CODEWITHSUNDEEP
c#recursionlambdafactorial
Andreas Grech
Andreas Grech

Reputation: 107950

C#: Recursive functions with Lambdas

The below does not compile:

Func<int, int> fac = n => (n <= 1) ? 1 : n * fac(n - 1);

Local variable 'fac' might not be initialized before accessing

How can you make a recursive function with lambdas?

[Update]

Here are also two links that I found interesting to read:

  1. Eric Lippert's "Why does a recursive lambda cause a definite assignment error?"
  2. Anonymous Recursion in C#

Upvotes: 29

Views: 8795

Answers (3)

user1859022
user1859022

Reputation: 2685

Since C# 7.0 you finally can do a similar thing by using a local function instead of a lambda.

int fac(int n) => (n <= 1) ? 1 : n * fac(n - 1);

Upvotes: -1

Dario
Dario

Reputation: 49218

You'll have to create fac first und assign it later (which is pretty unfunctional because it depends on multiple assignment) or use so called Y-combinators.

Example:

delegate Func<TIn, TOut> FixedPointFunction<TIn, TOut>(Func<TIn, TOut> f);

static Func<T, TRes> Fix<T, TRes>(FixedPointFunction<T, TRes> f) {
    return f(x => Fix(f)(x));
}

static void Main(string[] args) {

    var fact = Fix<int, int>(f => x => (x <= 1) ? x : x * f(x - 1));

    Console.WriteLine(fact(5));            
}

But note that this might be somewhat hard to read/understand.

Upvotes: 12

JaredPar
JaredPar

Reputation: 754715

This particular style of function is not supported by C# as a single line declaration. You have to separate out the declaration and definition into 2 lines

Func<int, int> fac = null;
fac = n => (n <= 1) ? 1 : n * fac(n - 1);

Upvotes: 50

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