using System;

using System.Collections.Generic;

public class Program

{

    // Problem 1: Implement the method Map (without using library methods).

    //

    // Map applies a function "f" to each element of "list" and returns the resulting list.

    //

    // Some equations it should satisfy:

    // Map([1,2,3], x => x + 1) = [2,3,4]

    // Map([1,2,3], x => x.ToString()) = ["1","2","3"]

    // Map([], ...) == []

    List<B> Map<A, B>(List<A> list, Func<A, B> f)

    {

        var res = new List<B>();

        foreach(A item in list)

        {

            res.Add(f(item));

        }

        return res;

    }

​

    // Problem 2: Implement the method Fold (without using library methods).

    //

    // Fold applies a function "f" to each element of "list", but in addition to the current element

    // the function accepts a state value, starting with "init", and subsequently,

    // the state returned by the previous call to "f".

    //

    // Some equations it should satisfy:

    // Fold([1,2,3], 0, (sum,x) => sum + x) == 6

    // Fold([1,2,3], "", (str,x) => str + x.ToString()) == "123"

    // Fold([], init, ...) == init

    B Fold<A, B>(List<A> list, B init, Func<B, A, B> f)

    {   

        foreach(A item in list)

        {

            init = f(init, item);

        }

        return init;

    }

    // Problem 3: implement Map in terms of Fold. In other words, implement

    // Map by calling Fold with appropriate parameters.

    List<B> Map2<A, B>(List<A> list, Func<A, B> f)

    {

        return Fold(list, new List<B>(), (lst, x) => {

            lst.Add(f(x));

            return lst;

        });

    }

​

​

    public static void Main()

    {

        Console.WriteLine("Hello World");

    }

}