Develop Reference

C#:How to get the longest common prefix from a list of strings LINQ expression

I am trying to learn LINQ
I would like to understand how to get the longest common prefix from a list of strings
{"a","abC","abcD"}
would return "ab". Common as in at least 1 other string has it. Even though "a" is common for all 3, I would like to get "ab" because 2 elements share this prefix and "ab" is longer than "a"

It was an interesting challenge and this is my solution:
var array = new []{"a","abC","abcD"};
var longestCommonPrefix = Enumerable.Range(1, array.Max(_ => _)!.Length)
.Select(i =>
{
var grouped = array.Where(x => x.Length >= i)
.GroupBy(x => x[..i])
.Where(x => x.Count() > 1)
.OrderByDescending(x => x.Count())
.Select(x => new { LongestCommonPrefix = x.Key })
.FirstOrDefault();
return grouped?.LongestCommonPrefix ?? string.Empty;
}).Max();

var longestCommonPrefix = (words.FirstOrDefault() ?? String.Empty)
.Substring(0,
Enumerable.Range(0, words.Any() ? words.Min(x => x.Length) + 1 : 0)
.Where(x => words.Select(w => w.Substring(0, x))
.Distinct().Count() == 1).DefaultIfEmpty(0).Max()
);

Group the indexes of the same elements in a array in C#

There is a int[] array that stores different numbers.
What I want is to group the indexes of those same numbers in the array to the same groups.
For exmaple, the array is int[5]{1,2,5,1,5}
I would like to see the output is List<List<int>> { {0,3}, {1}, {2,4} } // don't mind syntax
It's better if Linq (or a more efficient way) can be used, thanks for help.

You can simply use GroupBy and the position obtained from the Select overload:
int[] array;
var result = array.Select((v, idx) => new { Value = v, Index = idx })
.GroupBy(g => g.Value)
.Select(g => g.ToArray()) // inner array
.ToArray(); // outer array

One of ways:
var result = myArray.Select((elem, idx) => new { Value = elem, Idx = idx})
.GroupBy(proxy => proxy.Value);
foreach (var grouped in result)
{
Console.WriteLine("Element {0} has indexes: {1}",
grouped.Key,
string.Join(", ", grouped.Select(proxy => proxy.Idx).ToArray()));
}
var myFinalList = result.Select(proxy => proxy.ToArray()).ToList();

You can use Enumerable.Range combined with GroupBy:
int[] arr = { 1, 2, 5, 1, 5 };
var result = Enumerable.Range(0, arr.Length)
.GroupBy(i => arr[i])
.Select(x => x.ToList()).ToList();
DEMO HERE

Get Max occurence of element in Linq

Hello all i have 2 class for the example i will name it A and B
A is a list and every A element have a list of B element.
B element have a type
I want to get a B element by occurance of it in my list of B element.
var listB = A
.SelectMany(a => a.B);
var listBId = listB
.Where(b => b.Type == SelectedType)
.Select(b => b.Id);
var IdMaxoccur = listBId
.GroupBy(x => x)
.OrderByDescending(x => x.Count())
.First()
.Key;
I find this a bit heavy for just get the max occurence of an id in a list....
Do you know a better way to do this?

I think your code is good enough if you rewrite and simplify it a bit and handle case when your list is empty. This code assumes that 0 is not a valid Id.
var result = A
.SelectMany(x => x.B)
.Where(x => x.Type == selectedType)
.GroupBy(x => x.Id, new { Id = x.Key, Count = x.Count() })
.OrderByDescending(x => x.Count)
.FirstOrDefault();
Console.WriteLine("Max ID = {0}, Count = {1}", result.Id, result.Count);
If you still think that your existing code is too complex, you could write extension method to hide complexity
public static int TryGetBIdWithMaxOccur(this IEnumerable<A> input, SelectedTypeEnum selectedType)
{
var result = input
.SelectMany(x => x.B)
.Where(x => x.Type == selectedType)
.GroupBy(x => x.Id, new { Id = x.Key, Count = x.Count() })
.OrderByDescending(x => x.Count)
.Select(x => x.Id)
.FirstOrDefault();
return result;
}
Then you can use it like this:
var result = A.TryGetBIdWithMaxOccur(SelectedTypeEnum.CoolValue);
if(result != default(int))
{
//do stuff
}

just an idea
var bs = A.SelectMany().Where().Select(b=>b.Id).OrderBy();
int current = -1, maxB = -1; // make sure it is stub Id
int currentCount = 0, maxCount = 0;
foreach(var b in bs)
{
if (b != current)
{
// check if previous was max
if (currentCount > maxCount)
{
maxB = current;
maxCount = currentCount;
}
// change current
current = b;
currentCount = 0;
}
currentCount ++;
}

To make it a bit shorter you can put the Where selector inside SelectMany and use GroupBy overload:
var idMaxOccur = A
.SelectMany(a => a.B.Where(b => b.Type == selectedType))
.GroupBy(b => b.Id, b => b.Id)
.OrderByDescending(g => g.Count())
.First().Key;

LINQ - Select min count

I have a list of strings which contain X in them. I want to select list(s) with the minimum count of X in them. For example:
CountMin("AXBXX", "AAX") will return AAX.
How can I write this qith LINQ in a concise way ?

public static string CountMin(IList<string> inputList)
{
if (inputList == null || !inputList.Any()) return null;
var result = inputList.Select(s => new
{
Item = s,
Count => s.Count(ch => ch == 'X')
})
.OrderBy(item => item.Count).First().Item;
}
Snippet assumes that all elements on list are different to null. If you need it, it could be easily improved.
You can also omit temporary class:
inputList.OrderBy(s => s.Count(c => c == 'X')).First();

string[] list = {"AXBXX", "AAX", "AXX"};
string result = (from word in list
select new { word, wordLen = (word.Length - (word.Replace("X", "")).Length) })
.OrderBy(x => x.wordLen).First().word;
MessageBox.Show(result);

Here's an answer that will get you all of the minimum X strings from the list.
var listOfStrings = new List<string>()
{
"AXB",
"ABXXC",
"ABX",
};
var minimumXs =
listOfStrings
.GroupBy(x => x.Count(y => y == 'X'))
.OrderBy(x => x.Key)
.Take(1)
.SelectMany(x => x);
That gives me:
AXB
ABX

C# double for comprehension?

Is it possible to do a double for comprehension in C#? For example, the following works:
var a = new[] { 1, 2, 3 };
var b = Enumerable.Range(0, a.Length).Select(i => a[i]).ToArray();
But when I try and adapt this code for the two-dimensional case, things don't work. Below I'm trying to iterate over the pixels of a bitmap:
Color[] p = Enumerable.Range(0, Source.Width).Select(i => Enumerable.Range(0, Source.Height).Select(j => Source.GetPixel(i, j))).ToArray().
Is there any way to get what I want? The error I'm currently getting is:
Cannot implicitly convert type
System.Collections.Generic.IEnumerable[] to
System.Drawing.Color[]

The outer Select needs to be a SelectMany to flatten the projection:
Color[] p = Enumerable.Range(0, Source.Width)
.SelectMany(i => Enumerable.Range(0, Source.Height)
.Select(j => Source.GetPixel(i, j)))
.ToArray();
or to create a jagged 2-D array add an inner ToArray():
Color[][] p = Enumerable.Range(0, Source.Width)
.Select(i => Enumerable.Range(0, Source.Height)
.Select(j => Source.GetPixel(i, j))
.ToArray())
.ToArray();