Get the number from list using LINQ with specific conditions

Question

I have a list of numbers, I would like to get the next >= number than the one I pass, but there's a problem when I pass value that's bigger to the biggest in list. For example when I pass 41 and the biggest in list is 40 it won't work so I'd like it to return 40.

var numbers = new[] {30, 20, 40};

I would like it to work like:

numbers.GetNearest(45) -> 40
numbers.GetNearest(40) -> 40
numbers.GetNearest(31) -> 40
numbers.GetNearest(30) -> 30
numbers.GetNearest(29) -> 30
numbers.GetNearest(1) -> 20

Is this possible with LINQ alone, or is there any well-known algorithm for this?

What I have now is something like:

numbers.OrderBy(n => n).FirstOrDefault(n => n >= minute)

but it doesn't work when I pass value bigger than 40

I don't want the closest number, but the next one >= than the one I pass, but for certain numbers bigger than the biggest in list won't work so I'd like to return just the last one.

Your title talks about "specific conditions" but you don't describe what those are. We can *maybe* guess from the code that you've written, but as you've said that doesn't work, it's hard to say for sure. Please provide more details about what you're trying to achieve. — Jon Skeet, Sep 28 '18 at 09:27
Possible duplicate of [How to get the closest number from a List with LINQ?](https://stackoverflow.com/questions/5953552/how-to-get-the-closest-number-from-a-listint-with-linq) — FCin, Sep 28 '18 at 09:31
Do you want the nearest bigger number and in case it does not exist then the nearest smaller number? That is the only way your cae of 31->40 and 45 -> 40 can be explained? — peeyush singh, Sep 28 '18 at 09:33
What is there is no items in `numbers` array? There will be no last one number. — apocalypse, Sep 28 '18 at 09:47
@apocalypse fair point, I can just check if it's empty first and return 0, but it depends on the case. — Konrad, Sep 28 '18 at 09:50

fubo · Accepted Answer · 2018-09-28T09:39:58.287

5

Define that special case with DefaultIfEmpty()

var numbers = new[] { 30, 20, 40 };
Array.Sort(numbers);
int result = numbers.Where(n => n >= minute).DefaultIfEmpty(numbers.Last()).First();

Testcases: https://dotnetfiddle.net/nIEsRO

edited Sep 28 '18 at 09:39

answered Sep 28 '18 at 09:32

fubo

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This is not pure `Linq`. – apocalypse Sep 28 '18 at 09:41
1

@apocalypse since the input is a array, I'd prefer `Array.Sort(numbers);` over `numbers = numbers.OrderBy(x => x).ToArray();` because it's faster – fubo Sep 28 '18 at 09:43
@fubo: Yeah I know. But what if we have array of 1,000,000 numbers? What if we don't want to modify input array? Or we don't have array but IEnumerable? – apocalypse Sep 28 '18 at 09:46
2

The answer is [DefaultIfEmpty](https://learn.microsoft.com/en-us/dotnet/api/system.linq.queryable.defaultifempty?view=netframework-4.7.2). The fact that there is an `Array.Sort` is irrevelant. – Drag and Drop Sep 28 '18 at 09:49

TheGeneral · Answer 2 · 2018-09-28T09:47:05.750

3

Update

var numbers = new[]{30, 20, 40};
var number = 25;
var closest = numbers.OrderBy(n => n)
                     .Cast<int?>()
                     .FirstOrDefault(n => n >= number) ?? numbers.Last();
Console.WriteLine(closest);

Note : The Cast is just to handle 0 due to default(int) maybe a better way to do this though

output

Full Demo Here

Original

You could just order by the difference

var closest = numbers.OrderBy(x => Math.Abs((long)x - number))
                     .First();

edited Sep 28 '18 at 09:47

answered Sep 28 '18 at 09:27

TheGeneral

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1

what if I want it to return 30 when I pass 21,22,23 or 24? – Konrad Sep 28 '18 at 09:29
1

@Konrad why would you want that? That's not the nearest number. Update your question and explain what you actually want – Panagiotis Kanavos Sep 28 '18 at 09:30
Then that is not the nearest – TheGeneral Sep 28 '18 at 09:30
@PanagiotisKanavos I don't know the term for that – Konrad Sep 28 '18 at 09:30
@Konrad the term for *what*? In any case, if you want an arbitrary condition pass a different lambda to `OrderBy` – Panagiotis Kanavos Sep 28 '18 at 09:30
1

I think it's unfair to downvote this given that is answered the original question. – DavidG Sep 28 '18 at 09:39
1

Having said that, the `Cast` in the update is absolutely horrible. – DavidG Sep 28 '18 at 09:42
@DavidG Yeah i guess your right, i have pizza though :P – TheGeneral Sep 28 '18 at 09:45
Though default for an int = 0 – TheGeneral Sep 28 '18 at 09:45
1

Recommending awful code means you should have that pizza taken off you. You should know better... – DavidG Sep 28 '18 at 09:47

score 1 · Answer 3 · answered Sep 28 '18 at 10:17

You didn't provide enough details about specific conditions, so I can only give you this answer:

var numbers = new[] {30, 20, 40};

var ordered = numbers.Select(x => (int?)x).OrderBy(x => x);

int? value  = ordered.FirstOrDefault(x => x >= 41) ?? ordered.LastOrDefault();

score 0 · Answer 4 · answered Sep 28 '18 at 11:08

Someone posted an answer with Aggregate but not working exactly how I wanted it to work but deleted it. So I came up with my own solution using Aggregate which should be faster than using OrderBy first I think:

numbers.Aggregate((x, y) =>
{
    var min = Math.Min(x, y);
    return number <= min ? min : Math.Max(x, y);
});

It took me a while to figure it out but it solves my problem. If you can see any flaws in it, let me know.

Get the number from list using LINQ with specific conditions

4 Answers4