I'm trying to make a program like wordlist generator.
I want to add the items on the 2nd list next to each item on the 1st list.
`
List<string> list1 = new List<string>() {"tomato", "ball", "icecream", "blue"};
List<string> list2 = new List<string>() { "123", "yellow", "green" };
`
//Values to be added to Listing 3: tomato123, tomatoyellow, tomatogreen, ball123, ballyellow, ballgreen bla bla bla
To solve your problem, we will iterate over one of the lists, and for every item in it, we will create all the possible combinations with words from the other list. With LINQ, it would look something like this:
var list3 = list1.Select(w1 => list2.Select(w2 => w1 + w2)).ToList();
The problem is that now list3 is of type List<IEnumerable<string>> because we have a list of combinations for every word in list1. To flatten the result, all we need is to change the Select projection to a SelectMany flattened projection:
var list3 = list1.SelectMany(w1 => list2.Select(w2 => w1 + w2)).ToList();
Based on your requirement it might be useful. Please have a look.
static void AddLists()
{
List<string> list1 = new List<string>() { "tomato", "ball", "icecream", "blue" };
List<string> list2 = new List<string>() { "123", "yellow", "green" };
var resultList = from l1 in list1
from l2 in list2
select string.Concat(l1, l2);
}
My advice would be to create an extension method, instead of a LINQ statement that is difficult to understand: readers will immediately see what it does, it is easier to test and easier to change.
See extension methods demystified
public static IEnumerable<string> ConcatCombinations(
this.IEnumerable<string> sequenceA,
IEnumerable<string> sequenceB)
{
// TODO: invent a proper name
foreach (string textA in sequenceA)
foreach (string textB in sequenceB)
yield return textA + textB;
}
This code is way simpler than any solution using LINQ methods. Anyone will immediately see what it does.
Usage:
List<string> list1 = ...
string[] array1 = ...
List<string> concatenated = list1.ConcatCombinations(array1).ToList();
If you want to make a more generic method, consider this:
public static IEnumerable<TResult> MakeCombinations<TA, TB, TResult>(
this IEnumerable<TA> itemsA,
IEnumerable<TB> itemsB,
Func<TA, TB, TResult> resultSelector)
{
foreach (TA itemA in itemsA)
foreach (TB itemB in itemsB)
{
TResult result = resultSelector(itemA, itemB);
yield return result;
}
}
Usage:
List<string> list1 = ...
List<string> list2 = ...
List<string> concatenated = list1.ConcatCombinations(list2,
// parameter ResultSelector: concat every a and b:
(a, b) => a+b)
.ToList();
Or just change your ConcatCombinations:
public static IEnumerable<string> ConcatCombinations(
this.IEnumerable<string> sequenceA,
IEnumerable<string> sequenceB)
{
return sequenceA.MakeCombinations(sequenceB,
(a, b) => a + b);
}
Another completely different example, that shows you the reusability of the code:
var maleBallroomDancers = ...
var femaleBallroomDancers = ...
var danceCombinations = maleBallroomDancers.MakeCombinations(femaleBallroomDancers,
(male, female) => new
{
Male = male,
Female = female,
})
.ToList();
Related
I have a list of lists which I want to find the intersection for like this:
var list1 = new List<int>() { 1, 2, 3 };
var list2 = new List<int>() { 2, 3, 4 };
var list3 = new List<int>() { 3, 4, 5 };
var listOfLists = new List<List<int>>() { list1, list2, list3 };
// expected intersection is List<int>() { 3 };
Is there some way to do this with IEnumerable.Intersect()?
EDIT:
I should have been more clear on this: I really have a list of lists, I don't know how many there will be, the three lists above was just an example, what I have is actually an IEnumerable<IEnumerable<SomeClass>>
SOLUTION
Thanks for all great answers. It turned out there were four options for solving this: List+aggregate (#Marcel Gosselin), List+foreach (#JaredPar, #Gabe Moothart), HashSet+aggregate (#jesperll) and HashSet+foreach (#Tony the Pony). I did some performance testing on these solutions (varying number of lists, number of elements in each list and random number max size.
It turns out that for most situations the HashSet performs better than the List (except with large lists and small random number size, because of the nature of HashSet I guess.)
I couldn't find any real difference between the foreach method and the aggregate method (the foreach method performs slightly better.)
To me, the aggregate method is really appealing (and I'm going with that as the accepted answer) but I wouldn't say it's the most readable solution.. Thanks again all!
How about:
var intersection = listOfLists
.Skip(1)
.Aggregate(
new HashSet<T>(listOfLists.First()),
(h, e) => { h.IntersectWith(e); return h; }
);
That way it's optimized by using the same HashSet throughout and still in a single statement. Just make sure that the listOfLists always contains at least one list.
You can indeed use Intersect twice. However, I believe this will be more efficient:
HashSet<int> hashSet = new HashSet<int>(list1);
hashSet.IntersectWith(list2);
hashSet.IntersectWith(list3);
List<int> intersection = hashSet.ToList();
Not an issue with small sets of course, but if you have a lot of large sets it could be significant.
Basically Enumerable.Intersect needs to create a set on each call - if you know that you're going to be doing more set operations, you might as well keep that set around.
As ever, keep a close eye on performance vs readability - the method chaining of calling Intersect twice is very appealing.
EDIT: For the updated question:
public List<T> IntersectAll<T>(IEnumerable<IEnumerable<T>> lists)
{
HashSet<T> hashSet = null;
foreach (var list in lists)
{
if (hashSet == null)
{
hashSet = new HashSet<T>(list);
}
else
{
hashSet.IntersectWith(list);
}
}
return hashSet == null ? new List<T>() : hashSet.ToList();
}
Or if you know it won't be empty, and that Skip will be relatively cheap:
public List<T> IntersectAll<T>(IEnumerable<IEnumerable<T>> lists)
{
HashSet<T> hashSet = new HashSet<T>(lists.First());
foreach (var list in lists.Skip(1))
{
hashSet.IntersectWith(list);
}
return hashSet.ToList();
}
Try this, it works but I'd really like to get rid of the .ToList() in the aggregate.
var list1 = new List<int>() { 1, 2, 3 };
var list2 = new List<int>() { 2, 3, 4 };
var list3 = new List<int>() { 3, 4, 5 };
var listOfLists = new List<List<int>>() { list1, list2, list3 };
var intersection = listOfLists.Aggregate((previousList, nextList) => previousList.Intersect(nextList).ToList());
Update:
Following comment from #pomber, it is possible to get rid of the ToList() inside the Aggregate call and move it outside to execute it only once. I did not test for performance whether previous code is faster than the new one. The change needed is to specify the generic type parameter of the Aggregate method on the last line like below:
var intersection = listOfLists.Aggregate<IEnumerable<int>>(
(previousList, nextList) => previousList.Intersect(nextList)
).ToList();
You could do the following
var result = list1.Intersect(list2).Intersect(list3).ToList();
This is my version of the solution with an extension method that I called IntersectMany.
public static IEnumerable<TResult> IntersectMany<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, IEnumerable<TResult>> selector)
{
using (var enumerator = source.GetEnumerator())
{
if(!enumerator.MoveNext())
return new TResult[0];
var ret = selector(enumerator.Current);
while (enumerator.MoveNext())
{
ret = ret.Intersect(selector(enumerator.Current));
}
return ret;
}
}
So the usage would be something like this:
var intersection = (new[] { list1, list2, list3 }).IntersectMany(l => l).ToList();
This is my one-row solution for List of List (ListOfLists) without intersect function:
var intersect = ListOfLists.SelectMany(x=>x).Distinct().Where(w=> ListOfLists.TrueForAll(t=>t.Contains(w))).ToList()
This should work for .net 4 (or later)
After searching the 'net and not really coming up with something I liked (or that worked), I slept on it and came up with this. Mine uses a class (SearchResult) which has an EmployeeId in it and that's the thing I need to be common across lists. I return all records that have an EmployeeId in every list. It's not fancy, but it's simple and easy to understand, just what I like. For small lists (my case) it should perform just fine—and anyone can understand it!
private List<SearchResult> GetFinalSearchResults(IEnumerable<IEnumerable<SearchResult>> lists)
{
Dictionary<int, SearchResult> oldList = new Dictionary<int, SearchResult>();
Dictionary<int, SearchResult> newList = new Dictionary<int, SearchResult>();
oldList = lists.First().ToDictionary(x => x.EmployeeId, x => x);
foreach (List<SearchResult> list in lists.Skip(1))
{
foreach (SearchResult emp in list)
{
if (oldList.Keys.Contains(emp.EmployeeId))
{
newList.Add(emp.EmployeeId, emp);
}
}
oldList = new Dictionary<int, SearchResult>(newList);
newList.Clear();
}
return oldList.Values.ToList();
}
Here's an example just using a list of ints, not a class (this was my original implementation).
static List<int> FindCommon(List<List<int>> items)
{
Dictionary<int, int> oldList = new Dictionary<int, int>();
Dictionary<int, int> newList = new Dictionary<int, int>();
oldList = items[0].ToDictionary(x => x, x => x);
foreach (List<int> list in items.Skip(1))
{
foreach (int i in list)
{
if (oldList.Keys.Contains(i))
{
newList.Add(i, i);
}
}
oldList = new Dictionary<int, int>(newList);
newList.Clear();
}
return oldList.Values.ToList();
}
This is a simple solution if your lists are all small. If you have larger lists, it's not as performing as hash set:
public static IEnumerable<T> IntersectMany<T>(this IEnumerable<IEnumerable<T>> input)
{
if (!input.Any())
return new List<T>();
return input.Aggregate(Enumerable.Intersect);
}
This question already has answers here:
Generating all Possible Combinations
(12 answers)
Closed 4 years ago.
I need an algorithm that will take any number of List inside a List and generate a unique set of permutations. I prefer to find a LINQ solution.
I actually have a Javascript function that works well and I'm trying to recreate it in C# (see code at bottom)
C# (my attempt) - Visual Studio does not like my second Aggregate(). It says the arguments cannot be inferred from usage
public static void testit()
{
List<List<string>> master = new List<List<string>>();
List<string> voltages = new string[] { "208", "230", "460" }.ToList();
List<string> sysConfigs = new string[] { "10205", "10210", "10215", "10220" }.ToList();
master.Add(voltages);
master.Add(sysConfigs);
var amp = master.Aggregate(
(a, b) => a.Aggregate(
(r, v) => r.Concat(
b.Select(w => new List<string>().Concat(v, w))
), new List<string>()
)
);
}
The output of this new collection should look like this:
/*
OUTPUT (displayed as arrays - but will be lists):
[
["208", "10205"],
["208", "10210"],
["208", "10215"],
["208", "10220"],
["230", "10205"],
["230", "10210"],
["230", "10215"],
["230", "10220"],
["460", "10205"],
["460", "10210"],
["460", "10215"],
["460", "10220"]
];
Here's a Javascript function that works well that I'm trying to mimic in C#:
function getPermutations(arr) {
return arr.reduce(
(a, b) => a.reduce((r, v) => r.concat(b.map(w => [].concat(v, w))), [])
);
}
var voltages = ["208", "230", "460"];
var sysConfigs = ["10205", "10210", "10215", "10220"];
var master = [];
master.push(voltages);
master.push(sysConfigs);
var newArr = getPermutations(master);
console.log(newArr);
As noted in other questions, this is the Cartesian product, not a permutation.
Short version: Just go to my blog:
https://ericlippert.com/2010/06/28/computing-a-cartesian-product-with-linq/
Long version:
Cartesian product of two lists is built-in in C# in the form of SelectMany or a query comprehension. Let's start with that.
Before we get into that though, please do not ever do this:
List<string> voltages = new string[] { "208", "230", "460" }.ToList()
Either do this:
IEnumerable<string> voltages = new string[] { "208", "230", "460" };
Or do this:
List<string> voltages = new List<string>() { "208", "230", "460" };
But do not make an array and then to-list it! Just make a list from the beginning.
OK, onward. We have two sequences:
IEnumerable<string> voltages = new string[] { "208", "230", "460" };
IEnumerable<string> sysConfigs = new string[] { "10205", "10210", "10215", "10220" };
We want their Cartesian product:
IEnumerable<IEnumerable<string>> master =
from v in voltages
from s in sysConfigs
select new List<string>() { v, s };
And we're done.
If you don't like "comprehension form" then you can use "fluent form":
IEnumerable<IEnumerable<string>> master =
voltages.SelectMany(
v => sysConfigs,
(s, v) => new List<string>() { v, s });
If you want a list of lists:
List<List<string>> master =
voltages.SelectMany(
v => sysConfigs,
(v, s) => new List<string>() { v, s })
.ToList();
Easy peasy.
The meaning of this operation should be clear, but if it is not: the general form is:
var zs =
from x in xs
from y in f(x) // f takes x and returns a collection of ys
g(x, y) // do something with every combination of x and y to make a z
In your case, f(x) is just "always produce the second collection", but it need not be; the collection could depend on x. The result is a sequence of z.
Now, what you need is the Cartesian product of arbitrarily many sequences.
Your intuition that this is an aggregation of concatenations is correct. We can solve it like this:
static IEnumerable<IEnumerable<T>> CartesianProduct<T>(
this IEnumerable<IEnumerable<T>> sequences)
{
IEnumerable<IEnumerable<T>> emptyProduct = new[] { Enumerable.Empty<T>() };
return sequences.Aggregate(
emptyProduct,
(accumulator, sequence) =>
from accseq in accumulator
from item in sequence
select accseq.Concat(new[] {item}));
}
Notice how this combines the three operations: a select-many query, a concatenation, and an aggregation. Study this carefully to see how it works.
This is the correct version of your code
var amp = master.Aggregate(
new List<List<string>>(){new List<string>()},
(a, b) => a.Aggregate(
new List<List<string>>(new List<List<string>>()),
(r, v) => r.Concat(
b.Select(w => v.Concat(new List<string>{w}).ToList())
).ToList()));
I have 2 list of string, list A and list B. list A is a list of strings containing paths, and the other contains strings of folder. Examples:
List<string> listA = new List<string>{ "c:\myPath\FolderA\blabla\", "c:\myPath\FolderB\blabla2\", "c:\myPath\FolderA\blabla3\" "c:\myPath\FolderC\blabla\"};
List<string> listB = new List<string> { "FolderA, FolderC"};
I want to have a method that compares the 2 list. If listA contains any of listB it is valid, else I don't want it. So based on this logic I'd have:
List<string> listReturn = new List<string>{ "c:\myPath\FolderA\blabla\", "c:\myPath\FolderA\blabla3\" "c:\myPath\FolderC\blabla\"};
So far all I've done is a method that iterates through the first list and does a Contain call on the string with a Linq Any call, like this:
private static List<string> FilterList(List<string> listA, List<string> listB)
{
List<string> listReturn = new List<string>();
foreach (string val in listA)
{
if (listB.Any(item => val.Contains(item)))
{
listReturn.Add(val);
}
}
return listReturn;
}
It's not bad, but I want to use a Linq approach or a .NET approach if there's an Intersect method available for this. Thank you.
Use Where() against the listA to filter items in this list and Exists() on listB for the filter condition:
List<string> listA = new List<string> {#"c:\myPath\FolderA\blabla\", #"c:\myPath\FolderA\blabla2\", #"c:\myPath\Folder\blabla3\", #"c:\myPath\FolderC\blabla\"};
List<string> listB = new List<string> { "FolderA", "FolderC" };
var intersect = listA.Where(a => listB.Exists(b => a.Contains(b)));
Try this
var result = listA.Where(i => listB.Any(y => i.Contains(y)).ToList();
I have a list of lists which I want to find the intersection for like this:
var list1 = new List<int>() { 1, 2, 3 };
var list2 = new List<int>() { 2, 3, 4 };
var list3 = new List<int>() { 3, 4, 5 };
var listOfLists = new List<List<int>>() { list1, list2, list3 };
// expected intersection is List<int>() { 3 };
Is there some way to do this with IEnumerable.Intersect()?
EDIT:
I should have been more clear on this: I really have a list of lists, I don't know how many there will be, the three lists above was just an example, what I have is actually an IEnumerable<IEnumerable<SomeClass>>
SOLUTION
Thanks for all great answers. It turned out there were four options for solving this: List+aggregate (#Marcel Gosselin), List+foreach (#JaredPar, #Gabe Moothart), HashSet+aggregate (#jesperll) and HashSet+foreach (#Tony the Pony). I did some performance testing on these solutions (varying number of lists, number of elements in each list and random number max size.
It turns out that for most situations the HashSet performs better than the List (except with large lists and small random number size, because of the nature of HashSet I guess.)
I couldn't find any real difference between the foreach method and the aggregate method (the foreach method performs slightly better.)
To me, the aggregate method is really appealing (and I'm going with that as the accepted answer) but I wouldn't say it's the most readable solution.. Thanks again all!
How about:
var intersection = listOfLists
.Skip(1)
.Aggregate(
new HashSet<T>(listOfLists.First()),
(h, e) => { h.IntersectWith(e); return h; }
);
That way it's optimized by using the same HashSet throughout and still in a single statement. Just make sure that the listOfLists always contains at least one list.
You can indeed use Intersect twice. However, I believe this will be more efficient:
HashSet<int> hashSet = new HashSet<int>(list1);
hashSet.IntersectWith(list2);
hashSet.IntersectWith(list3);
List<int> intersection = hashSet.ToList();
Not an issue with small sets of course, but if you have a lot of large sets it could be significant.
Basically Enumerable.Intersect needs to create a set on each call - if you know that you're going to be doing more set operations, you might as well keep that set around.
As ever, keep a close eye on performance vs readability - the method chaining of calling Intersect twice is very appealing.
EDIT: For the updated question:
public List<T> IntersectAll<T>(IEnumerable<IEnumerable<T>> lists)
{
HashSet<T> hashSet = null;
foreach (var list in lists)
{
if (hashSet == null)
{
hashSet = new HashSet<T>(list);
}
else
{
hashSet.IntersectWith(list);
}
}
return hashSet == null ? new List<T>() : hashSet.ToList();
}
Or if you know it won't be empty, and that Skip will be relatively cheap:
public List<T> IntersectAll<T>(IEnumerable<IEnumerable<T>> lists)
{
HashSet<T> hashSet = new HashSet<T>(lists.First());
foreach (var list in lists.Skip(1))
{
hashSet.IntersectWith(list);
}
return hashSet.ToList();
}
Try this, it works but I'd really like to get rid of the .ToList() in the aggregate.
var list1 = new List<int>() { 1, 2, 3 };
var list2 = new List<int>() { 2, 3, 4 };
var list3 = new List<int>() { 3, 4, 5 };
var listOfLists = new List<List<int>>() { list1, list2, list3 };
var intersection = listOfLists.Aggregate((previousList, nextList) => previousList.Intersect(nextList).ToList());
Update:
Following comment from #pomber, it is possible to get rid of the ToList() inside the Aggregate call and move it outside to execute it only once. I did not test for performance whether previous code is faster than the new one. The change needed is to specify the generic type parameter of the Aggregate method on the last line like below:
var intersection = listOfLists.Aggregate<IEnumerable<int>>(
(previousList, nextList) => previousList.Intersect(nextList)
).ToList();
You could do the following
var result = list1.Intersect(list2).Intersect(list3).ToList();
This is my version of the solution with an extension method that I called IntersectMany.
public static IEnumerable<TResult> IntersectMany<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, IEnumerable<TResult>> selector)
{
using (var enumerator = source.GetEnumerator())
{
if(!enumerator.MoveNext())
return new TResult[0];
var ret = selector(enumerator.Current);
while (enumerator.MoveNext())
{
ret = ret.Intersect(selector(enumerator.Current));
}
return ret;
}
}
So the usage would be something like this:
var intersection = (new[] { list1, list2, list3 }).IntersectMany(l => l).ToList();
This is my one-row solution for List of List (ListOfLists) without intersect function:
var intersect = ListOfLists.SelectMany(x=>x).Distinct().Where(w=> ListOfLists.TrueForAll(t=>t.Contains(w))).ToList()
This should work for .net 4 (or later)
After searching the 'net and not really coming up with something I liked (or that worked), I slept on it and came up with this. Mine uses a class (SearchResult) which has an EmployeeId in it and that's the thing I need to be common across lists. I return all records that have an EmployeeId in every list. It's not fancy, but it's simple and easy to understand, just what I like. For small lists (my case) it should perform just fine—and anyone can understand it!
private List<SearchResult> GetFinalSearchResults(IEnumerable<IEnumerable<SearchResult>> lists)
{
Dictionary<int, SearchResult> oldList = new Dictionary<int, SearchResult>();
Dictionary<int, SearchResult> newList = new Dictionary<int, SearchResult>();
oldList = lists.First().ToDictionary(x => x.EmployeeId, x => x);
foreach (List<SearchResult> list in lists.Skip(1))
{
foreach (SearchResult emp in list)
{
if (oldList.Keys.Contains(emp.EmployeeId))
{
newList.Add(emp.EmployeeId, emp);
}
}
oldList = new Dictionary<int, SearchResult>(newList);
newList.Clear();
}
return oldList.Values.ToList();
}
Here's an example just using a list of ints, not a class (this was my original implementation).
static List<int> FindCommon(List<List<int>> items)
{
Dictionary<int, int> oldList = new Dictionary<int, int>();
Dictionary<int, int> newList = new Dictionary<int, int>();
oldList = items[0].ToDictionary(x => x, x => x);
foreach (List<int> list in items.Skip(1))
{
foreach (int i in list)
{
if (oldList.Keys.Contains(i))
{
newList.Add(i, i);
}
}
oldList = new Dictionary<int, int>(newList);
newList.Clear();
}
return oldList.Values.ToList();
}
This is a simple solution if your lists are all small. If you have larger lists, it's not as performing as hash set:
public static IEnumerable<T> IntersectMany<T>(this IEnumerable<IEnumerable<T>> input)
{
if (!input.Any())
return new List<T>();
return input.Aggregate(Enumerable.Intersect);
}
I have the following scenario:
List 1 has 20 items of type TItem, List 2 has 5 items of the same type. List 1 already contains the items from List 2 but in a different state. I want to overwrite the 5 items in List 1 with the items from List 2.
I thought a join might work, but I want to overwrite the items in List 1, not join them together and have duplicates.
There is a unique key that can be used to find which items to overwrite in List 1 the key is of type int
You could use the built in Linq .Except() but it wants an IEqualityComparer so use a fluid version of .Except() instead.
Assuming an object with an integer key as you indicated:
public class Item
{
public int Key { get; set; }
public int Value { get; set; }
public override string ToString()
{
return String.Format("{{{0}:{1}}}", Key, Value);
}
}
The original list of objects can be merged with the changed one as follows:
IEnumerable<Item> original = new[] { 1, 2, 3, 4, 5 }.Select(x => new Item
{
Key = x,
Value = x
});
IEnumerable<Item> changed = new[] { 2, 3, 5 }.Select(x => new Item
{
Key = x,
Value = x * x
});
IEnumerable<Item> result = original.Except(changed, x => x.Key).Concat(changed);
result.ForEach(Console.WriteLine);
output:
{1:1}
{4:4}
{2:4}
{3:9}
{5:25}
LINQ isn't used to perform actual modifications to the underlying data sources; it's strictly a query language. You could, of course, do an outer join on List2 from List1 and select List2's entity if it's not null and List1's entity if it is, but that is going to give you an IEnumerable<> of the results; it won't actually modify the collection. You could do a ToList() on the result and assign it to List1, but that would change the reference; I don't know if that would affect the rest of your application.
Taking your question literally, in that you want to REPLACE the items in List1 with those from List2 if they exist, then you'll have to do that manually in a for loop over List1, checking for the existence of a corresponding entry in List2 and replacing the List1 entry by index with that from List2.
As Adam says, LINQ is about querying. However, you can create a new collection in the right way using Enumerable.Union. You'd need to create an appropriate IEqualityComparer though - it would be nice to have UnionBy. (Another one for MoreLINQ perhaps?)
Basically:
var list3 = list2.Union(list1, keyComparer);
Where keyComparer would be an implementation to compare the two keys. MiscUtil contains a ProjectionEqualityComparer which would make this slightly easier.
Alternatively, you could use DistinctBy from MoreLINQ after concatenation:
var list3 = list2.Concat(list1).DistinctBy(item => item.Key);
Here's a solution with GroupJoin.
List<string> source = new List<string>() { "1", "22", "333" };
List<string> modifications = new List<string>() { "4", "555"};
//alternate implementation
//List<string> result = source.GroupJoin(
// modifications,
// s => s.Length,
// m => m.Length,
// (s, g) => g.Any() ? g.First() : s
//).ToList();
List<string> result =
(
from s in source
join m in modifications
on s.Length equals m.Length into g
select g.Any() ? g.First() : s
).ToList();
foreach (string s in result)
Console.WriteLine(s);
Hmm, how about a re-usable extension method while I'm at it:
public static IEnumerable<T> UnionBy<T, U>
(
this IEnumerable<T> source,
IEnumerable<T> otherSource,
Func<T, U> selector
)
{
return source.GroupJoin(
otherSource,
selector,
selector,
(s, g) => g.Any() ? g.First() : s
);
}
Which is called by:
List<string> result = source
.UnionBy(modifications, s => s.Length)
.ToList();