I have a Person object with a Nullable DateOfBirth property. Is there a way to use LINQ to query a list of Person objects for the one with the earliest/smallest DateOfBirth value?
Here's what I started with:
var firstBornDate = People.Min(p => p.DateOfBirth.GetValueOrDefault(DateTime.MaxValue));
Null DateOfBirth values are set to DateTime.MaxValue in order to rule them out of the Min consideration (assuming at least one has a specified DOB).
But all that does for me is to set firstBornDate to a DateTime value. What I'd like to get is the Person object that matches that. Do I need to write a second query like so:
var firstBorn = People.Single(p=> (p.DateOfBirth ?? DateTime.MaxValue) == firstBornDate);
Or is there a leaner way of doing it?
People.Aggregate((curMin, x) => (curMin == null || (x.DateOfBirth ?? DateTime.MaxValue) <
curMin.DateOfBirth ? x : curMin))
Unfortunately there isn't a built-in method to do this, but it's easy enough to implement for yourself. Here are the guts of it:
public static TSource MinBy<TSource, TKey>(this IEnumerable<TSource> source,
Func<TSource, TKey> selector)
{
return source.MinBy(selector, null);
}
public static TSource MinBy<TSource, TKey>(this IEnumerable<TSource> source,
Func<TSource, TKey> selector, IComparer<TKey> comparer)
{
if (source == null) throw new ArgumentNullException("source");
if (selector == null) throw new ArgumentNullException("selector");
comparer ??= Comparer<TKey>.Default;
using (var sourceIterator = source.GetEnumerator())
{
if (!sourceIterator.MoveNext())
{
throw new InvalidOperationException("Sequence contains no elements");
}
var min = sourceIterator.Current;
var minKey = selector(min);
while (sourceIterator.MoveNext())
{
var candidate = sourceIterator.Current;
var candidateProjected = selector(candidate);
if (comparer.Compare(candidateProjected, minKey) < 0)
{
min = candidate;
minKey = candidateProjected;
}
}
return min;
}
}
Example usage:
var firstBorn = People.MinBy(p => p.DateOfBirth ?? DateTime.MaxValue);
Note that this will throw an exception if the sequence is empty, and will return the first element with the minimal value if there's more than one.
Alternatively, you can use the implementation we've got in MoreLINQ, in MinBy.cs. (There's a corresponding MaxBy, of course.)
Install via package manager console:
PM> Install-Package morelinq
NOTE: I include this answer for completeness since the OP didn't mention what the data source is and we shouldn't make any assumptions.
This query gives the correct answer, but could be slower since it might have to sort all the items in People, depending on what data structure People is:
var oldest = People.OrderBy(p => p.DateOfBirth ?? DateTime.MaxValue).First();
UPDATE: Actually I shouldn't call this solution "naive", but the user does need to know what he is querying against. This solution's "slowness" depends on the underlying data. If this is a array or List<T>, then LINQ to Objects has no choice but to sort the entire collection first before selecting the first item. In this case it will be slower than the other solution suggested. However, if this is a LINQ to SQL table and DateOfBirth is an indexed column, then SQL Server will use the index instead of sorting all the rows. Other custom IEnumerable<T> implementations could also make use of indexes (see i4o: Indexed LINQ, or the object database db4o) and make this solution faster than Aggregate() or MaxBy()/MinBy() which need to iterate the whole collection once. In fact, LINQ to Objects could have (in theory) made special cases in OrderBy() for sorted collections like SortedList<T>, but it doesn't, as far as I know.
People.OrderBy(p => p.DateOfBirth.GetValueOrDefault(DateTime.MaxValue)).First()
Would do the trick
So you are asking for ArgMin or ArgMax. C# doesn't have a built-in API for those.
I've been looking for a clean and efficient (O(n) in time) way to do this. And I think I found one:
The general form of this pattern is:
var min = data.Select(x => (key(x), x)).Min().Item2;
^ ^ ^
the sorting key | take the associated original item
Min by key(.)
Specially, using the example in original question:
For C# 7.0 and above that supports value tuple:
var youngest = people.Select(p => (p.DateOfBirth, p)).Min().Item2;
For C# version before 7.0, anonymous type can be used instead:
var youngest = people.Select(p => new {age = p.DateOfBirth, ppl = p}).Min().ppl;
They work because both value tuple and anonymous type have sensible default comparers: for (x1, y1) and (x2, y2), it first compares x1 vs x2, then y1 vs y2. That's why the built-in .Min can be used on those types.
And since both anonymous type and value tuple are value types, they should be both very efficient.
NOTE
In my above ArgMin implementations I assumed DateOfBirth to take type DateTime for simplicity and clarity. The original question asks to exclude those entries with null DateOfBirth field:
Null DateOfBirth values are set to DateTime.MaxValue in order to rule them out of the Min consideration (assuming at least one has a specified DOB).
It can be achieved with a pre-filtering
people.Where(p => p.DateOfBirth.HasValue)
So it's immaterial to the question of implementing ArgMin or ArgMax.
NOTE 2
The above approach has a caveat that when there are two instances that have the same min value, then the Min() implementation will try to compare the instances as a tie-breaker. However, if the class of the instances does not implement IComparable, then a runtime error will be thrown:
At least one object must implement IComparable
Luckily, this can still be fixed rather cleanly. The idea is to associate a distanct "ID" with each entry that serves as the unambiguous tie-breaker. We can use an incremental ID for each entry. Still using the people age as example:
var youngest = Enumerable.Range(0, int.MaxValue)
.Zip(people, (idx, ppl) => (ppl.DateOfBirth, idx, ppl)).Min().Item3;
.NET 6 supports MaxBy/MinBy natively. So you will be able to do this with a simple
People.MinBy(p => p.DateOfBirth)
Solution with no extra packages:
var min = lst.OrderBy(i => i.StartDate).FirstOrDefault();
var max = lst.OrderBy(i => i.StartDate).LastOrDefault();
also you can wrap it into extension:
public static class LinqExtensions
{
public static T MinBy<T, TProp>(this IEnumerable<T> source, Func<T, TProp> propSelector)
{
return source.OrderBy(propSelector).FirstOrDefault();
}
public static T MaxBy<T, TProp>(this IEnumerable<T> source, Func<T, TProp> propSelector)
{
return source.OrderBy(propSelector).LastOrDefault();
}
}
and in this case:
var min = lst.MinBy(i => i.StartDate);
var max = lst.MaxBy(i => i.StartDate);
By the way... O(n^2) is not the best solution. Paul Betts gave fatster solution than my. But my is still LINQ solution and it's more simple and more short than other solutions here.
From .Net 6 (Preview 7) or later, there are new build-in method Enumerable.MaxBy and Enumerable.MinBy to achieve this.
var lastBorn = people.MaxBy(p => p.DateOfBirth);
var firstBorn = people.MinBy(p => p.DateOfBirth);
public class Foo {
public int bar;
public int stuff;
};
void Main()
{
List<Foo> fooList = new List<Foo>(){
new Foo(){bar=1,stuff=2},
new Foo(){bar=3,stuff=4},
new Foo(){bar=2,stuff=3}};
Foo result = fooList.Aggregate((u,v) => u.bar < v.bar ? u: v);
result.Dump();
}
Perfectly simple use of aggregate (equivalent to fold in other languages):
var firstBorn = People.Aggregate((min, x) => x.DateOfBirth < min.DateOfBirth ? x : min);
The only downside is that the property is accessed twice per sequence element, which might be expensive. That's hard to fix.
You can just do it like order by and limit/fetch only trick in SQL. So you order by DateOfBirth ascending and then just fetch first row.
var query = from person in People
where person.DateOfBirth!=null
orderby person.DateOfBirth
select person;
var firstBorn = query.Take(1).toList();
The following is the more generic solution. It essentially does the same thing (in O(N) order) but on any IEnumerable types and can mixed with types whose property selectors could return null.
public static class LinqExtensions
{
public static T MinBy<T>(this IEnumerable<T> source, Func<T, IComparable> selector)
{
if (source == null)
{
throw new ArgumentNullException(nameof(source));
}
if (selector == null)
{
throw new ArgumentNullException(nameof(selector));
}
return source.Aggregate((min, cur) =>
{
if (min == null)
{
return cur;
}
var minComparer = selector(min);
if (minComparer == null)
{
return cur;
}
var curComparer = selector(cur);
if (curComparer == null)
{
return min;
}
return minComparer.CompareTo(curComparer) > 0 ? cur : min;
});
}
}
Tests:
var nullableInts = new int?[] {5, null, 1, 4, 0, 3, null, 1};
Assert.AreEqual(0, nullableInts.MinBy(i => i));//should pass
Try the following idea:
var firstBornDate = People.GroupBy(p => p.DateOfBirth).Min(g => g.Key).FirstOrDefault();
I was looking for something similar myself, preferably without using a library or sorting the entire list. My solution ended up similar to the question itself, just simplified a bit.
var min = People.Min(p => p.DateOfBirth);
var firstBorn = People.FirstOrDefault(p => p.DateOfBirth == min);
EDIT again:
Sorry. Besides missing the nullable I was looking at the wrong function,
Min<(Of <(TSource, TResult>)>)(IEnumerable<(Of <(TSource>)>), Func<(Of <(TSource, TResult>)>)) does return the result type as you said.
I would say one possible solution is to implement IComparable and use Min<(Of <(TSource>)>)(IEnumerable<(Of <(TSource>)>)), which really does return an element from the IEnumerable. Of course, that doesn't help you if you can't modify the element. I find MS's design a bit weird here.
Of course, you can always do a for loop if you need to, or use the MoreLINQ implementation Jon Skeet gave.
Another implementation, which could work with nullable selector keys, and for the collection of reference type returns null if no suitable elements found.
This could be helpful then processing database results for example.
public static class IEnumerableExtensions
{
/// <summary>
/// Returns the element with the maximum value of a selector function.
/// </summary>
/// <typeparam name="TSource">The type of the elements of source.</typeparam>
/// <typeparam name="TKey">The type of the key returned by keySelector.</typeparam>
/// <param name="source">An IEnumerable collection values to determine the element with the maximum value of.</param>
/// <param name="keySelector">A function to extract the key for each element.</param>
/// <exception cref="System.ArgumentNullException">source or keySelector is null.</exception>
/// <exception cref="System.InvalidOperationException">source contains no elements.</exception>
/// <returns>The element in source with the maximum value of a selector function.</returns>
public static TSource MaxBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector) => MaxOrMinBy(source, keySelector, 1);
/// <summary>
/// Returns the element with the minimum value of a selector function.
/// </summary>
/// <typeparam name="TSource">The type of the elements of source.</typeparam>
/// <typeparam name="TKey">The type of the key returned by keySelector.</typeparam>
/// <param name="source">An IEnumerable collection values to determine the element with the minimum value of.</param>
/// <param name="keySelector">A function to extract the key for each element.</param>
/// <exception cref="System.ArgumentNullException">source or keySelector is null.</exception>
/// <exception cref="System.InvalidOperationException">source contains no elements.</exception>
/// <returns>The element in source with the minimum value of a selector function.</returns>
public static TSource MinBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector) => MaxOrMinBy(source, keySelector, -1);
private static TSource MaxOrMinBy<TSource, TKey>
(IEnumerable<TSource> source, Func<TSource, TKey> keySelector, int sign)
{
if (source == null) throw new ArgumentNullException(nameof(source));
if (keySelector == null) throw new ArgumentNullException(nameof(keySelector));
Comparer<TKey> comparer = Comparer<TKey>.Default;
TKey value = default(TKey);
TSource result = default(TSource);
bool hasValue = false;
foreach (TSource element in source)
{
TKey x = keySelector(element);
if (x != null)
{
if (!hasValue)
{
value = x;
result = element;
hasValue = true;
}
else if (sign * comparer.Compare(x, value) > 0)
{
value = x;
result = element;
}
}
}
if ((result != null) && !hasValue)
throw new InvalidOperationException("The source sequence is empty");
return result;
}
}
Example:
public class A
{
public int? a;
public A(int? a) { this.a = a; }
}
var b = a.MinBy(x => x.a);
var c = a.MaxBy(x => x.a);
IF you want to select object with minimum or maximum property value. another way is to use Implementing IComparable.
public struct Money : IComparable<Money>
{
public Money(decimal value) : this() { Value = value; }
public decimal Value { get; private set; }
public int CompareTo(Money other) { return Value.CompareTo(other.Value); }
}
Max Implementation will be.
var amounts = new List<Money> { new Money(20), new Money(10) };
Money maxAmount = amounts.Max();
Min Implementation will be.
var amounts = new List<Money> { new Money(20), new Money(10) };
Money maxAmount = amounts.Min();
In this way, you can compare any object and get the Max and Min while returning the object type.
Hope This will help someone.
A way via extension function on IEnumerable that returns both the object and the minimum found. It takes a Func that can do any operation on the object in the collection:
public static (double min, T obj) tMin<T>(this IEnumerable<T> ienum,
Func<T, double> aFunc)
{
var okNull = default(T);
if (okNull != null)
throw new ApplicationException("object passed to Min not nullable");
(double aMin, T okObj) best = (double.MaxValue, okNull);
foreach (T obj in ienum)
{
double q = aFunc(obj);
if (q < best.aMin)
best = (q, obj);
}
return (best);
}
Example where object is an Airport and we want to find closest Airport to a given (latitude, longitude). Airport has a dist(lat, lon) function.
(double okDist, Airport best) greatestPort = airPorts.tMin(x => x.dist(okLat, okLon));
You can use existing linq extension out there like MoreLinq. But if you just need only these methods, then you can use the simple code here:
public static IEnumerable<T> MinBys<T>(this IEnumerable<T> collection, Func<T, IComparable> selector)
{
var dict = collection.GroupBy(selector).ToDictionary(g => g.Key);
return dict[dict.Keys.Min()];
}
public static IEnumerable<T> MaxBys<T>(this IEnumerable<T> collection, Func<T, IComparable> selector)
{
var dict = collection.GroupBy(selector).ToDictionary(g => g.Key);
return dict[dict.Keys.Max()];
}
This is a simple way to get the minimum and maximum value:
`dbcontext.tableName.Select(x=>x.Feild1).Min()`
I have the following records
The last 2 are children of record 3 and 4, I would like to be able to sort the records by amount but it should be that the non interest(parents) ones are sorted first then their children should show up after so for example it would be like this
2000
2000
20001
99.84 (child of the above)
50000
249.58 (child of the above)
Basically I would like my sort by amount to disregard the one with "IsInterest" set to true but make them show up after their parent.
I can do this by first taking all the parents into a new collection.. then go through the parent to see if there is any children then insert them after the parent in the new collection but I feel this is not efficient and dirty code so I thought I would ask maybe someone knows black magic.
The sort should also be aware of asc/desc on the amount.
I can post my code of ripping the collection apart and putting it together if it helps but I am trying not to use that code if possible.
My sort method takes a string for "ascending" or "descending" if that helps
Thank you
UPDATE2
I will point out that there is only ever going to be 2 levels, and that the children will ever only have one parent (no grand parents) and that each parent will have a maximum of 1 child
UPDATE code as requested (fields name may differ from the db fields..)
switch (sortMember.ToUpper())
{
case "AMOUNT":
{
//check to see if any imputed interests exist
if (contributions.Any(x => x.IsImputedInterest))
{
var children = contributions.Where(x => x.IsImputedInterest);
var sortedColl = contributions.Where(x => x.IsImputedInterest == false).OrderByWithDirection(x => x.ContributionAmount, sortDirection.ToUpper() == "DESCENDING").ToList();
foreach (var child in children )
{
//find the parent
var parentIndex = sortedColl.FindIndex(x => x.ContributionId == child.ParentContirbutionId);
sortedColl.Insert(parentIndex+1, child);
}
}
else
{
contributions = contributions.OrderByWithDirection(x => x.ContributionAmount, sortDirection.ToUpper() == "DESCENDING");
}
break;
}
}
.................
public static IOrderedEnumerable<TSource> OrderByWithDirection<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, bool descending)
{
return descending ? source.OrderByDescending(keySelector)
: source.OrderBy(keySelector);
}
public static IOrderedQueryable<TSource> OrderByWithDirection<TSource, TKey>(this IQueryable<TSource> source, Expression<Func<TSource, TKey>> keySelector, bool descending)
{
return descending ? source.OrderByDescending(keySelector)
: source.OrderBy(keySelector);
}
Here's a single statement Linq solution:
var desc = order == "descending";
var result = list
//group parents with it's children
.GroupBy(x => x.ParentId ?? x.Id)
//move the parent to the first position in each group
.Select(g => g.OrderBy(x => x.ParentId.HasValue).ThenBy(x => desc ? -x.Amount : x.Amount))
//sort the groups by parents' amounts
.OrderBy(g => desc ? -g.First().Amount : g.First().Amount)
//retrieve the items from each group
.SelectMany(g => g);
Some performance hints:
You can drop the ThenBy(...) if there's always going to be at most one child or you don't care about children order
Use an if statement to check the order and have two versions of the statement - the second one using OrderByDescending/ThenByDescending, and drop the ternary operator (desc ? ... : ...) - otherwise it will be evaluated for each item
I'm not giving any guarantees on performance in relation to your current solution - it might as well turn out to be slower.
You can use the following generic method (not limited by levels or number of parent/children):
public static class Extensions
{
public static IEnumerable<T> ThenByHierarchy<T, TKey>(this IOrderedEnumerable<T> source, Func<T, TKey> keySelector, Func<T, TKey> parentKeySelector)
{
var itemByKey = source.ToDictionary(keySelector);
var processSet = new HashSet<T>();
var stack = new Stack<T>();
foreach (var item in itemByKey.Values)
{
for (var next = item; processSet.Add(next); )
{
stack.Push(next);
if (!itemByKey.TryGetValue(parentKeySelector(next), out next)) break;
}
while (stack.Count != 0)
yield return stack.Pop();
}
}
}
Just append it at the end of your OrderBy sequence like this
var result = contributions
.OrderByWithDirection(x => x.ContributionAmount, sortDirection.ToUpper() == "DESCENDING")
.ThenByHierarchy(x => x.ContributionId, x => x.ParentContirbutionId);
UPDATE: It turns out that it's not so simple. While the method above provides a correct order for the leaf elements as well for the element to its parent, it does not order correctly the parents. The correct one is as follows (using another reusable method from here How to flatten tree via LINQ?, so if we don't count that it isn't really much bigger than the previous):
public static class Extensions
{
public static IEnumerable<T> HierarchicalOrder<T, TKey>(this IEnumerable<T> source, Func<T, TKey> keySelector, Func<T, TKey> parentKeySelector, Func<IEnumerable<T>, IOrderedEnumerable<T>> order)
{
// Collect parent/child relation info
var itemById = source.ToDictionary(keySelector);
var childListById = new Dictionary<TKey, List<T>>();
var rootList = new List<T>();
foreach (var item in itemById.Values)
{
var parentKey = parentKeySelector(item);
List<T> childList;
if (parentKey == null || !itemById.ContainsKey(parentKey))
childList = rootList;
else if (!childListById.TryGetValue(parentKey, out childList))
childListById.Add(parentKey, childList = new List<T>());
childList.Add(item);
}
// Traverse the tree using in-order DFT and applying the sort on each sublist
return order(rootList).Expand(item =>
{
List<T> childList;
return childListById.TryGetValue(keySelector(item), out childList) ? order(childList) : null;
});
}
public static IEnumerable<T> Expand<T>(this IEnumerable<T> source, Func<T, IEnumerable<T>> elementSelector)
{
var stack = new Stack<IEnumerator<T>>();
var e = source.GetEnumerator();
try
{
while (true)
{
while (e.MoveNext())
{
var item = e.Current;
yield return item;
var elements = elementSelector(item);
if (elements == null) continue;
stack.Push(e);
e = elements.GetEnumerator();
}
if (stack.Count == 0) break;
e.Dispose();
e = stack.Pop();
}
}
finally
{
e.Dispose();
while (stack.Count != 0) stack.Pop().Dispose();
}
}
}
and the usage in your case is simple
var result = contributions
.HierarchicalOrder(x => x.ContributionId, x => x.ParentContirbutionId, c =>
.OrderByWithDirection(x => x.ContributionAmount, sortDirection.ToUpper() == "DESCENDING"));
I have the following: (C# code, VS2013)
class DailyTemp
{
public int Day;
public int LowTemp;
public int HighTemp;
}
List<DailyTemp> dailyTemps = new List<DailyTemp>();
//I fill this list with DailyTemp objects
My questions is, how do I write a lambda expression to go through my list and return the day that has the smallest difference between high and low temps? Thank you for your time.
Try:
return dailyTemps.OrderBy(dt => dt.HighTemp - dt.LowTemp).Select(dt => dt.Day).First();
As noted by #Colin DeClue, it's more efficient to call Select before First so that we're retrieving the minimum needed information from the data source (which can matter on larger data sets coming from a DB).
If you want to do an approach with a single pass over the sequence and using built-in methods, you can leverage Enumerable.Aggregate and keep the argument with the lowest temperature difference, discarding the other.
Func<DailyTemp, int> tempDiff = x => x.HighTemp - x.LowTemp;
var day = dailyTemps.Aggregate((a, b) => tempDiff(a) < tempDiff(b) ? a : b);
Otherwise, MoreLinq's MinBy might read simpler (externally available API), as well as the OrderBy method available within the BCL (that requires sorting), each approach being highlighted in other visible answers.
We can use the MinBy method of MoreLINQ (argument validation removed below) to get the min value of a collection based on the value of a selector:
public static TSource MinBy<TSource, TKey>(this IEnumerable<TSource> source,
Func<TSource, TKey> selector)
{
return source.MinBy(selector, Comparer<TKey>.Default);
}
public static TSource MinBy<TSource, TKey>(this IEnumerable<TSource> source,
Func<TSource, TKey> selector, IComparer<TKey> comparer)
{
using (IEnumerator<TSource> sourceIterator = source.GetEnumerator())
{
if (!sourceIterator.MoveNext())
{
throw new InvalidOperationException("Sequence was empty");
}
TSource min = sourceIterator.Current;
TKey minKey = selector(min);
while (sourceIterator.MoveNext())
{
TSource candidate = sourceIterator.Current;
TKey candidateProjected = selector(candidate);
if (comparer.Compare(candidateProjected, minKey) < 0)
{
min = candidate;
minKey = candidateProjected;
}
}
return min;
}
}
This lets us write:
var day = dailyTemps.MinBy(dt => dt.HighTemp - dt.LowTemp);
I would like to pass in a different function to Aggregate for the last element in the collection.
A use for this would be:
List<string> listString = new List{"1", "2", "3"};
string joined = listString.Aggregate(new StringBuilder(),
(sb,s) => sb.Append(s).Append(", "),
(sb,s) => sb.Append(s)).ToString();
//joined => "1, 2, 3"
What would be a custom implementation if no other exists?
P.S. I would like to do this w/ composable functions iterating once through the collection. In other words, I do not want to do a Select wrapped in a String.Join
Aggregate does not allow that in natural way.
You can carry previous element and do you final handling after Aggregate. Also I think your best bet would be to write custom method that does that custom handling for last (and possibly first) element.
Some approximate code to special case last item with Aggregate (does not handle most special case like empty/short list):
var firstLast = seq.Aggregate(
Tuple.Create(new StringBuilder(), default(string)),
(sum, cur) =>
{
if (sum.Item2 != null)
{
sum.Item1.Append(",");
sum.Item1.Append(sum.Item2);
}
return Tuple.Create(sum.Item1, cur);
});
firstLast.Item1.Append(SpecialProcessingForLast(sum.Item2));
return firstLast.Item1.ToString();
Aggregate with special case for "last". Sample is ready to copy/paste to LinqPad/console app, uncomment "this" when making extension function. Main shows aggregating array with summing all but last element, last one is subtracted from result:
void Main()
{
Console.WriteLine(AggregateWithLast(new[] {1,1,1,-3}, 0, (s,c)=>s+c, (s,c)=>s-c));
Console.WriteLine(AggregateWithLast(new[] {1,1,1,+3}, 0, (s,c)=>s+c, (s,c)=>s-c));
}
public static TAccumulate AggregateWithLast<TSource, TAccumulate>(
/*this */ IEnumerable<TSource> source,
TAccumulate seed,
Func<TAccumulate, TSource, TAccumulate> funcAll,
Func<TAccumulate, TSource, TAccumulate> funcLast)
{
using (IEnumerator<TSource> sourceIterator = source.GetEnumerator())
{
if (!sourceIterator.MoveNext())
{
return seed;
}
TSource last = sourceIterator.Current;
TAccumulate total = seed;
while (sourceIterator.MoveNext())
{
total = funcAll(total, last);
last = sourceIterator.Current;
}
return funcLast(total, last);
}
}
Note: if you need just String.Join than one in .Net 4.0+ takes IEnumerable<T> - so it will iterate sequence only once without need to ToList/ToArray.
Another approach for your particular example, is to skip the comma for the first element and prepend it to the tail elements, like this:
List<string> listString = new() { "1", "2", "3" };
string joined = listString
.Select((value, index) => (value, index))
.Aggregate(new StringBuilder(), (sb, s) =>
s.index == 0
? sb.Append(s.value)
: sb.Append(", ").Append(s.value))
.ToString();
I know this does not address the question in the title, but for most "join things with some infix" problems, this works well. That is, when string.Join is not the solution.
I have a List. I want to print the longest address.
Pseudocode
foreach (var i in listAddr)
{
// access listAddr.Address.Length
// print longest address
// print shortest address
}
Very roughly, with no foreach:
var sortedAddr = listAddr.OrderBy(x => x.Address.Length);
var longestAddr = sortedAddr.Last();
var shortedAddr = sortedAddr.First();
As Jon said, this has O(n log n) complexity.
But could be reasonable if you don't have extreme performance need.
EDIT:
If you have a lot of same-length addresses you can do this:
var sortedGroups = listAddr.GroupBy(x => x.Address.Length).OrderBy(x => x.Key);
var longestAddresses = sortedGroups.Last();
var shortestAddresses = sortedGroups.First();
// just print iterating over longestAddresses and shortestAddresses ...
It sounds like you want MaxBy and MinBy functionality, e.g.
var maxEntry = listAddr.MaxBy(entry => entry.Address.Length);
Console.WriteLine(maxEntry.Address);
var minEntry = listAddr.MinBy(entry => entry.Address.Length);
Console.WriteLine(minEntry.Address);
Unfortunately there's nothing like this in plain LINQ to Objects, but we have an implementation in MoreLINQ and I believe Reactive Extensions has one in System.Interactive too.
Obviously you can sort by address size descending and then take the first result... that will be O(n log n) instead of O(n) complexity... that may well be fine in most cases. It feels inelegant to me though :)
Code from the MoreLINQ implementation of MaxBy (without comments :)
public static TSource MaxBy<TSource, TKey>(
this IEnumerable<TSource> source,
Func<TSource, TKey> selector)
{
return source.MaxBy(selector, Comparer<TKey>.Default);
}
public static TSource MaxBy<TSource, TKey>(
this IEnumerable<TSource> source,
Func<TSource, TKey> selector, IComparer<TKey> comparer)
{
source.ThrowIfNull("source");
selector.ThrowIfNull("selector");
comparer.ThrowIfNull("comparer");
using (IEnumerator<TSource> sourceIterator = source.GetEnumerator())
{
if (!sourceIterator.MoveNext())
{
throw new InvalidOperationException("Sequence was empty");
}
TSource max = sourceIterator.Current;
TKey maxKey = selector(max);
while (sourceIterator.MoveNext())
{
TSource candidate = sourceIterator.Current;
TKey candidateProjected = selector(candidate);
if (comparer.Compare(candidateProjected, maxKey) > 0)
{
max = candidate;
maxKey = candidateProjected;
}
}
return max;
}
}
}
This should work list.Max(x => x.Address) and list.Min(x => x.Address)
For ex, if you have a list like that
List<int> myList = new List<int>();
you are able to use myList.Max() and myList.Min() to get max and min values