Lambda expression for custom sort of list

Question

I have a list of MyClass and one of its properties is Position. Initially, the positions could be like this:

2,-3,1,3,4,5,-2,-1,

Using the Sort (NOT .OrderBy) method, I need a lambda expression that will result in the above numbers being ordered from lowest positive number and then from lowest negative number, so the output would be this:

1,2,3,4,5,-1,-2,-3

I'm new to lambda expressions, I got this far: myList.Sort((x, y) => x.Position.CompareTo(y.Position)) but it doesn't quite do what I want.

Cheers!

Aditya Santoso · Accepted Answer · 2015-09-15 07:29:14Z

4

When x.Position and y.Position are both positive or both negative, you can order them by their absolute value. Otherwise, use the inverted value of CompareTo function

myList.Sort((x,y) => x.Position > 0 && y.Position > 0 || x.Position < 0 && y.Position < 0 ? 
                      Math.Abs(x.Position).CompareTo(Math.Abs(y.Position)) :
                      -x.Position.CompareTo(y.Position));

edited Sep 15, 2015 at 7:29

answered Sep 15, 2015 at 4:31

Aditya Santoso

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1 Comment

Yabbie Over a year ago

-x.CompareTo should be -x.Position.CompareTo. Once I made that change, it worked PERFECTLY! Thank you!

Denis · Accepted Answer · 2015-09-15 04:33:25Z

2

static int Cmp(int x, int y)
{
    if (x > 0 && y < 0)
    {
        return -1;
    }

    if (x < 0 && y > 0)
    {
        return 1;
    }

    if (x < 0 && y < 0)
    {
        return y.CompareTo(x);
    }

    return x.CompareTo(y);
}
//---
List<int> list = new List<int> { 2, -3, 1, 3, 4, 5, -2, -1 };
list.Sort(Cmp);

edited Sep 15, 2015 at 4:33

answered Sep 15, 2015 at 4:27

Denis

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Comments

123 456 789 0 · Accepted Answer · 2015-09-15 04:37:43Z

You need to implement your own custom IComparable to your MyClass and implement CompareTo. More information can be found here. Here's a direct sample without using IComparable.

using System;
using System.Linq;
using System.Collections.Generic;
public class Test
{
    public static void Main()
    {
        var myList = new List<int>() { 2,-3,1,3,4,5,-2,-1,};
        myList.Sort(Compare);
        foreach(var item in myList){
            Console.WriteLine(item);
        }
    }

    static int Compare(int x, int y)
    {
     if (x > 0 && y < 0)
     {
        return -1;
     }

     if (x < 0 && y > 0)
     {
        return 1;
     }

     if (x < 0 && y < 0)
     {
        return y.CompareTo(x);
     }
     return x.CompareTo(y);
    }
}

For objects A, B and C, the following must be true: A.CompareTo(A) must return zero. If A.CompareTo(B) returns zero, then B.CompareTo(A) must return zero. If A.CompareTo(B) returns zero and B.CompareTo(C) returns zero, then A.CompareTo(C) must return zero. If A.CompareTo(B) returns a value other than zero, then B.CompareTo(A) must return a value of the opposite sign. If A.CompareTo(B) returns a value x not equal to zero, and B.CompareTo(C) returns a value y of the same sign as x, then A.CompareTo(C) must return a value of the same sign as x and y. Notes to Callers Use the CompareTo method to determine the ordering of instances of a class.

ravi chandra · Accepted Answer · 2017-03-28 13:34:35Z

 class ListOperations
{
    public static void Main(string[] args)
    {
        List<Book> books = new List<Book>
       {
           new Book { Name="book1",ISBN= 554654,Category= "Mathematics",Price=12.45,Index=1},
           new Book { Name="book2",ISBN= 454654, Category="English", Price=-6.45, Index=3},
           new Book {Name="book3",ISBN= 754654, Category="English", Price=7.45, Index=4 },
           new Book { Name = "book4", ISBN = 854654,Category= "History", Price=8.45,Index= 5 },
           new Book { Name = "book5", ISBN = 154654, Category="Mathematics", Price=17.45, Index=2 },
           new Book { Name = "book6", ISBN = 354654, Category="History", Price=-15.45, Index=6 },
           new Book { Name = "book7", ISBN = 354653, Category="History", Price=0, Index=6 }
        };
        books.Sort((x, y) =>( x.Price > 0 && y.Price > 0 || x.Price < 0 && y.Price < 0 )? Math.Abs(x.Price).CompareTo(Math.Abs(y.Price)) : -x.Price.CompareTo(y.Price));
    }

    class Book
    {
        public string Name { get; set; }
        public int ISBN { get; set; }
        public string Category { get; set; }
        public double Price { get; set; }
        public double Index { get; set; }
        public int RowNum { get; set; }
    }
}

Hari Prasad · Accepted Answer · 2015-09-15 04:35:42Z

-2

This code works for your case.

var values = new[] {2,-3,1,3,4,5,-2,-1};
var solution = values.GroupBy(i => i > 0)
    .SelectMany(g => g.Key? g.OrderBy(c=>c) : g.OrderByDescending(c=>c))
    .ToArray();

Working sample attached here.

answered Sep 15, 2015 at 4:35

Hari Prasad

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5 Comments

123 456 789 0 Over a year ago

Your code won't work. It will produce the output 2 -3 1 3 4 5 -2

Hari Prasad Over a year ago

@lll Can you check sample that I have attached ?

123 456 789 0 Over a year ago

I did. it doesn't compile.

Hari Prasad Over a year ago

@lll What do you mean it doesn't compile? it is working. I don't see it is fair to down vote without even verifying the code and links.

123 456 789 0 Over a year ago

I did verify the code and link. Also, it was broken so you must have fixed it and I didn't down vote.

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