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Is it possible to always take 3 objects and if only 2 exists it returns 3 but one has null values in?

I need to return the 3 latest elements in a collection... If use Linq e.g. .OrderByDescending(a => a.Year).Take(3) then this is fine as long as the collection contains at least 3 elements. What I want is for it always to return 3, so for example if there are only 2 items then the last item would be a blank/initialised element (ideally where I could configure what was returned)
Is this possible?

You can concatenate the sequence with another (lazily created) sequence of 3 elements:
var result = query
.OrderByDescending(a => a.Year)
.Concat(Enumerable.Range(0, 3).Select(_ => new ResultElement()))
.Take(3);
Or perhaps:
var result = query
.OrderByDescending(a => a.Year)
.Concat(Enumerable.Repeat(new ResultElement(), 3))
.Take(3);
(The latter will end up with duplicate references and will always create an empty element, so I'd probably recommend the former... but it depends on the context. You might want to use Enumerable.Repeat(null, 3) and handle null elements instead.)

You could write your own extension method:
public static IEnumerable<T> TakeAndCreate<T>(this IEnumerable<T> input, int amount, Func<T> defaultElement)
{
int counter = 0;
foreach(T element in input.Take(amount))
{
yield return element;
counter++;
}
for(int i = 0; i < amount - counter; i++)
{
yield return defaultElement.Invoke();
}
}
Usage is
var result = input.OrderByDescending(a => a.Year).TakeAndCreate(3, () => new ResultElement());
One advantage of this solution is that it will create new elements only if they are acutally needed, which might be good for performance if you have a lot of elements to be created or their creation is not trivial.
Online demo: https://dotnetfiddle.net/HHexGd

How to filter a list<int> by comparing against a single int variable?

I have a list<int> with values like 10, 20, 30, 56. I also have a local variable int _marks = 30.
How can I print out the values in the list that are less than the value of _marks?

You can use Where() from System.Linq namespace to filter the array. It returns IEnumerable<int> object. And for printing the elements in this collection we can use List<T>.ForEach method. It performs the specified action on each element of the List<T>. And in case of single argument you can pass function by itself:
marks.Where(x => x < _marks).ToList().Foreach(Console.WriteLine);
By the way if you are newbie you can use non-LINQ solution also:
foreach(int item in marks)
{
if(item < _marks)
Console.WriteLine(item);
}
Also, as #Kjartan said, if the list is ordered, then it may be good option to use TakeWhile() or SkipWhile() functions to get desired elements:
// If the list is ordered in ascending order
marks.TakeWhile(x => x < _marks).ToList().Foreach(Console.WriteLine);
// If the list is ordered in descending order
marks.SkipWhile(x => x >= _marks).ToList().Foreach(Console.WriteLine);

You've got several options here. A couple of examples:
var yourList = new List<int>{10, 20, 30, 40, 50, 56, 60, 70};
var _marks = 55;
// Get a IEnumerable containing values matching your condition ( ie. < _marks):
var selected = yourList.Where(i => i < _marks);
Alternative if you know the values are sorted by increasing values (this will avoid iterating through the whole list unnecessarily):
var selected = yourList.TakeWhile(i => i < _marks);
To print, do e.g.:
selected.ToList().ForEach(i => Console.WriteLine(i));

Using LINQ to take the top 100 and bottom 100?

I would like to do something like this (below) but not sure if there is a formal/optimized syntax to do so?
.Orderby(i => i.Value1)
.Take("Bottom 100 & Top 100")
.Orderby(i => i.Value2);
basically, I want to sort by one variable, then take the top 100 and bottom 100, and then sort those results by another variable.
Any suggestions?

var sorted = list.OrderBy(i => i.Value);
var top100 = sorted.Take(100);
var last100 = sorted.Reverse().Take(100);
var result = top100.Concat(last100).OrderBy(i => i.Value2);
I don't know if you want Concat or Union at the end. Concat will combine all entries of both lists even if there are similar entries which would be the case if your original list contains less than 200 entries. Union would only add stuff from last100 that is not already in top100.
Some things that are not clear but that should be considered:
If list is an IQueryable to a db, it probably is advisable to use ToArray() or ToList(), e.g.
var sorted = list.OrderBy(i => i.Value).ToArray();
at the beginning. This way only one query to the database is done while the rest is done in memory.
The Reverse method is not optimized the way I hoped for, but it shouldn't be a problem, since ordering the list is the real deal here. For the record though, the skip method explained in other answers here is probably a little bit faster but needs to know the number of elements in list.
If list would be a LinkedList or another class implementing IList, the Reverse method could be done in an optimized way.

You can use an extension method like this:
public static IEnumerable<T> TakeFirstAndLast<T>(this IEnumerable<T> source, int count)
{
var first = new List<T>();
var last = new LinkedList<T>();
foreach (var item in source)
{
if (first.Count < count)
first.Add(item);
if (last.Count >= count)
last.RemoveFirst();
last.AddLast(item);
}
return first.Concat(last);
}
(I'm using a LinkedList<T> for last because it can remove items in O(1))
You can use it like this:
.Orderby(i => i.Value1)
.TakeFirstAndLast(100)
.Orderby(i => i.Value2);
Note that it doesn't handle the case where there are less then 200 items: if it's the case, you will get duplicates. You can remove them using Distinct if necessary.

Take the top 100 and bottom 100 separately and union them:
var tempresults = yourenumerable.OrderBy(i => i.Value1);
var results = tempresults.Take(100);
results = results.Union(tempresults.Skip(tempresults.Count() - 100).Take(100))
.OrderBy(i => i.Value2);

You can do it with in one statement also using this .Where overload, if you have the number of elements available:
var elements = ...
var count = elements.Length; // or .Count for list
var result = elements
.OrderBy(i => i.Value1)
.Where((v, i) => i < 100 || i >= count - 100)
.OrderBy(i => i.Value2)
.ToArray(); // evaluate
Here's how it works:
| first 100 elements | middle elements | last 100 elements |
i < 100 i < count - 100 i >= count - 100

You can write your own extension method like Take(), Skip() and other methods from Enumerable class. It will take the numbers of elements and the total length in list as input. Then it will return first and last N elements from the sequence.
var result = yourList.OrderBy(x => x.Value1)
.GetLastAndFirst(100, yourList.Length)
.OrderBy(x => x.Value2)
.ToList();
Here is the extension method:
public static class SOExtensions
{
public static IEnumerable<T> GetLastAndFirst<T>(
this IEnumerable<T> seq, int number, int totalLength
)
{
if (totalLength < number*2)
throw new Exception("List length must be >= (number * 2)");
using (var en = seq.GetEnumerator())
{
int i = 0;
while (en.MoveNext())
{
i++;
if (i <= number || i >= totalLength - number)
yield return en.Current;
}
}
}
}

Simplest way to check db values against values in an array

I'm looping through the items in my database using C# .NET and I'm attempting to display different data dependant on if a column value matches any of the values in an array. Because my array could potentially have hundreds of values, I'd rather not create hundreds of different IF statements, if possible. Is there a simpler way to achieve this?
Here's some example code, where "Items" is my db data and "Categories" is a column of said data:
var categoryList = new List<int> { 1, 2, 3, 4 };
foreach(var item in Items){
if(item.Categories.Any(x => #categoryList.Equals(x))){
<p>Yes</p>
}else{
<p>No</p>
}
}

The answer I give is based on the answer of this question. I modified the code to your situation.
foreach(var item in Items)
{
bool hasCategory = categoryList.Any(x => item.Categories.Any(c => c.Id == x));
}
or for larger collections (performance-wise):
bool hasCategory = item.Categories.Select(c => c.Id)
.Intersect(categoryList)
.Any();
Edit:
At first I thought item.Categories was a collection of IDs or something but then I started doubting. If item.Categories is just a single integer, following code will work:
foreach(var item in Items)
{
if(categoryList.Any(x => x == item.Categories))
<p>Yes</p>
else
<p>No</p>
}

LINQ changing local "let" variable

Using a LINQ query (with C#) how would I go about do something like this (pseudocode)?
I'd look to do something like this is in places where, for example, I might generate 1000's of lists of 100's of random (bounded) integers, where I want to track the smallest of them as they're generated.
Best <- null value
Foreach N in Iterations
NewList <- List of 100 randomly generated numbers
If Best is null
Best <- NewList
If Sum(NewList) < Sum(Best)
Best <- NewList
Select Best
I've tried all sorts of things, but I can't really get it working. This isn't for any kind of project or work, just for my own curiosity!
Example of what I was thinking:
let R = new Random()
let Best = Enumerable.Range(0, 100).Select(S => R.Next(-100, 100)).ToArray()
//Where this from clause is acting like a for loop
from N in Iterations
let NewList = Enumerable.Range(0, 100).Select(S => R.Next(-100, 100))
Best = (NewList.Sum() < Best.Sum())? NewList : Best;
select Best

I believe you are looking for fold (aka "reduce") which is known as Aggregate in LINQ.
(IEnumerable.Min/Max are special-cases, but can be written in terms of fold/Aggregate.)
int Max (IEnumerable<int> x) {
return x.Aggregate(int.MinValue, (prev, cur) => prev > cur ? prev : cur);
}
Max(new int[] { 1, 42, 2, 3 }); // 42
Happy coding.

Looks like you're just selecting the minimum value.
var minimum = collection.Min( c => c );

You are effectively finding the minimum value in the collection, if it exists:
int? best = null;
if (collection != null && collection.Length > 0) best = collection.Min();