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Getting ALL possible combination of integers from an integer collection without fixed length in C# [duplicate]

I have a list of integers List<int> in my C# program. However, I know the number of items I have in my list only at runtime.
Let us say, for the sake of simplicity, my list is {1, 2, 3}
Now I need to generate all possible combinations as follows.
{1, 2, 3}
{1, 2}
{1, 3}
{2, 3}
{1}
{2}
{3}
Can somebody please help with this?

try this:
static void Main(string[] args)
{
GetCombination(new List<int> { 1, 2, 3 });
}
static void GetCombination(List<int> list)
{
double count = Math.Pow(2, list.Count);
for (int i = 1; i <= count - 1; i++)
{
string str = Convert.ToString(i, 2).PadLeft(list.Count, '0');
for (int j = 0; j < str.Length; j++)
{
if (str[j] == '1')
{
Console.Write(list[j]);
}
}
Console.WriteLine();
}
}

Assuming that all items within the initial collection are distinct, we can try using Linq in order to query; let's generalize the solution:
Code:
public static IEnumerable<T[]> Combinations<T>(IEnumerable<T> source) {
if (null == source)
throw new ArgumentNullException(nameof(source));
T[] data = source.ToArray();
return Enumerable
.Range(0, 1 << (data.Length))
.Select(index => data
.Where((v, i) => (index & (1 << i)) != 0)
.ToArray());
}
Demo:
var data = new char[] { 'A', 'B', 'C' };
var result = Combinations(data);
foreach (var item in result)
Console.WriteLine($"[{string.Join(", ", item)}]");
Outcome:
[]
[A]
[B]
[A, B]
[C]
[A, C]
[B, C]
[A, B, C]
If you want to exclude the initial empty array, put .Range(1, (1 << (data.Length)) - 1) instead of .Range(0, 1 << (data.Length))
Algorithm explanation:
Having a collection of collection.Length distinct items we get 2 ** collection.Length combinations (we can compute it as 1 << collection.Length):
mask - comments
------------------------------------
00..0000 - empty, no items are taken
00..0001 - 1st item taken
00..0010 - 2nd item taken
00..0011 - 1st and 2nd items are taken
00..0100 - 3d item taken
...
11..1111 - all items are taken
To generate all masks we can use direct Enumerable.Range(0, 1 << (data.Length)) Linq query. Now having index mask we should take item from the collection if and only if corresponding bit within index is set to 1:
011001001
^^ ^ ^
take 7, 6, 3, 0-th items from the collection
The code can be
.Select(index => data.Where((v, i) => (index & (1 << i)) != 0)
here for each item (v) in the collection data we check if i-th bit is set in the index (mask).

Here are two generic solutions for strongly typed lists that will return all unique combinations of list members (if you can solve this with simpler code, I salute you):
// Recursive
public static List<List<T>> GetAllCombos<T>(List<T> list)
{
List<List<T>> result = new List<List<T>>();
// head
result.Add(new List<T>());
result.Last().Add(list[0]);
if (list.Count == 1)
return result;
// tail
List<List<T>> tailCombos = GetAllCombos(list.Skip(1).ToList());
tailCombos.ForEach(combo =>
{
result.Add(new List<T>(combo));
combo.Add(list[0]);
result.Add(new List<T>(combo));
});
return result;
}
// Iterative, using 'i' as bitmask to choose each combo members
public static List<List<T>> GetAllCombos<T>(List<T> list)
{
int comboCount = (int) Math.Pow(2, list.Count) - 1;
List<List<T>> result = new List<List<T>>();
for (int i = 1; i < comboCount + 1; i++)
{
// make each combo here
result.Add(new List<T>());
for (int j = 0; j < list.Count; j++)
{
if ((i >> j) % 2 != 0)
result.Last().Add(list[j]);
}
}
return result;
}
// Example usage
List<List<int>> combos = GetAllCombos(new int[] { 1, 2, 3 }.ToList());

This answer uses the same algorithm as ojlovecd and (for his iterative solution) jaolho. The only thing I'm adding is an option to filter the results for a minimum number of items in the combinations. This can be useful, for example, if you are only interested in the combinations that contain at least two items.
Edit: As requested by #user3610374 a filter for the maximum number of items has been added.
Edit 2: As suggested by #stannius the algorithm has been changed to make it more efficient for cases where not all combinations are wanted.
/// <summary>
/// Method to create lists containing possible combinations of an input list of items. This is
/// basically copied from code by user "jaolho" on this thread:
/// http://stackoverflow.com/questions/7802822/all-possible-combinations-of-a-list-of-values
/// </summary>
/// <typeparam name="T">type of the items on the input list</typeparam>
/// <param name="inputList">list of items</param>
/// <param name="minimumItems">minimum number of items wanted in the generated combinations,
/// if zero the empty combination is included,
/// default is one</param>
/// <param name="maximumItems">maximum number of items wanted in the generated combinations,
/// default is no maximum limit</param>
/// <returns>list of lists for possible combinations of the input items</returns>
public static List<List<T>> ItemCombinations<T>(List<T> inputList, int minimumItems = 1,
int maximumItems = int.MaxValue)
{
int nonEmptyCombinations = (int)Math.Pow(2, inputList.Count) - 1;
List<List<T>> listOfLists = new List<List<T>>(nonEmptyCombinations + 1);
// Optimize generation of empty combination, if empty combination is wanted
if (minimumItems == 0)
listOfLists.Add(new List<T>());
if (minimumItems <= 1 && maximumItems >= inputList.Count)
{
// Simple case, generate all possible non-empty combinations
for (int bitPattern = 1; bitPattern <= nonEmptyCombinations; bitPattern++)
listOfLists.Add(GenerateCombination(inputList, bitPattern));
}
else
{
// Not-so-simple case, avoid generating the unwanted combinations
for (int bitPattern = 1; bitPattern <= nonEmptyCombinations; bitPattern++)
{
int bitCount = CountBits(bitPattern);
if (bitCount >= minimumItems && bitCount <= maximumItems)
listOfLists.Add(GenerateCombination(inputList, bitPattern));
}
}
return listOfLists;
}
/// <summary>
/// Sub-method of ItemCombinations() method to generate a combination based on a bit pattern.
/// </summary>
private static List<T> GenerateCombination<T>(List<T> inputList, int bitPattern)
{
List<T> thisCombination = new List<T>(inputList.Count);
for (int j = 0; j < inputList.Count; j++)
{
if ((bitPattern >> j & 1) == 1)
thisCombination.Add(inputList[j]);
}
return thisCombination;
}
/// <summary>
/// Sub-method of ItemCombinations() method to count the bits in a bit pattern. Based on this:
/// https://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetKernighan
/// </summary>
private static int CountBits(int bitPattern)
{
int numberBits = 0;
while (bitPattern != 0)
{
numberBits++;
bitPattern &= bitPattern - 1;
}
return numberBits;
}

Here's a generic solution using recursion
public static ICollection<ICollection<T>> Permutations<T>(ICollection<T> list) {
var result = new List<ICollection<T>>();
if (list.Count == 1) { // If only one possible permutation
result.Add(list); // Add it and return it
return result;
}
foreach (var element in list) { // For each element in that list
var remainingList = new List<T>(list);
remainingList.Remove(element); // Get a list containing everything except of chosen element
foreach (var permutation in Permutations<T>(remainingList)) { // Get all possible sub-permutations
permutation.Add(element); // Add that element
result.Add(permutation);
}
}
return result;
}
I know this is an old post, but someone might find this helpful.

Another solution using Linq and recursion...
static void Main(string[] args)
{
List<List<long>> result = new List<List<long>>();
List<long> set = new List<long>() { 1, 2, 3, 4 };
GetCombination<long>(set, result);
result.Add(set);
IOrderedEnumerable<List<long>> sorted = result.OrderByDescending(s => s.Count);
sorted.ToList().ForEach(l => { l.ForEach(l1 => Console.Write(l1 + " ")); Console.WriteLine(); });
}
private static void GetCombination<T>(List<T> set, List<List<T>> result)
{
for (int i = 0; i < set.Count; i++)
{
List<T> temp = new List<T>(set.Where((s, index) => index != i));
if (temp.Count > 0 && !result.Where(l => l.Count == temp.Count).Any(l => l.SequenceEqual(temp)))
{
result.Add(temp);
GetCombination<T>(temp, result);
}
}
}

This is an improvement of #ojlovecd answer without using strings.
static void Main(string[] args)
{
GetCombination(new List<int> { 1, 2, 3 });
}
private static void GetCombination(List<int> list)
{
double count = Math.Pow(2, list.Count);
for (int i = 1; i <= count - 1; i++)
{
for (int j = 0; j < list.Count; j++)
{
int b = i & (1 << j);
if (b > 0)
{
Console.Write(list[j]);
}
}
Console.WriteLine();
}
}

Firstly, given a set of n elements, you compute all combinations of k elements out of it (nCk). You have to change the value of k from 1 to n to meet your requirement.
See this codeproject article for C# code for generating combinations.
In case, you are interested in developing the combination algorithm by yourself, check this SO question where there are a lot of links to the relevant material.

protected List<List<T>> AllCombos<T>(Func<List<T>, List<T>, bool> comparer, params T[] items)
{
List<List<T>> results = new List<List<T>>();
List<T> workingWith = items.ToList();
results.Add(workingWith);
items.ToList().ForEach((x) =>
{
results.Add(new List<T>() { x });
});
for (int i = 0; i < workingWith.Count(); i++)
{
T removed = workingWith[i];
workingWith.RemoveAt(i);
List<List<T>> nextResults = AllCombos2(comparer, workingWith.ToArray());
results.AddRange(nextResults);
workingWith.Insert(i, removed);
}
results = results.Where(x => x.Count > 0).ToList();
for (int i = 0; i < results.Count; i++)
{
List<T> list = results[i];
if (results.Where(x => comparer(x, list)).Count() > 1)
{
results.RemoveAt(i);
}
}
return results;
}
protected List<List<T>> AllCombos2<T>(Func<List<T>, List<T>, bool> comparer, params T[] items)
{
List<List<T>> results = new List<List<T>>();
List<T> workingWith = items.ToList();
if (workingWith.Count > 1)
{
results.Add(workingWith);
}
for (int i = 0; i < workingWith.Count(); i++)
{
T removed = workingWith[i];
workingWith.RemoveAt(i);
List<List<T>> nextResults = AllCombos2(comparer, workingWith.ToArray());
results.AddRange(nextResults);
workingWith.Insert(i, removed);
}
results = results.Where(x => x.Count > 0).ToList();
for (int i = 0; i < results.Count; i++)
{
List<T> list = results[i];
if (results.Where(x => comparer(x, list)).Count() > 1)
{
results.RemoveAt(i);
}
}
return results;
}
This worked for me, it's slightly more complex and actually takes a comparer callback function, and it's actually 2 functions, the difference being that the AllCombos adds the single item lists explicitly. It is very raw and can definitely be trimmed down but it gets the job done. Any refactoring suggestions are welcome. Thanks,

public class CombinationGenerator{
private readonly Dictionary<int, int> currentIndexesWithLevels = new Dictionary<int, int>();
private readonly LinkedList<List<int>> _combinationsList = new LinkedList<List<int>>();
private readonly int _combinationLength;
public CombinationGenerator(int combinationLength)
{
_combinationLength = combinationLength;
}
private void InitializeLevelIndexes(List<int> list)
{
for (int i = 0; i < _combinationLength; i++)
{
currentIndexesWithLevels.Add(i+1, i);
}
}
private void UpdateCurrentIndexesForLevels(int level)
{
int index;
if (level == 1)
{
index = currentIndexesWithLevels[level];
for (int i = level; i < _combinationLength + 1; i++)
{
index = index + 1;
currentIndexesWithLevels[i] = index;
}
}
else
{
int previousLevelIndex;
for (int i = level; i < _combinationLength + 1; i++)
{
if (i > level)
{
previousLevelIndex = currentIndexesWithLevels[i - 1];
currentIndexesWithLevels[i] = previousLevelIndex + 1;
}
else
{
index = currentIndexesWithLevels[level];
currentIndexesWithLevels[i] = index + 1;
}
}
}
}
public void FindCombinations(List<int> list, int level, Stack<int> stack)
{
int currentIndex;
InitializeLevelIndexes(list);
while (true)
{
currentIndex = currentIndexesWithLevels[level];
bool levelUp = false;
for (int i = currentIndex; i < list.Count; i++)
{
if (level < _combinationLength)
{
currentIndex = currentIndexesWithLevels[level];
MoveToUpperLevel(ref level, stack, list, currentIndex);
levelUp = true;
break;
}
levelUp = false;
stack.Push(list[i]);
if (stack.Count == _combinationLength)
{
AddCombination(stack);
stack.Pop();
}
}
if (!levelUp)
{
MoveToLowerLevel(ref level, stack, list, ref currentIndex);
while (currentIndex >= list.Count - 1)
{
if (level == 1)
{
AdjustStackCountToCurrentLevel(stack, level);
currentIndex = currentIndexesWithLevels[level];
if (currentIndex >= list.Count - 1)
{
return;
}
UpdateCurrentIndexesForLevels(level);
}
else
{
MoveToLowerLevel(ref level, stack, list, ref currentIndex);
}
}
}
}
}
private void AddCombination(Stack<int> stack)
{
List<int> listNew = new List<int>();
listNew.AddRange(stack);
_combinationsList.AddLast(listNew);
}
private void MoveToUpperLevel(ref int level, Stack<int> stack, List<int> list, int index)
{
stack.Push(list[index]);
level++;
}
private void MoveToLowerLevel(ref int level, Stack<int> stack, List<int> list, ref int currentIndex)
{
if (level != 1)
{
level--;
}
AdjustStackCountToCurrentLevel(stack, level);
UpdateCurrentIndexesForLevels(level);
currentIndex = currentIndexesWithLevels[level];
}
private void AdjustStackCountToCurrentLevel(Stack<int> stack, int currentLevel)
{
while (stack.Count >= currentLevel)
{
if (stack.Count != 0)
stack.Pop();
}
}
public void PrintPermutations()
{
int count = _combinationsList.Where(perm => perm.Count() == _combinationLength).Count();
Console.WriteLine("The number of combinations is " + count);
}
}

We can use recursion for combination/permutation problems involving string or integers.
public static void Main(string[] args)
{
IntegerList = new List<int> { 1, 2, 3, 4 };
PrintAllCombination(default(int), default(int));
}
public static List<int> IntegerList { get; set; }
public static int Length { get { return IntegerList.Count; } }
public static void PrintAllCombination(int position, int prefix)
{
for (int i = position; i < Length; i++)
{
Console.WriteLine(prefix * 10 + IntegerList[i]);
PrintAllCombination(i + 1, prefix * 10 + IntegerList[i]);
}
}

What about
static void Main(string[] args)
{
Combos(new [] { 1, 2, 3 });
}
static void Combos(int[] arr)
{
for (var i = 0; i <= Math.Pow(2, arr.Length); i++)
{
Console.WriteLine();
var j = i;
var idx = 0;
do
{
if ((j & 1) == 1) Console.Write($"{arr[idx]} ");
} while ((j >>= 1) > 0 && ++idx < arr.Length);
}
}

A slightly more generalised version for Linq using C# 7. Here filtering by items that have two elements.
static void Main(string[] args)
{
foreach (var vals in Combos(new[] { "0", "1", "2", "3" }).Where(v => v.Skip(1).Any() && !v.Skip(2).Any()))
Console.WriteLine(string.Join(", ", vals));
}
static IEnumerable<IEnumerable<T>> Combos<T>(T[] arr)
{
IEnumerable<T> DoQuery(long j, long idx)
{
do
{
if ((j & 1) == 1) yield return arr[idx];
} while ((j >>= 1) > 0 && ++idx < arr.Length);
}
for (var i = 0; i < Math.Pow(2, arr.Length); i++)
yield return DoQuery(i, 0);
}

Here is how I did it.
public static List<List<int>> GetCombination(List<int> lst, int index, int count)
{
List<List<int>> combinations = new List<List<int>>();
List<int> comb;
if (count == 0 || index == lst.Count)
{
return null;
}
for (int i = index; i < lst.Count; i++)
{
comb = new List<int>();
comb.Add(lst.ElementAt(i));
combinations.Add(comb);
var rest = GetCombination(lst,i + 1, count - 1);
if (rest != null)
{
foreach (var item in rest)
{
combinations.Add(comb.Union(item).ToList());
}
}
}
return combinations;
}
You call it as :
List<int> lst= new List<int>(new int[]{ 1, 2, 3, 4 });
var combinations = GetCombination(lst, 0, lst.Length)

I just run into a situation where I needed to do this, this is what I came up with:
private static List<string> GetCombinations(List<string> elements)
{
List<string> combinations = new List<string>();
combinations.AddRange(elements);
for (int i = 0; i < elements.Count - 1; i++)
{
combinations = (from combination in combinations
join element in elements on 1 equals 1
let value = string.Join(string.Empty, $"{combination}{element}".OrderBy(c => c).Distinct())
select value).Distinct().ToList();
}
return combinations;
}
It may be not too efficient, and it sure has room for improvement, but gets the job done!
List<string> elements = new List<string> { "1", "2", "3" };
List<string> combinations = GetCombinations(elements);
foreach (string combination in combinations)
{
System.Console.Write(combination);
}

This is an improved version based on the answer from ojlovecd using extension methods:
public static class ListExtensions
{
public static IEnumerable<List<T>> GetCombinations<T>(
this List<T> valuesToCombine)
{
var count = Math.Pow(2, valuesToCombine.Count);
for(var i = 1; i <= count; i++)
{
var itemFlagList = i.ToBinaryString(valuesToCombine.Count())
.Select(x => x == '1').ToList();
yield return GetCombinationByFlagList(valuesToCombine, itemFlagList)
.ToList();
}
}
private static IEnumerable<T> GetCombinationByFlagList<T>(
List<T> valuesToCombine, List<bool> flagList)
{
for (var i = 0; i < valuesToCombine.Count; i++)
{
if (!flagList[i]) continue;
yield return valuesToCombine.ElementAt(i);
}
}
}
public static class IntegerExtensions
{
public static string ToBinaryString(this int value, int length)
{
return Convert.ToString(value, 2).ToString().PadLeft(length, '0');
}
}
Usage:
var numbersList = new List<int>() { 1, 2, 3 };
var combinations = numbersList.GetCombinations();
foreach (var combination in combinations)
{
System.Console.WriteLine(string.Join(",", combination));
}
Output:
3
2
2,3
1
1,3
1,2
1,2,3
The idea is to basically use some flags to keep track of which items were already added to the combination. So in case of 1, 2 & 3, the following binary strings are generated in order to indicate whether an item should be included or excluded:
001, 010, 011, 100, 101, 110 & 111

I'd like to suggest an approach that I find to be quite intuitive and easy to read. (Note: It is slower than the currently accepted solution.)
It is built on the idea that for each integer in the list, we need to extend the so-far-aggregated resulting combination list with
all currently existing combinations, each extended with the given integer
a single "combination" of that integer alone
Here, I am using .Aggregate() with a seed that is an IEnumerable<IEnumerable<int>> containing a single, empty collection entry. That empty entry lets us easily do the two steps above simultaneously. The empty collection entry can be skipped after the resulting combination collection has been aggregated.
It goes like this:
var emptyCollection = Enumerable.Empty<IEnumerable<int>>();
var emptyCombination = Enumerable.Empty<int>();
IEnumerable<int[]> combinations = list
.Aggregate(emptyCollection.Append(emptyCombination),
( acc, next ) => acc.Concat(acc.Select(entry => entry.Append(next))))
.Skip(1) // skip the initial, empty combination
.Select(comb => comb.ToArray());
For each entry in the input integer list { 1, 2, 3 }, the accumulation progresses as follows:
next = 1
{ { } }.Concat({ { }.Append(1) })
{ { } }.Concat({ { 1 } })
{ { }, { 1 } } // acc
next = 2
{ { }, { 1 } }.Concat({ { }.Append(2), { 1 }.Append(2) })
{ { }, { 1 } }.Concat({ { 2 }, { 1, 2 } })
{ { }, { 1 }, { 2 }, { 1, 2 } } // acc
next = 3
{ { }, { 1 }, { 2 }, { 1, 2 } }.Concat({ { }.Append(3), { 1 }.Append(3), { 2 }.Append(3), { 1, 2 }.Append(3) })
{ { }, { 1 }, { 2 }, { 1, 2 } }.Concat({ { 3 }, { 1, 3 }, { 2, 3 }, { 1, 2, 3 } })
{ { }, { 1 }, { 2 }, { 1, 2 }, { 3 }, { 1, 3 }, { 2, 3 }, { 1, 2, 3 } } // acc
Skipping the first (empty) entry, we are left with the following collection:
1
2
1 2
3
1 3
2 3
1 2 3
, which can easily be ordered by collection length and collection entry sum for a clearer overview.
Example fiddle here.

Some of the solutions here are truly ingenious; especially the ones that use bitmaps.
But I found that in practice these algos
aren't easy to modify if a specific range of lengths needed (e.g. all variations of 3 to 5 choices from an input set of 8 elements)
can't handle LARGE input lists (and return empty or singleton results instead of throwing exception); and
can be tricky to debug.
So I decided to write something not as clever as the other people here.
My more basic approach recognises that the set of Variations(1 to maxLength) is simply a UNION of all fixed-length Variations of each length 1 to maxLength:
i.e
Variations(1 to maxLength) = Variations(1) + Variations(2) + ... + Variations(maxLength)
So you can do a "choose K from N" for each required length (for each K in (1, 2, 3, ..., maxLength)) and then just do a Union of these separate results to yield a List of Lists.
This resulting code intends to be easy to understand and to maintain:
/// <summary>
/// Generates ALL variations of length between minLength and maxLength (inclusive)
/// Relies on Combinatorics library to generate each set of Variations
/// Nuget https://www.nuget.org/packages/Combinatorics/
/// Excellent more general references (without this solution):
/// https://www.codeproject.com/Articles/26050/Permutations-Combinations-and-Variations-using-C-G
/// Self-authored solution.
/// </summary>
/// <typeparam name="T">Any type without any constraints.</typeparam>
/// <param name="sourceList">The source list of elements to be combined.</param>
/// <param name="minLength">The minimum length of variation required.</param>
/// <param name="maxLength">The maximum length of variation required.</param>
/// <returns></returns>
public static List<List<T>> GenerateVariations<T>(this IEnumerable<T> sourceList, int minLength, int maxLength)
{
List<List<T>> finalUnion = new();
foreach (int length in Enumerable.Range(minLength, maxLength))
{
Variations<T> variations = new Variations<T>(sourceList, length, GenerateOption.WithoutRepetition);
foreach (var variation in variations)
{
var list = variation.ToList<T>();
finalUnion.Add(list);
}
}
Debug.WriteLine(sourceList.Count() + " source " + typeof(T).Name + " yielded " + finalUnion.Count());
return finalUnion;
}
Happy to receive comments (good and bad). Maybe there's a more succint way to achieve this in LINQ? Maybe the really smart people here can marry their approach with my more basic one?

Please find very very simple solution without recursion and which dont eat RAM.
Unique Combinations

Extension method Average() C#

First of all you all need to know that I don't want to use the LINQ library in C#.
Now I want to write an Extension method that returns the average of different lists. This are the lists
integers = new List<int> { 5, 76, 3, 93, 143, 5, 11, 67, 5 };
doubles = new List<double> { 1.23, 68.256, 44.55, 96.127, 393.4567, 2.45, 4.1 };
persons = new List<Person>
{
new Person {Firstname = "John", Lastname = "last", Age = 66, Sallary = 1513},
new Person {Firstname = "Donald", Lastname = "last", Age = 77, Sallary = 3100}
};
This is what I already got:
public static double Average(this IEnumerable<int> source)
{
IList<int> list = source as IList<int>;
if (source == null) throw new ArgumentNullException("Err");
long sum = 0;
for (int i = 0; i < list.Count; i++)
{
sum += list[i];
}
return sum / list.Count;
}
I am only able to calculate the average of my integer list. I also want to calculate the average of the double list and the average salary of my persons list.
This is how I will call the function where the persons list average salary should be calculated
persons.Average(x => x.Sallary).ShowSingle("persons.Average(x => x.Sallary)");

First, your Average method returns the wrong result for the integer list since you are using integer division (it returns 45 instead of 45.3333).
To make it work with double, you need to create an overload that is using IEnumerable<double> etc.
public static double Average(this IEnumerable<double> source)
{
IList<double> list = source as IList<double>;
if (source == null || source.Count == 0)
throw new ArgumentNullException("Err");
double sum = 0;
for (int i = 0; i < list.Count; i++)
{
sum += list[i];
}
return sum / list.Count;
}
You can simplify the method a bit by using foreach:
public static double Average(this IEnumerable<double> source)
{
if (source == null)
throw new ArgumentNullException("Err");
double sum = 0;
int count = 0;
foreach (double entry in source)
{
sum += entry;
count+=1;
}
if (count == 0) return 0;
return sum / count;
}

Like written in my comment you can use the implementation from the Linq repository. I have just changed the way how they process it on 0 count of items.
public static double Average(this IEnumerable<int> source)
{
if (source == null)
{
throw new ArgumentNullException(nameof(source));
}
long sum = 0;
long count = 0;
checked
{
foreach (int v in source)
{
sum += v;
count++;
}
}
if (count == 0)
{
return 0;
}
return (double)sum / count;
}
public static double Average<T>(this IEnumerable<T> source)
{
if (source == null)
{
throw new ArgumentNullException(nameof(source));
}
if(typeof(T) != typeof(int)
&& typeof(T) != typeof(double)
&& typeof(T) != typeof(float)
&& typeof(T) != typeof(decimal)
&& typeof(T) != typeof(long))
{
throw new InvalidOperationException($"Wrong type specified: {typeof(T)}");
}
double sum = 0;
long count = 0;
checked
{
foreach (T v in source)
{
sum += Convert.ToDouble(v);
count++;
}
}
if (count == 0)
{
return 0;
}
return (double)sum / count;
}
A working example can be opened here
And of course the original Linq code can be found here

I am only able to calculate the average of my integer list. I also want to calculate the average of the double list and the average salary of my persons list.
As already mentioned in the comments by a few user's, it would be best to extract this into separate routines. As Klaus mentioned above, Compile-time checking is always better than run-time checking (better performance, better and earlier diagnostics). I agree, but as I mentioned on another answer, does this mean it's bad design from the programmer and or bad object oriented design on the authors?
Here's a little sample using generics (<T>), one routine does the int and double list and the other (for Person) is extracted out to it's own routine. The first Average routine uses the help of Convert.ChangeType to return specified type whose value is equivalent to a specified object.
public static class Extensions
{
public static T Average<T>(this IEnumerable<T> source)
{
double dblSum = 0;
int intSum = 0;
if (source == null)
throw new ArgumentNullException("Err");
if (typeof(T) == typeof(int) && source is List<int> intList && intList.Count > 0)
{
for (int i = 0; i < intList.Count; i++)
{
intSum += intList[i];
}
return (T)Convert.ChangeType((intSum / intList.Count), typeof(T));
}
else if (typeof(T) == typeof(double) && source is List<double> dblList && dblList.Count > 0)
{
for (int i = 0; i < dblList.Count; i++)
{
dblSum += dblList[i];
}
return (T)Convert.ChangeType((dblSum / dblList.Count), typeof(T));
}
else
{
return default;
}
}
public static int Average(this IEnumerable<Person> people)
{
int intSum = 0;
if (people != null && people is List<Person> perList && perList.Count > 0)
{
for (int i = 0; i < perList.Count; i++)
{
intSum += perList[i].Sallary;
}
return intSum / perList.Count;
}
else
{
return default;
// throw new ArgumentNullException("Err");
}
}
}
Please note: I am not sure the type of Sallary as this wasn't provided in your post. You may need to change the Average routine (Person) slightly to adjust to your Sallary type.

We can leverage on generic types in c# to find the average. This allows us to use only one method to calculate for both integers and double.
public static double Average<T>(this List<T> source)
{
IList<T> list = source as IList<T>;
if (source == null)
throw new ArgumentNullException("Err");
if(source.GetType() == typeof(List<int>) || source.GetType() == typeof(List<double>))
{
double sum = 0;
for (int i = 0; i < list.Count; i++)
{
sum += Convert.ToDouble((object)list[i]);
}
return sum / list.Count;
}
else
{
throw new InvalidOperationException("Not possible to find average");
}
}
And to call the method
var integers = new List<int> { 5, 76, 3, 93, 143, 5, 11, 67, 5 };
var doubles = new List<double> { 1.23, 68.256, 44.55, 96.127, 393.4567, 2.45, 4.1 };
Console.WriteLine(doubles.Average());
Console.WriteLine(integers.Average());

Create a sliding window from a list

I need to iterate over a list while getting the next and previous Items. I'm currently doing it like so:
var items = collection.ToList();
for (int index = 0; index < items.Count; index++)
{
var prevItem = index == 0 ? null : items[index - 1];
var currentItem = items[index];
var nextItem = (index + 1) == items.Count ? null : items[index + 1];
// do work
}
This work but it is not as nice and readable as I would like. Is there a more readable way of creating a sliding window? which doesn't involve all of the ugly ternary checks. I feel like there is a more friendly way using select, skip, take, and default if empty

This sort of problem is best solved using generators if you want it to be reusable IMHO.
public static IEnumerable<(T PrevItem, T CurrentItem, T NextItem)>
SlidingWindow<T>(this IEnumerable<T> source, T emptyValue = default)
{
using (var iter = source.GetEnumerator())
{
if (!iter.MoveNext())
yield break;
var prevItem = emptyValue;
var currentItem = iter.Current;
while (iter.MoveNext())
{
var nextItem = iter.Current;
yield return (prevItem, currentItem, nextItem);
prevItem = currentItem;
currentItem = nextItem;
}
yield return (prevItem, currentItem, emptyValue);
}
}
Then use it:
foreach (var (prevItem, currentItem, nextItem) in collection.SlidingWindow())
{
// do stuff with prevItem, currentItem, nextItem
}

Generic window size:
public IEnumerable<IEnumerable<T>> CreateSlidingWindow<T>(IEnumerable<T> input, int windowSize)
{
if (input == null || input.Count() < windowSize)
{
return new List<IEnumerable<T>>();
}
var first = new[] { input.Take(windowSize) };
var rest = CreateSlidingWindow(input.Skip(1), windowSize);
return first.Union(rest);
}

One way to make it more nicer is to transform your collection in a List of objects which contain the item, prevItem and nextItem:
static void Main(string[] args)
{
var collection = new List<string> { "A", "B", "C", "D", "E" };
var items = collection.Select((item, index) => new
{
Item = item,
PreVItem = index > 0 ? collection[index - 1] : null,
NextItem = index < collection.Count-1 ? collection[index + 1] : null
});
foreach (var item in items)
{
Console.WriteLine($"{item.PreVItem} \t {item.Item} \t {item.NextItem}");
}
Console.ReadLine();
}

Created the following solution that is using an enumerator.
public static IEnumerable<IList<T>> SlidingWindowValues<T>(this IEnumerable<T> source, int windowSize)
{
var windows = Enumerable.Range(0, windowSize)
.Select(_ => new List<T>())
.ToList();
int i = 0;
using (var iter = source.GetEnumerator())
{
while (iter.MoveNext())
{
var c = Math.Min(i + 1, windowSize);
for (var j = 0; j < c; j++)
{
windows[(i - j) % windowSize].Add(iter.Current);
}
if (i >= windowSize - 1)
{
var previous = (i + 1) % windowSize;
yield return windows[previous];
windows[previous] = new List<T>();
}
i++;
}
}
}

Calculating the Power set of large string array in C# [duplicate]

This question already has answers here:
Memory efficient power set algorithm
(5 answers)
Closed 6 years ago.
I want to calculate power set of string array (consider it as a set). When I am exceeding above 26 elements it is thowing out of memory exception.
List<int> ff = new List<int>();
double length = Math.Pow(2, 29);
for (int i = 0; i < length; i++)
{
ff.Add(1);
}
Above code will produce the that exception if you run it. The size of the set may go up to 1000. So the size of power set of that set will be 2^1000.
How can I deal with this?
EDIT:
I know that above code is not a function of power set. I was just checking how big array c# will be able to hold.
private static Dictionary<int, object> PowerSetB(string[] input)
{
int n = input.Length;
// Power set contains 2^N subsets.
int powerSetCount = 1 << n;
var ans = new Dictionary<int, object>();
for (int setMask = 0; setMask < powerSetCount; setMask++)
{
var s = new ArrayList();
for (int i = 0; i < n; i++)
{
// Checking whether i'th element of input collection should go to the current subset.
if ((setMask & (1 << i)) > 0)
s.Add(input[i]);
}
ans[setMask] = s;
}
return ans;
}
Above code is my function of power set.
Thank you in advance.

Do you really want to store all the items in memory? I suggest using IEnumerable<int> instead of materialized List<int>:
// just enumeration, coefficients aren't stored
public static IEnumerable<int> Serie(Func<int, int> coefByIndex) {
if (null == coefByIndex)
throw new ArgumentNullException("coefByIndex");
for (int i = 0; ; ++i)
yield return coefByIndex(i);
}
// Let's sum up all 2**29 values,
// i.e. compute f(1) summing up 2**29 items (it's a long process...)
// sum = 1.44115187606094E+17 (diverges, as we might have expected)
Double sum = Serie(index => index)
.Select(x => x * 1.0)
.Take(1 << 29)
.Sum();
Edit: Once agian, do not materialize (Dictionary<int, object>) huge results! Provide an IReadOnlyDictionary<int, int[]> interface but not implementation as Dictionary<int, object>,
Something like this:
// ArrayList is an obsolete collection;
// int[] far more natural here
public sealed class PowerSet: IReadOnlyDictionary<int, int[]> {
private int m_Power;
private int[] getItem(int index) {
int[] result = new int[m_Power];
for (int i = 0; i < m_Power; ++i) {
result[i] = index % 2;
index /= 2;
}
return result;
}
public PowerSet(int power) {
m_Power = power;
}
public int[] this[int key] {
get {
if (key >= 0 && key < Count)
return getItem(key);
else
throw new ArgumentOutOfRangeException("key");
}
}
public int Count {
get {
return 1 << m_Power;
}
}
public IEnumerable<int> Keys {
get {
return Enumerable.Range(0, Count);
}
}
public IEnumerable<int[]> Values {
get {
return Enumerable.Range(0, Count).Select(index => getItem(index));
}
}
public bool ContainsKey(int key) {
return key >= 0 && key < Count;
}
public IEnumerator<KeyValuePair<int, int[]>> GetEnumerator() {
return Enumerable
.Range(0, Count)
.Select(index => new KeyValuePair<int, int[]>(index, getItem(index)))
.GetEnumerator();
}
public bool TryGetValue(int key, out int[] value) {
if (key >= 0 && key < Count) {
value = getItem(key);
return true;
}
value = null;
return false;
}
IEnumerator IEnumerable.GetEnumerator() {
return this.GetEnumerator();
}
}
...
// Just an easy call
private static IDictionary<int, int[]> PowerSetB(string[] input) {
return new PowerSet(input.Length);
}

c# permutation without repetition when order is important [duplicate]

I have a list of Offers, from which I want to create "chains" (e.g. permutations) with limited chain lengths.
I've gotten as far as creating the permutations using the Kw.Combinatorics project.
However, the default behavior creates permutations in the length of the list count. I'm not sure how to limit the chain lengths to 'n'.
Here's my current code:
private static List<List<Offers>> GetPerms(List<Offers> list, int chainLength)
{
List<List<Offers>> response = new List<List<Offers>>();
foreach (var row in new Permutation(list.Count).GetRows())
{
List<Offers> innerList = new List<Offers>();
foreach (var mix in Permutation.Permute(row, list))
{
innerList.Add(mix);
}
response.Add(innerList);
innerList = new List<Offers>();
}
return response;
}
Implemented by:
List<List<AdServer.Offers>> lst = GetPerms(offers, 2);
I'm not locked in KWCombinatorics if someone has a better solution to offer.

Here's another implementation which I think should be faster than the accepted answer (and it's definitely less code).
public static IEnumerable<IEnumerable<T>> GetVariationsWithoutDuplicates<T>(IList<T> items, int length)
{
if (length == 0 || !items.Any()) return new List<List<T>> { new List<T>() };
return from item in items.Distinct()
from permutation in GetVariationsWithoutDuplicates(items.Where(i => !EqualityComparer<T>.Default.Equals(i, item)).ToList(), length - 1)
select Prepend(item, permutation);
}
public static IEnumerable<IEnumerable<T>> GetVariations<T>(IList<T> items, int length)
{
if (length == 0 || !items.Any()) return new List<List<T>> { new List<T>() };
return from item in items
from permutation in GetVariations(Remove(item, items).ToList(), length - 1)
select Prepend(item, permutation);
}
public static IEnumerable<T> Prepend<T>(T first, IEnumerable<T> rest)
{
yield return first;
foreach (var item in rest) yield return item;
}
public static IEnumerable<T> Remove<T>(T item, IEnumerable<T> from)
{
var isRemoved = false;
foreach (var i in from)
{
if (!EqualityComparer<T>.Default.Equals(item, i) || isRemoved) yield return i;
else isRemoved = true;
}
}
On my 3.1 GHz Core 2 Duo, I tested with this:
public static void Test(Func<IList<int>, int, IEnumerable<IEnumerable<int>>> getVariations)
{
var max = 11;
var timer = System.Diagnostics.Stopwatch.StartNew();
for (int i = 1; i < max; ++i)
for (int j = 1; j < i; ++j)
getVariations(MakeList(i), j).Count();
timer.Stop();
Console.WriteLine("{0,40}{1} ms", getVariations.Method.Name, timer.ElapsedMilliseconds);
}
// Make a list that repeats to guarantee we have duplicates
public static IList<int> MakeList(int size)
{
return Enumerable.Range(0, size/2).Concat(Enumerable.Range(0, size - size/2)).ToList();
}
Unoptimized
GetVariations 11894 ms
GetVariationsWithoutDuplicates 9 ms
OtherAnswerGetVariations 22485 ms
OtherAnswerGetVariationsWithDuplicates 243415 ms
With compiler optimizations
GetVariations 9667 ms
GetVariationsWithoutDuplicates 8 ms
OtherAnswerGetVariations 19739 ms
OtherAnswerGetVariationsWithDuplicates 228802 ms

You're not looking for a permutation, but for a variation. Here is a possible algorithm. I prefer iterator methods for functions that can potentially return very many elements. This way, the caller can decide if he really needs all elements:
IEnumerable<IList<T>> GetVariations<T>(IList<T> offers, int length)
{
var startIndices = new int[length];
var variationElements = new HashSet<T>(); //for duplicate detection
while (startIndices[0] < offers.Count)
{
var variation = new List<T>(length);
var valid = true;
for (int i = 0; i < length; ++i)
{
var element = offers[startIndices[i]];
if (variationElements.Contains(element))
{
valid = false;
break;
}
variation.Add(element);
variationElements.Add(element);
}
if (valid)
yield return variation;
//Count up the indices
startIndices[length - 1]++;
for (int i = length - 1; i > 0; --i)
{
if (startIndices[i] >= offers.Count)
{
startIndices[i] = 0;
startIndices[i - 1]++;
}
else
break;
}
variationElements.Clear();
}
}
The idea for this algorithm is to use a number in offers.Count base. For three offers, all digits are in the range 0-2. We then basically increment this number step by step and return the offers that reside at the specified indices. If you want to allow duplicates, you can remove the check and the HashSet<T>.
Update
Here is an optimized variant that does the duplicate check on the index level. In my tests it is a lot faster than the previous variant:
IEnumerable<IList<T>> GetVariations<T>(IList<T> offers, int length)
{
var startIndices = new int[length];
for (int i = 0; i < length; ++i)
startIndices[i] = i;
var indices = new HashSet<int>(); // for duplicate check
while (startIndices[0] < offers.Count)
{
var variation = new List<T>(length);
for (int i = 0; i < length; ++i)
{
variation.Add(offers[startIndices[i]]);
}
yield return variation;
//Count up the indices
AddOne(startIndices, length - 1, offers.Count - 1);
//duplicate check
var check = true;
while (check)
{
indices.Clear();
for (int i = 0; i <= length; ++i)
{
if (i == length)
{
check = false;
break;
}
if (indices.Contains(startIndices[i]))
{
var unchangedUpTo = AddOne(startIndices, i, offers.Count - 1);
indices.Clear();
for (int j = 0; j <= unchangedUpTo; ++j )
{
indices.Add(startIndices[j]);
}
int nextIndex = 0;
for(int j = unchangedUpTo + 1; j < length; ++j)
{
while (indices.Contains(nextIndex))
nextIndex++;
startIndices[j] = nextIndex++;
}
break;
}
indices.Add(startIndices[i]);
}
}
}
}
int AddOne(int[] indices, int position, int maxElement)
{
//returns the index of the last element that has not been changed
indices[position]++;
for (int i = position; i > 0; --i)
{
if (indices[i] > maxElement)
{
indices[i] = 0;
indices[i - 1]++;
}
else
return i;
}
return 0;
}

If I got you correct here is what you need
this will create permutations based on the specified chain limit
public static List<List<T>> GetPerms<T>(List<T> list, int chainLimit)
{
if (list.Count() == 1)
return new List<List<T>> { list };
return list
.Select((outer, outerIndex) =>
GetPerms(list.Where((inner, innerIndex) => innerIndex != outerIndex).ToList(), chainLimit)
.Select(perms => (new List<T> { outer }).Union(perms).Take(chainLimit)))
.SelectMany<IEnumerable<IEnumerable<T>>, List<T>>(sub => sub.Select<IEnumerable<T>, List<T>>(s => s.ToList()))
.Distinct(new PermComparer<T>()).ToList();
}
class PermComparer<T> : IEqualityComparer<List<T>>
{
public bool Equals(List<T> x, List<T> y)
{
return x.SequenceEqual(y);
}
public int GetHashCode(List<T> obj)
{
return (int)obj.Average(o => o.GetHashCode());
}
}
and you'll call it like this
List<List<AdServer.Offers>> lst = GetPerms<AdServer.Offers>(offers, 2);
I made this function is pretty generic so you may use it for other purpose too
eg
List<string> list = new List<string>(new[] { "apple", "banana", "orange", "cherry" });
List<List<string>> perms = GetPerms<string>(list, 2);
result