation and length at which this is explained on this page.
One example: You have a function compare(criteria1, criteria2, option1, option2, left, right). But when you want to supply the function compare to some method with sorts a list, then compare() must only take two arguments, compare(left, right). With curry you then bind the criteria arguments as you need it for sorting this list, and then finally this highly configurable function presents to the sort algorithm as any other plain compare(left,right).
Detail: .NET delegates employ implicit currying. Each non-static member function of a class has an implicit this reference, still, when you write delegates, you do not need to manually use some currying to bind this to the function. Instead C# cares for the syntactic sugar, automatically binds this, and returns a function which only requires the arguments left.
In C++ boost::bind et al. are used for the same. And as always, in C++ everything is a little bit more explicit (for instance, if you want to pass a instance-member function as a callback, you need to explicitly bind this).
I have this silly example:
Uncurry version:
void print(string name, int age, DateTime dob)
{
Console.Out.WriteLine(name);
Console.Out.WriteLine(age);
Console.Out.WriteLine(dob.ToShortDateString());
Console.Out.WriteLine();
}
Curry Function:
public Func<string, Func<int, Action<DateTime>>> curry(Action<string, int, DateTime> f)
{
return (name) => (age) => (dob) => f(name, age, dob);
}
Usage:
var curriedPrint = curry(print);
curriedPrint("Jaider")(29)(new DateTime(1983, 05, 10)); // Console Displays the values
Have fun!
here's another example of how you might use a Curry function. Depending on some condition (e.g. day of week) you could decide what archive policy to apply before updating a file.
void ArchiveAndUpdate(string[] files)
{
Func<string, bool> archiveCurry1 = (file) =>
Archive1(file, "archiveDir", 30, 20000000, new[] { ".tmp", ".log" });
Func<string, bool> archiveCurry2 = (file) =>
Archive2("netoworkServer", "admin", "nimda", new FileInfo(file));
Func<string, bool> archvieCurry3 = (file) => true;
// backup locally before updating
UpdateFiles(files, archiveCurry1);
// OR backup to network before updating
UpdateFiles(files, archiveCurry2);
// OR do nothing before updating
UpdateFiles(files, archvieCurry3);
}
void UpdateFiles(string[] files, Func<string, bool> archiveCurry)
{
foreach (var file in files)
{
if (archiveCurry(file))
{
// update file //
}
}
}
bool Archive1(string fileName, string archiveDir,
int maxAgeInDays, long maxSize, string[] excludedTypes)
{
// backup to local disk
return true;
}
bool Archive2(string sereverName, string username,
string password, FileInfo fileToArchvie)
{
// backup to network
return true;
}