C#•2w ago

LINQ help

I have a collection of ints, how can I take the last three that are bigger than 100? is there a better way than this?

using System;
using System.Linq;

class Program
{
    static void Main()
    {
        int[] numbers = { 80, 120, 110, 90, 10, 140, 150, 50, 22, -800 };
 
        var result = numbers.Where(num => num > 100).Reverse().Take(3).Reverse().ToArray();

        Console.WriteLine("Last 3 elements over 100:");
        foreach (var num in result)
        {
            Console.WriteLine(num);
        }
    }
}

using System;
using System.Linq;

class Program
{
    static void Main()
    {
        int[] numbers = { 80, 120, 110, 90, 10, 140, 150, 50, 22, -800 };
 
        var result = numbers.Where(num => num > 100).Reverse().Take(3).Reverse().ToArray();

        Console.WriteLine("Last 3 elements over 100:");
        foreach (var num in result)
        {
            Console.WriteLine(num);
        }
    }
}

17 Replies

Jason_Bjorn•2w ago

reversing the entire numbers array to take the first (last then) 3 seems kinda ineffecient

daniel2•2w ago

I suggest u use for loop reversely

Alex_is_coding•2w ago

I was curious to try, and got this

            int[] numbers = { 80, 120, 110, 90, 10, 140, 150, 50, 22, -800 };

            IOrderedEnumerable<int> sortedNum = numbers.OrderDescending();
            string aStr = string.Empty;
            for(int i = 0; i <= 2; i++)
            {
                if (sortedNum.ElementAt(i) >= 100)
                {
                    aStr += sortedNum.ElementAt(i).ToString() + "\r\n";
                }
            }
            MessageBox.Show( aStr );

            int[] numbers = { 80, 120, 110, 90, 10, 140, 150, 50, 22, -800 };

            IOrderedEnumerable<int> sortedNum = numbers.OrderDescending();
            string aStr = string.Empty;
            for(int i = 0; i <= 2; i++)
            {
                if (sortedNum.ElementAt(i) >= 100)
                {
                    aStr += sortedNum.ElementAt(i).ToString() + "\r\n";
                }
            }
            MessageBox.Show( aStr );

Jason_Bjorn•2w ago

Why sort them seems like a waste of time? Just traverse in reverse then and pick until you have 3 or reach end (start) like this?

int[] numbers = [80, 120, 110, 90, 10, 140, 150, 50, 22, -800];

int[] threeOver100 = new int[3];
int insertIndex = threeOver100.Length - 1;

for (int i = numbers.Length - 1; i >= 0; i--)
{
    if (numbers[i] > 100)
    {
        threeOver100[insertIndex] = numbers[i];
        insertIndex--;
        if (insertIndex < 0)
        {
            break;
        }
    }
}

Array.ForEach(threeOver100, Console.WriteLine);

int[] numbers = [80, 120, 110, 90, 10, 140, 150, 50, 22, -800];

int[] threeOver100 = new int[3];
int insertIndex = threeOver100.Length - 1;

for (int i = numbers.Length - 1; i >= 0; i--)
{
    if (numbers[i] > 100)
    {
        threeOver100[insertIndex] = numbers[i];
        insertIndex--;
        if (insertIndex < 0)
        {
            break;
        }
    }
}

Array.ForEach(threeOver100, Console.WriteLine);

2spooky2play•2w ago

ForEach really sucks, dont use it

Jason_Bjorn•2w ago

can you elaborate?

2spooky2play•2w ago

the last Reverse().ToArray() seems unnecessary its a long story, but 1) its slower (iirc) 2) it requires an allocated list (sometimes you dont need to allocate) var result = numbers.Where(num => num > 100).Reverse().Take(3) is all you need

Jason_Bjorn•2w ago

but isn't reverse expensive? sound like O(n) more space when I only need O(3) more space

2spooky2play•2w ago

linq is lazy, so i doubt it allocates

Jason_Bjorn•2w ago

any way to know for sure?

2spooky2play•2w ago

ask someone in #advanced maybe?

Jason_Bjorn•2w ago

okay

TheBoxyBear•2w ago

My guess would be to put Reverse first. Linq also has a lot of shortcuts for common collection types and I wouldn't be surprised if Reverse didn't simply enumerate with a decreasing index for arrays. It's always possible to look into the underlying code of .NET to confirm for yourself, in this case by looking at the body of Reverse

Angius•2w ago

If you don't want to reverse:

var result = nubers
    .Where(n => n > 100)
    .Skip(numbers.Length - 3);

var result = nubers
    .Where(n => n > 100)
    .Skip(numbers.Length - 3);

Joschi•2w ago

Reverse() indeed completely enumerates the IEnumerable<T> and copies the result into a new T[]. It then returns the elements of the newly created array in reverse order.

Jason_Bjorn•2w ago

Yeah, we talked about it in #advanced @Joschi

Joschi•2w ago

Did they by any chance elaborate on why? Because for IList<T> implementations shouldn't it be possible to just yield the content in reverse order using the index?

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