Linq select and add multiple values from the same object

Question

Let's say I have a class with two numerical properties and I create a bunch of them.

public class TwoNumbers
{
    public TwoNumbers(int first, int second)
    {
        First = first;
        Second = second;
    }

    public int First { get; set; }
    public int Second { get; set; }
}

Is there a way using LINQ or lambda queries to add both First and Second of each object to a list of integers? For example:

List<TwoNumbers> twoNumbersList = new List<TwoNumbers>()
{
    new TwoNumbers(1,1),
    new TwoNumbers(32,59),
    new TwoNumbers(-12,200),
}

// Can I populate a list of integers upon assignment with a LINQ or lambda query?
List<int> integersVerbose = new List<int>()
foreach (var tn in twoNumbersList)
{
    integersVerbose .Add(tn.First);
    integersVerbose .Add(tn.Second);
}

// Desired syntax
var integersConcise = twoNumbersList. /* some expression */ .ToList(); // Automatically infers type from query

// Print the numbers
integersVerbose.Foreach(x => Console.WriteLine(x));
integersConcise.Foreach(x => Console.WriteLine(x));

// Expected output:
// 1
// 1
// 32
// 59
// -12
// 200 
// 1
// 1
// 32
// 59
// -12
// 200

I am aware that there's a SelectMany method but it doesn't seem useful here because I'm not trying to extract a collection from an object, I just want a flat list with numbers.

stuartd · Accepted Answer · 2020-08-27 12:32:59Z

2

You can use SelectMany if you project the two numbers into a collection:

List<TwoNumbers> twoNumbersList = new List<TwoNumbers> {
    new TwoNumbers(1,1),
    new TwoNumbers(32,59),
    new TwoNumbers(-12,200),
};

var results = twoNumbersList.SelectMany(nl => new[] { nl.First, nl.Second });

Output:

1
1
32
59
-12
200

answered Aug 27, 2020 at 12:32

stuartd

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Comments

Johnathan Barclay · Accepted Answer · 2020-08-27 12:40:25Z

2

You could use SelectMany like this:

var integersConcise = twoNumbersList.SelectMany(x => new[] { x.First, x.Second });

And to avoid the array allocations you could create an IEnumerable property on your TwoNumbers class:

public class TwoNumbers
{
    //...

    public IEnumerable<int> Both { get { yield return First; yield return Second; } }
}

var integersConcise = twoNumbersList.SelectMany(x => x.Both);

Or even implement IEnumerable<int>:

public class TwoNumbers : IEnumerable<int>
{
    //...

    public IEnumerator<int> GetEnumerator()
    {
        yield return First;
        yield return Second;
    }

    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

var integersConcise = twoNumbersList.SelectMany(x => x);

edited Aug 27, 2020 at 12:40

answered Aug 27, 2020 at 12:35

Johnathan Barclay

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Comments

jdweng · Accepted Answer · 2020-08-27 12:39:29Z

0

Try GroupBy

            List<int> inputs = new List<int> { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

            List<List<int>> outputs = inputs
                .Select((x, i) => new { x = x, i = i })
                .GroupBy(x => x.i / 2)
                .Select(x => new List<int>() {x.First().x, x.Last().x})
                .ToList();

answered Aug 27, 2020 at 12:39

jdweng

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1 Comment

JansthcirlU Over a year ago

You have it the other way around, I'm starting with a list of TwoNumbers and I want to make a flat list of integers from it.

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