Say I have a list of integers:
List<int> myInts = new List<int>() {1,2,3,5,8,13,21};
I would like to get the next available integer, ordered by increasing integer. Not the last or highest one, but in this case the next integer that is not in this list. In this case the number is 4.
Is there a LINQ statement that would give me this? As in:
var nextAvailable = myInts.SomeCoolLinqMethod();
Edit: Crap. I said the answer should be 2 but I meant 4. I apologize for that!
For example: Imagine that you are responsible for handing out process IDs. You want to get the list of current process IDs, and issue a next one, but the next one should not just be the highest value plus one. Rather, it should be the next one available from an ordered list of process IDs. You could get the next available starting with the highest, it does not really matter.
I see a lot of answers that write a custom extension method, but it is possible to solve this problem with the standard linq extension methods and the static Enumerable class:
List<int> myInts = new List<int>() {1,2,3,5,8,13,21};
// This will set firstAvailable to 4.
int firstAvailable = Enumerable.Range(1, Int32.MaxValue).Except(myInts).First();
The answer provided by #Kevin has a undesirable performance profile. The logic will access the source sequence numerous times: once for the .Count call, once for the .FirstOrDefault call, and once for each .Contains call. If the IEnumerable<int> instance is a deferred sequence, such as the result of a .Select call, this will cause at least 2 calculations of the sequence, along with once for each number. Even if you pass a list to the method, it will potentially go through the entire list for each checked number. Imagine running it on the sequence { 1, 1000000 } and you can see how it would not perform well.
LINQ strives to iterate source sequences no more than once. This is possible in general and can have a big impact on the performance of your code. Below is an extension method which will iterate the sequence exactly once. It does so by looking for the difference between each successive pair, then adds 1 to the first lower number which is more than 1 away from the next number:
public static int? FirstMissing(this IEnumerable<int> numbers)
{
int? priorNumber = null;
foreach(var number in numbers.OrderBy(n => n))
{
var difference = number - priorNumber;
if(difference != null && difference > 1)
{
return priorNumber + 1;
}
priorNumber = number;
}
return priorNumber == null ? (int?) null : priorNumber + 1;
}
Since this extension method can be called on any arbitrary sequence of integers, we make sure to order them before we iterate. We then calculate the difference between the current number and the prior number. If this is the first number in the list, priorNumber will be null and thus difference will be null. If this is not the first number in the list, we check to see if the difference from the prior number is exactly 1. If not, we know there is a gap and we can add 1 to the prior number.
You can adjust the return statement to handle sequences with 0 or 1 items as you see fit; I chose to return null for empty sequences and n + 1 for the sequence { n }.
This will be fairly efficient:
static int Next(this IEnumerable<int> source)
{
int? last = null;
foreach (var next in source.OrderBy(_ => _))
{
if (last.HasValue && last.Value + 1 != next)
{
return last.Value + 1;
}
last = next;
}
return last.HasValue ? last.Value + 1 : Int32.MaxValue;
}
public static class IntExtensions
{
public static int? SomeCoolLinqMethod(this IEnumerable<int> ints)
{
int counter = ints.Count() > 0 ? ints.First() : -1;
while (counter < int.MaxValue)
{
if (!ints.Contains(++counter)) return counter;
}
return null;
}
}
Usage:
var nextAvailable = myInts.SomeCoolLinqMethod();
Ok, here is the solution that I came up with that works for me.
var nextAvailableInteger = Enumerable.Range(myInts.Min(),myInts.Max()).FirstOrDefault( r=> !myInts.Contains(r));
If anyone has a more elegant solution I would be happy to accept that one. But for now, this is what I'm putting in my code and moving on.
Edit: this is what I implemented after Kevin's suggestion to add an extension method. And that was the real answer - that no single LINQ extension would do so it makes more sense to add my own. That is really what I was looking for.
public static int NextAvailableInteger(this IEnumerable<int> ints)
{
return NextAvailableInteger(ints, 1); // by default we use one
}
public static int NextAvailableInteger(this IEnumerable<int> ints, int defaultValue)
{
if (ints == null || ints.Count() == 0) return defaultValue;
var ordered = ints.OrderBy(v => v);
int counter = ints.Min();
int max = ints.Max();
while (counter < max)
{
if (!ordered.Contains(++counter)) return counter;
}
return (++counter);
}
Not sure if this qualifies as a cool Linq method, but using the left outer join idea from This SO Answer
var thelist = new List<int> {1,2,3,4,5,100,101};
var nextAvailable = (from curr in thelist
join next in thelist
on curr + 1 equals next into g
from newlist in g.DefaultIfEmpty()
where !g.Any ()
orderby curr
select curr + 1).First();
This puts the processing on the sql server side if you're using Linq to Sql, and allows you to not have to pull the ID lists from the server to memory.
var nextAvailable = myInts.Prepend(0).TakeWhile((x,i) => x == i).Last() + 1;
It is 7 years later, but there are better ways of doing this than the selected answer or the answer with the most votes.
The list is already in order, and based on the example 0 doesn't count. We can just prepend 0 and check if each item matches it's index. TakeWhile will stop evaluating once it hits a number that doesn't match, or at the end of the list.
The answer is the last item that matches, plus 1.
TakeWhile is more efficient than enumerating all the possible numbers then excluding the existing numbers using Except, because we TakeWhile will only go through the list until it finds the first available number, and the resulting Enumerable collection is at most n.
The answer using Except generates an entire enumerable of answers that are not needed just to grab the first one. Linq can do some optimization with First(), but it still much slower and more memory intensive than TakeWhile.
Related
there's an exercise i need to do, given a List i need to sort the content using ONLY recursive methods (no while, do while, for, foreach).
So... i'm struggling (for over 2 hours now) and i dont know how to even begin.
The function must be
List<int> SortHighestToLowest (List<int> list) {
}
I THINK i should check if the previous number is greater than the actual number and so on but what if the last number is greater than the first number on the list?, that's why im having a headache.
I appreciate your help, thanks a lot.
[EDIT]
I delivered the exercise but then teacher said i shouldn't use external variables like i did here:
List<int> _tempList2 = new List<int>();
int _actualListIndex = 0;
int _actualMaxNumber = 0;
int _actualMaxNumberIndex = 0;
List<int> SortHighestToLowest(List<int> list)
{
if (list.Count == 0)
return _tempList2;
if (_actualListIndex == 0)
_actualMaxNumber = list[0];
if (_actualListIndex < list.Count -1)
{
_actualListIndex++;
if (list[_actualListIndex] > _actualMaxNumber)
{
_actualMaxNumberIndex = _actualListIndex;
_actualMaxNumber = list[_actualListIndex];
}
return SortHighestToLowest(list);
}
_tempList2.Add(_actualMaxNumber);
list.RemoveAt(_actualMaxNumberIndex);
_actualListIndex = 0;
_actualMaxNumberIndex = 0;
return SortHighestToLowest(list);
}
Exercise is done and i approved (thanks to other exercises as well) but i was wondering if there's a way of doing this without external variables and without using System.Linq like String.Empty's response (im just curious, the community helped me to solve my issue and im thankful).
I am taking your instructions to the letter here.
Only recursive methods
No while, do while, for, foreach
Signature must be List<int> SortHighestToLowest(List<int> list)
Now, I do assume you may use at least the built-in properties and methods of the List<T> type. If not, you would have a hard time even reading the elements of your list.
That said, any calls to Sort or OrderBy methods would be beyond the point here, since they would render any recursive method useless.
I also assume it is okay to use other lists in the process, since you didn't mention anything in regards to that.
With all that in mind, I came to this piece below, making use of Max and Remove methods from List<T> class, and a new list of integers for each recursive call:
public static List<int> SortHighestToLowest(List<int> list)
{
// recursivity breaker
if (list.Count <= 1)
return list;
// remove highest item
var max = list.Max();
list.Remove(max);
// append highest item to recursive call for the remainder of the list
return new List<int>(SortHighestToLowest(list)) { max };
}
For solving this problem, try to solve smaller subsets. Consider the following list
[1,5,3,2]
Let's take the last element out of list, and consider the rest as sorted which will be [1,3,5] and 2. Now the problem reduces to another problem of inserting this 2 in its correct position. If we can insert it in correct position then the array becomes sorted. This can be applied recursively.
For every recursive problem there should be a base condition w.r.t the hypothesis we make. For the first problem the base condition is array with single element. A single element array is always sorted.
For the second insert problem the base condition will be an empty array or the last element in array is less than the element to be inserted. In both cases the element is inserted at the end.
Algorithm
---------
Sort(list)
if(list.count==1)
return
temp = last element of list
temp_list = list with last element removed
Sort(temp_list)
Insert(temp_list, temp)
Insert(list, temp)
if(list.count ==0 || list[n-1] <= temp)
list.insert(temp)
return
insert_temp = last element of list
insert_temp_list = list with last element removed
Insert(insert_temo_list, insert_temp)
For Insert after base condition its calling recursively till it find the correct position for the last element which is removed.
This is my first question here so I hope I'm doing right.
I have to create a List of array of integer:
List<int[]> finalList = new List<int[]>();
in order to store all the combinations of K elements with N numbers.
For example:
N=5, K=2 => {1,2},{1,3},{1,4},...
Everything is all right but I want to avoid the repetitions of the same combination in the list({1,2} and {2,1} for example). So before adding the tmpArray (where I temporally store the new combination) in the list, I want to check if it's already stored.
Here it's what I'm doing:
create the tmpArray with the next combination (OK)
sort tmpArray (OK)
check if the List already contains tmpArray with the following code:
if (!finalList.Contains(tmpArray))
finalList.Add(tmpArray);
but it doesn't work. Can anyone help me with this issue?
Array is a reference type - your Contains query will not do what you want (compare all members in order).
You may use something like this:
if (!finalList.Any(x => x.SequenceEqual(tmpArray))
{
finalList.Add(tmpArray);
}
(Make sure you add a using System.Linq to the top of your file)
I suggest you learn more about value vs. reference types, Linq and C# data structure fundamentals. While above query should work it will be slow - O(n*m) where n = number of arrays in finalList and m length of each array.
For larger arrays some precomputing (e.g. a hashcode for each of the arrays) that allows you a faster comparison might be beneficial.
If I remember correctly, contains will either check the value for value data types or it will check the address for object types. An array is an object type, so the contains is only checking if the address in memory is stored in your list. You'll have to check each item in this list and perform some type of algorithm to check that the values of the array are in the list.
Linq, Lambda, or brute force checking comes to mind.
BrokenGlass gives a good suggestion with Linq and Lambda.
Brute Force:
bool itemExists = true;
foreach (int[] ints in finalList)
{
if (ints.Length != tmpArray.Length)
{
itemExists = false;
break;
}
else
{
// Compare each element
for (int i = 0; i < tmpArray.Length; i++)
{
if (ints[i] != tmpArray[i])
{
itemExists = false;
break;
}
}
// Have to check to break from the foreach loop
if (itemExists == false)
{
break;
}
}
}
if (itemExists == false)
{
finalList.add(tmpArray);
}
I want to count the occurences of objects within a List<T> that match a certain condition.
For example like this
int List<T>.Count(Predicate<T> match)
So for example if have a list of chores, I can see how many are overdue.
int overdue = listOfChores.Count((element) => { return element.DueDate <= DateTime.Today; });
I know that does not exist and so far I solve problems like that in the following way:
int overdue = listOfChores.FindAll([...]).Count;
However that allocates and initializes a new List etc. only to get the count.
A way to do this with less allocation overhead etc.:
int good = 0;
foreach(chore element in listOfChores)
if(element.DueDate <= DateTime.Today)
good++;
The last approach can also be exandend to count several conditions without iterating over the loop more than once. (I already found that getting the count property only takes O(1), but making the List to count from still eats a lot of time)
int a = 0;
int b = 0;
foreach(chore element in listOfChores)
if(element.CondA)
a++;
if(element.CondB)
b++;
Given this I could even imagine something like
int[] List<T>.Count(Predicate<T>[] matches)
My question(s):
Is there such a thing, just I haven't found it yet?
If not: What would be way to implement such functionality?
EDIT :
Adding LINQ looks like it fixes it.
You just have your syntax slightly off. This is how to use Count :
int overdue = listOfChores.Count(element => element.DueDate <= DateTime.Today);
If you already have a Predicate<T> and want to pass it to Count just call it like a function:
Predicate<Chore> p = (element) => element.DueDate <= DateTime.Today;
int overdue = listOfChores.Count(element => p(element));
There's is a count method using a predicate : see Enumerable.Count Method (IEnumerable, Func)
Note that this method is an extension method and you can use it only if you add a reference to the System.Linq namespace.
What is the quickest (and least resource intensive) to compare two massive (>50.000 items) and as a result have two lists like the ones below:
items that show up in the first list but not in the second
items that show up in the second list but not in the first
Currently I'm working with the List or IReadOnlyCollection and solve this issue in a linq query:
var list1 = list.Where(i => !list2.Contains(i)).ToList();
var list2 = list2.Where(i => !list.Contains(i)).ToList();
But this doesn't perform as good as i would like.
Any idea of making this quicker and less resource intensive as i need to process a lot of lists?
Use Except:
var firstNotSecond = list1.Except(list2).ToList();
var secondNotFirst = list2.Except(list1).ToList();
I suspect there are approaches which would actually be marginally faster than this, but even this will be vastly faster than your O(N * M) approach.
If you want to combine these, you could create a method with the above and then a return statement:
return !firstNotSecond.Any() && !secondNotFirst.Any();
One point to note is that there is a difference in results between the original code in the question and the solution here: any duplicate elements which are only in one list will only be reported once with my code, whereas they'd be reported as many times as they occur in the original code.
For example, with lists of [1, 2, 2, 2, 3] and [1], the "elements in list1 but not list2" result in the original code would be [2, 2, 2, 3]. With my code it would just be [2, 3]. In many cases that won't be an issue, but it's worth being aware of.
Enumerable.SequenceEqual Method
Determines whether two sequences are equal according to an equality comparer.
MS.Docs
Enumerable.SequenceEqual(list1, list2);
This works for all primitive data types. If you need to use it on custom objects you need to implement IEqualityComparer
Defines methods to support the comparison of objects for equality.
IEqualityComparer Interface
Defines methods to support the comparison of objects for equality.
MS.Docs for IEqualityComparer
More efficient would be using Enumerable.Except:
var inListButNotInList2 = list.Except(list2);
var inList2ButNotInList = list2.Except(list);
This method is implemented by using deferred execution. That means you could write for example:
var first10 = inListButNotInList2.Take(10);
It is also efficient since it internally uses a Set<T> to compare the objects. It works by first collecting all distinct values from the second sequence, and then streaming the results of the first, checking that they haven't been seen before.
If you want the results to be case insensitive, the following will work:
List<string> list1 = new List<string> { "a.dll", "b1.dll" };
List<string> list2 = new List<string> { "A.dll", "b2.dll" };
var firstNotSecond = list1.Except(list2, StringComparer.OrdinalIgnoreCase).ToList();
var secondNotFirst = list2.Except(list1, StringComparer.OrdinalIgnoreCase).ToList();
firstNotSecond would contain b1.dll
secondNotFirst would contain b2.dll
using System.Collections.Generic;
using System.Linq;
namespace YourProject.Extensions
{
public static class ListExtensions
{
public static bool SetwiseEquivalentTo<T>(this List<T> list, List<T> other)
where T: IEquatable<T>
{
if (list.Except(other).Any())
return false;
if (other.Except(list).Any())
return false;
return true;
}
}
}
Sometimes you only need to know if two lists are different, and not what those differences are. In that case, consider adding this extension method to your project. Note that your listed objects should implement IEquatable!
Usage:
public sealed class Car : IEquatable<Car>
{
public Price Price { get; }
public List<Component> Components { get; }
...
public override bool Equals(object obj)
=> obj is Car other && Equals(other);
public bool Equals(Car other)
=> Price == other.Price
&& Components.SetwiseEquivalentTo(other.Components);
public override int GetHashCode()
=> Components.Aggregate(
Price.GetHashCode(),
(code, next) => code ^ next.GetHashCode()); // Bitwise XOR
}
Whatever the Component class is, the methods shown here for Car should be implemented almost identically.
It's very important to note how we've written GetHashCode. In order to properly implement IEquatable, Equals and GetHashCode must operate on the instance's properties in a logically compatible way.
Two lists with the same contents are still different objects, and will produce different hash codes. Since we want these two lists to be treated as equal, we must let GetHashCode produce the same value for each of them. We can accomplish this by delegating the hashcode to every element in the list, and using the standard bitwise XOR to combine them all. XOR is order-agnostic, so it doesn't matter if the lists are sorted differently. It only matters that they contain nothing but equivalent members.
Note: the strange name is to imply the fact that the method does not consider the order of the elements in the list. If you do care about the order of the elements in the list, this method is not for you!
try this way:
var difList = list1.Where(a => !list2.Any(a1 => a1.id == a.id))
.Union(list2.Where(a => !list1.Any(a1 => a1.id == a.id)));
Not for this Problem, but here's some code to compare lists for equal and not! identical objects:
public class EquatableList<T> : List<T>, IEquatable<EquatableList<T>> where T : IEquatable<T>
/// <summary>
/// True, if this contains element with equal property-values
/// </summary>
/// <param name="element">element of Type T</param>
/// <returns>True, if this contains element</returns>
public new Boolean Contains(T element)
{
return this.Any(t => t.Equals(element));
}
/// <summary>
/// True, if list is equal to this
/// </summary>
/// <param name="list">list</param>
/// <returns>True, if instance equals list</returns>
public Boolean Equals(EquatableList<T> list)
{
if (list == null) return false;
return this.All(list.Contains) && list.All(this.Contains);
}
If only combined result needed, this will work too:
var set1 = new HashSet<T>(list1);
var set2 = new HashSet<T>(list2);
var areEqual = set1.SetEquals(set2);
where T is type of lists element.
While Jon Skeet's answer is an excellent advice for everyday's practice with small to moderate number of elements (up to a few millions) it is nevertheless not the fastest approach and not very resource efficient. An obvious drawback is the fact that getting the full difference requires two passes over the data (even three if the elements that are equal are of interest as well). Clearly, this can be avoided by a customized reimplementation of the Except method, but it remains that the creation of a hash set requires a lot of memory and the computation of hashes requires time.
For very large data sets (in the billions of elements) it usually pays off to consider the particular circumstances. Here are a few ideas that might provide some inspiration:
If the elements can be compared (which is almost always the case in practice), then sorting the lists and applying the following zip approach is worth consideration:
/// <returns>The elements of the specified (ascendingly) sorted enumerations that are
/// contained only in one of them, together with an indicator,
/// whether the element is contained in the reference enumeration (-1)
/// or in the difference enumeration (+1).</returns>
public static IEnumerable<Tuple<T, int>> FindDifferences<T>(IEnumerable<T> sortedReferenceObjects,
IEnumerable<T> sortedDifferenceObjects, IComparer<T> comparer)
{
var refs = sortedReferenceObjects.GetEnumerator();
var diffs = sortedDifferenceObjects.GetEnumerator();
bool hasNext = refs.MoveNext() && diffs.MoveNext();
while (hasNext)
{
int comparison = comparer.Compare(refs.Current, diffs.Current);
if (comparison == 0)
{
// insert code that emits the current element if equal elements should be kept
hasNext = refs.MoveNext() && diffs.MoveNext();
}
else if (comparison < 0)
{
yield return Tuple.Create(refs.Current, -1);
hasNext = refs.MoveNext();
}
else
{
yield return Tuple.Create(diffs.Current, 1);
hasNext = diffs.MoveNext();
}
}
}
This can e.g. be used in the following way:
const int N = <Large number>;
const int omit1 = 231567;
const int omit2 = 589932;
IEnumerable<int> numberSequence1 = Enumerable.Range(0, N).Select(i => i < omit1 ? i : i + 1);
IEnumerable<int> numberSequence2 = Enumerable.Range(0, N).Select(i => i < omit2 ? i : i + 1);
var numberDiffs = FindDifferences(numberSequence1, numberSequence2, Comparer<int>.Default);
Benchmarking on my computer gave the following result for N = 1M:
Method
Mean
Error
StdDev
Ratio
Gen 0
Gen 1
Gen 2
Allocated
DiffLinq
115.19 ms
0.656 ms
0.582 ms
1.00
2800.0000
2800.0000
2800.0000
67110744 B
DiffZip
23.48 ms
0.018 ms
0.015 ms
0.20
-
-
-
720 B
And for N = 100M:
Method
Mean
Error
StdDev
Ratio
Gen 0
Gen 1
Gen 2
Allocated
DiffLinq
12.146 s
0.0427 s
0.0379 s
1.00
13000.0000
13000.0000
13000.0000
8589937032 B
DiffZip
2.324 s
0.0019 s
0.0018 s
0.19
-
-
-
720 B
Note that this example of course benefits from the fact that the lists are already sorted and integers can be very efficiently compared. But this is exactly the point: If you do have favourable circumstances, make sure that you exploit them.
A few further comments: The speed of the comparison function is clearly relevant for the overall performance, so it may be beneficial to optimize it. The flexibility to do so is a benefit of the zipping approach. Furthermore, parallelization seems more feasible to me, although by no means easy and maybe not worth the effort and the overhead. Nevertheless, a simple way to speed up the process by roughly a factor of 2, is to split the lists respectively in two halfs (if it can be efficiently done) and compare the parts in parallel, one processing from front to back and the other in reverse order.
I have used this code to compare two list which has million of records.
This method will not take much time
//Method to compare two list of string
private List<string> Contains(List<string> list1, List<string> list2)
{
List<string> result = new List<string>();
result.AddRange(list1.Except(list2, StringComparer.OrdinalIgnoreCase));
result.AddRange(list2.Except(list1, StringComparer.OrdinalIgnoreCase));
return result;
}
I compared 3 different methods for comparing different data sets. Tests below create a string collection of all the numbers from 0 to length - 1, then another collection with the same range, but with even numbers. I then pick out the odd numbers from the first collection.
Using Linq Except
public void TestExcept()
{
WriteLine($"Except {DateTime.Now}");
int length = 20000000;
var dateTime = DateTime.Now;
var array = new string[length];
for (int i = 0; i < length; i++)
{
array[i] = i.ToString();
}
Write("Populate set processing time: ");
WriteLine(DateTime.Now - dateTime);
var newArray = new string[length/2];
int j = 0;
for (int i = 0; i < length; i+=2)
{
newArray[j++] = i.ToString();
}
dateTime = DateTime.Now;
Write("Count of items: ");
WriteLine(array.Except(newArray).Count());
Write("Count processing time: ");
WriteLine(DateTime.Now - dateTime);
}
Output
Except 2021-08-14 11:43:03 AM
Populate set processing time: 00:00:03.7230479
2021-08-14 11:43:09 AM
Count of items: 10000000
Count processing time: 00:00:02.9720879
Using HashSet.Add
public void TestHashSet()
{
WriteLine($"HashSet {DateTime.Now}");
int length = 20000000;
var dateTime = DateTime.Now;
var hashSet = new HashSet<string>();
for (int i = 0; i < length; i++)
{
hashSet.Add(i.ToString());
}
Write("Populate set processing time: ");
WriteLine(DateTime.Now - dateTime);
var newHashSet = new HashSet<string>();
for (int i = 0; i < length; i+=2)
{
newHashSet.Add(i.ToString());
}
dateTime = DateTime.Now;
Write("Count of items: ");
// HashSet Add returns true if item is added successfully (not previously existing)
WriteLine(hashSet.Where(s => newHashSet.Add(s)).Count());
Write("Count processing time: ");
WriteLine(DateTime.Now - dateTime);
}
Output
HashSet 2021-08-14 11:42:43 AM
Populate set processing time: 00:00:05.6000625
Count of items: 10000000
Count processing time: 00:00:01.7703057
Special HashSet test:
public void TestLoadingHashSet()
{
int length = 20000000;
var array = new string[length];
for (int i = 0; i < length; i++)
{
array[i] = i.ToString();
}
var dateTime = DateTime.Now;
var hashSet = new HashSet<string>(array);
Write("Time to load hashset: ");
WriteLine(DateTime.Now - dateTime);
}
> TestLoadingHashSet()
Time to load hashset: 00:00:01.1918160
Using .Contains
public void TestContains()
{
WriteLine($"Contains {DateTime.Now}");
int length = 20000000;
var dateTime = DateTime.Now;
var array = new string[length];
for (int i = 0; i < length; i++)
{
array[i] = i.ToString();
}
Write("Populate set processing time: ");
WriteLine(DateTime.Now - dateTime);
var newArray = new string[length/2];
int j = 0;
for (int i = 0; i < length; i+=2)
{
newArray[j++] = i.ToString();
}
dateTime = DateTime.Now;
WriteLine(dateTime);
Write("Count of items: ");
WriteLine(array.Where(a => !newArray.Contains(a)).Count());
Write("Count processing time: ");
WriteLine(DateTime.Now - dateTime);
}
Output
Contains 2021-08-14 11:19:44 AM
Populate set processing time: 00:00:03.1046998
2021-08-14 11:19:49 AM
Count of items: Hosting process exited with exit code 1.
(Didnt complete. Killed it after 14 minutes)
Conclusion:
Linq Except ran approximately 1 second slower on my device than using HashSets (n=20,000,000).
Using Where and Contains ran for a very long time
Closing remarks on HashSets:
Unique data
Make sure to override GetHashCode (correctly) for class types
May need up to 2x the memory if you make a copy of the data set, depending on implementation
HashSet is optimized for cloning other HashSets using the IEnumerable constructor, but it is slower to convert other collections to HashSets (see special test above)
First approach:
if (list1 != null && list2 != null && list1.Select(x => list2.SingleOrDefault(y => y.propertyToCompare == x.propertyToCompare && y.anotherPropertyToCompare == x.anotherPropertyToCompare) != null).All(x => true))
return true;
Second approach if you are ok with duplicate values:
if (list1 != null && list2 != null && list1.Select(x => list2.Any(y => y.propertyToCompare == x.propertyToCompare && y.anotherPropertyToCompare == x.anotherPropertyToCompare)).All(x => true))
return true;
Both Jon Skeet's and miguelmpn's answers are good. It depends on whether the order of the list elements is important or not:
// take order into account
bool areEqual1 = Enumerable.SequenceEqual(list1, list2);
// ignore order
bool areEqual2 = !list1.Except(list2).Any() && !list2.Except(list1).Any();
One line:
var list1 = new List<int> { 1, 2, 3 };
var list2 = new List<int> { 1, 2, 3, 4 };
if (list1.Except(list2).Count() + list2.Except(list1).Count() == 0)
Console.WriteLine("same sets");
I did the generic function for comparing two lists.
public static class ListTools
{
public enum RecordUpdateStatus
{
Added = 1,
Updated = 2,
Deleted = 3
}
public class UpdateStatu<T>
{
public T CurrentValue { get; set; }
public RecordUpdateStatus UpdateStatus { get; set; }
}
public static List<UpdateStatu<T>> CompareList<T>(List<T> currentList, List<T> inList, string uniqPropertyName)
{
var res = new List<UpdateStatu<T>>();
res.AddRange(inList.Where(a => !currentList.Any(x => x.GetType().GetProperty(uniqPropertyName).GetValue(x)?.ToString().ToLower() == a.GetType().GetProperty(uniqPropertyName).GetValue(a)?.ToString().ToLower()))
.Select(a => new UpdateStatu<T>
{
CurrentValue = a,
UpdateStatus = RecordUpdateStatus.Added,
}));
res.AddRange(currentList.Where(a => !inList.Any(x => x.GetType().GetProperty(uniqPropertyName).GetValue(x)?.ToString().ToLower() == a.GetType().GetProperty(uniqPropertyName).GetValue(a)?.ToString().ToLower()))
.Select(a => new UpdateStatu<T>
{
CurrentValue = a,
UpdateStatus = RecordUpdateStatus.Deleted,
}));
res.AddRange(currentList.Where(a => inList.Any(x => x.GetType().GetProperty(uniqPropertyName).GetValue(x)?.ToString().ToLower() == a.GetType().GetProperty(uniqPropertyName).GetValue(a)?.ToString().ToLower()))
.Select(a => new UpdateStatu<T>
{
CurrentValue = a,
UpdateStatus = RecordUpdateStatus.Updated,
}));
return res;
}
}
I think this is a simple and easy way to compare two lists element by element
x=[1,2,3,5,4,8,7,11,12,45,96,25]
y=[2,4,5,6,8,7,88,9,6,55,44,23]
tmp = []
for i in range(len(x)) and range(len(y)):
if x[i]>y[i]:
tmp.append(1)
else:
tmp.append(0)
print(tmp)
Maybe it's funny, but this works for me:
string.Join("",List1) != string.Join("", List2)
This is the best solution you'll found
var list3 = list1.Where(l => list2.ToList().Contains(l));
Context: C# 3.0, .Net 3.5
Suppose I have a method that generates random numbers (forever):
private static IEnumerable<int> RandomNumberGenerator() {
while (true) yield return GenerateRandomNumber(0, 100);
}
I need to group those numbers in groups of 10, so I would like something like:
foreach (IEnumerable<int> group in RandomNumberGenerator().Slice(10)) {
Assert.That(group.Count() == 10);
}
I have defined Slice method, but I feel there should be one already defined. Here is my Slice method, just for reference:
private static IEnumerable<T[]> Slice<T>(IEnumerable<T> enumerable, int size) {
var result = new List<T>(size);
foreach (var item in enumerable) {
result.Add(item);
if (result.Count == size) {
yield return result.ToArray();
result.Clear();
}
}
}
Question: is there an easier way to accomplish what I'm trying to do? Perhaps Linq?
Note: above example is a simplification, in my program I have an Iterator that scans given matrix in a non-linear fashion.
EDIT: Why Skip+Take is no good.
Effectively what I want is:
var group1 = RandomNumberGenerator().Skip(0).Take(10);
var group2 = RandomNumberGenerator().Skip(10).Take(10);
var group3 = RandomNumberGenerator().Skip(20).Take(10);
var group4 = RandomNumberGenerator().Skip(30).Take(10);
without the overhead of regenerating number (10+20+30+40) times. I need a solution that will generate exactly 40 numbers and break those in 4 groups by 10.
Are Skip and Take of any use to you?
Use a combination of the two in a loop to get what you want.
So,
list.Skip(10).Take(10);
Skips the first 10 records and then takes the next 10.
I have done something similar. But I would like it to be simpler:
//Remove "this" if you don't want it to be a extension method
public static IEnumerable<IList<T>> Chunks<T>(this IEnumerable<T> xs, int size)
{
var curr = new List<T>(size);
foreach (var x in xs)
{
curr.Add(x);
if (curr.Count == size)
{
yield return curr;
curr = new List<T>(size);
}
}
}
I think yours are flawed. You return the same array for all your chunks/slices so only the last chunk/slice you take would have the correct data.
Addition: Array version:
public static IEnumerable<T[]> Chunks<T>(this IEnumerable<T> xs, int size)
{
var curr = new T[size];
int i = 0;
foreach (var x in xs)
{
curr[i % size] = x;
if (++i % size == 0)
{
yield return curr;
curr = new T[size];
}
}
}
Addition: Linq version (not C# 2.0). As pointed out, it will not work on infinite sequences and will be a great deal slower than the alternatives:
public static IEnumerable<T[]> Chunks<T>(this IEnumerable<T> xs, int size)
{
return xs.Select((x, i) => new { x, i })
.GroupBy(xi => xi.i / size, xi => xi.x)
.Select(g => g.ToArray());
}
Using Skip and Take would be a very bad idea. Calling Skip on an indexed collection may be fine, but calling it on any arbitrary IEnumerable<T> is liable to result in enumeration over the number of elements skipped, which means that if you're calling it repeatedly you're enumerating over the sequence an order of magnitude more times than you need to be.
Complain of "premature optimization" all you want; but that is just ridiculous.
I think your Slice method is about as good as it gets. I was going to suggest a different approach that would provide deferred execution and obviate the intermediate array allocation, but that is a dangerous game to play (i.e., if you try something like ToList on such a resulting IEnumerable<T> implementation, without enumerating over the inner collections, you'll end up in an endless loop).
(I've removed what was originally here, as the OP's improvements since posting the question have since rendered my suggestions here redundant.)
Let's see if you even need the complexity of Slice. If your random number generates is stateless, I would assume each call to it would generate unique random numbers, so perhaps this would be sufficient:
var group1 = RandomNumberGenerator().Take(10);
var group2 = RandomNumberGenerator().Take(10);
var group3 = RandomNumberGenerator().Take(10);
var group4 = RandomNumberGenerator().Take(10);
Each call to Take returns a new group of 10 numbers.
Now, if your random number generator re-seeds itself with a specific value each time it's iterated, this won't work. You'll simply get the same 10 values for each group. So instead, you would use:
var generator = RandomNumberGenerator();
var group1 = generator.Take(10);
var group2 = generator.Take(10);
var group3 = generator.Take(10);
var group4 = generator.Take(10);
This maintains an instance of the generator so that you can continue retrieving values without re-seeding the generator.
You could use the Skip and Take methods with any Enumerable object.
For your edit :
How about a function that takes a slice number and a slice size as a parameter?
private static IEnumerable<T> Slice<T>(IEnumerable<T> enumerable, int sliceSize, int sliceNumber) {
return enumerable.Skip(sliceSize * sliceNumber).Take(sliceSize);
}
It seems like we'd prefer for an IEnumerable<T> to have a fixed position counter so that we can do
var group1 = items.Take(10);
var group2 = items.Take(10);
var group3 = items.Take(10);
var group4 = items.Take(10);
and get successive slices rather than getting the first 10 items each time. We can do that with a new implementation of IEnumerable<T> which keeps one instance of its Enumerator and returns it on every call of GetEnumerator:
public class StickyEnumerable<T> : IEnumerable<T>, IDisposable
{
private IEnumerator<T> innerEnumerator;
public StickyEnumerable( IEnumerable<T> items )
{
innerEnumerator = items.GetEnumerator();
}
public IEnumerator<T> GetEnumerator()
{
return innerEnumerator;
}
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
return innerEnumerator;
}
public void Dispose()
{
if (innerEnumerator != null)
{
innerEnumerator.Dispose();
}
}
}
Given that class, we could implement Slice with
public static IEnumerable<IEnumerable<T>> Slices<T>(this IEnumerable<T> items, int size)
{
using (StickyEnumerable<T> sticky = new StickyEnumerable<T>(items))
{
IEnumerable<T> slice;
do
{
slice = sticky.Take(size).ToList();
yield return slice;
} while (slice.Count() == size);
}
yield break;
}
That works in this case, but StickyEnumerable<T> is generally a dangerous class to have around if the consuming code isn't expecting it. For example,
using (var sticky = new StickyEnumerable<int>(Enumerable.Range(1, 10)))
{
var first = sticky.Take(2);
var second = sticky.Take(2);
foreach (int i in second)
{
Console.WriteLine(i);
}
foreach (int i in first)
{
Console.WriteLine(i);
}
}
prints
1
2
3
4
rather than
3
4
1
2
Take a look at Take(), TakeWhile() and Skip()
I think the use of Slice() would be a bit misleading. I think of that as a means to give me a chuck of an array into a new array and not causing side effects. In this scenario you would actually move the enumerable forward 10.
A possible better approach is to just use the Linq extension Take(). I don't think you would need to use Skip() with a generator.
Edit: Dang, I have been trying to test this behavior with the following code
Note: this is wasn't really correct, I leave it here so others don't fall into the same mistake.
var numbers = RandomNumberGenerator();
var slice = numbers.Take(10);
public static IEnumerable<int> RandomNumberGenerator()
{
yield return random.Next();
}
but the Count() for slice is alway 1. I also tried running it through a foreach loop since I know that the Linq extensions are generally lazily evaluated and it only looped once. I eventually did the code below instead of the Take() and it works:
public static IEnumerable<int> Slice(this IEnumerable<int> enumerable, int size)
{
var list = new List<int>();
foreach (var count in Enumerable.Range(0, size)) list.Add(enumerable.First());
return list;
}
If you notice I am adding the First() to the list each time, but since the enumerable that is being passed in is the generator from RandomNumberGenerator() the result is different every time.
So again with a generator using Skip() is not needed since the result will be different. Looping over an IEnumerable is not always side effect free.
Edit: I'll leave the last edit just so no one falls into the same mistake, but it worked fine for me just doing this:
var numbers = RandomNumberGenerator();
var slice1 = numbers.Take(10);
var slice2 = numbers.Take(10);
The two slices were different.
I had made some mistakes in my original answer but some of the points still stand. Skip() and Take() are not going to work the same with a generator as it would a list. Looping over an IEnumerable is not always side effect free. Anyway here is my take on getting a list of slices.
public static IEnumerable<int> RandomNumberGenerator()
{
while(true) yield return random.Next();
}
public static IEnumerable<IEnumerable<int>> Slice(this IEnumerable<int> enumerable, int size, int count)
{
var slices = new List<List<int>>();
foreach (var iteration in Enumerable.Range(0, count)){
var list = new List<int>();
list.AddRange(enumerable.Take(size));
slices.Add(list);
}
return slices;
}
I got this solution for the same problem:
int[] ints = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
IEnumerable<IEnumerable<int>> chunks = Chunk(ints, 2, t => t.Dump());
//won't enumerate, so won't do anything unless you force it:
chunks.ToList();
IEnumerable<T> Chunk<T, R>(IEnumerable<R> src, int n, Func<IEnumerable<R>, T> action){
IEnumerable<R> head;
IEnumerable<R> tail = src;
while (tail.Any())
{
head = tail.Take(n);
tail = tail.Skip(n);
yield return action(head);
}
}
if you just want the chunks returned, not do anything with them, use chunks = Chunk(ints, 2, t => t). What I would really like is to have to have t=>t as default action, but I haven't found out how to do that yet.