LINQ Aggregate algorithm explained

Question

This might sound lame, but I have not been able to find a really good explanation of Aggregate. Good means short, descriptive, comprehensive with a small and clear example.

Answer

The easiest to understand definition of Aggregate is that it performs an operation on each element of the list taking into account the operations that have gone before. That is to say it performs the action on the first and second element and carries the result forward. Then it operates on the previous result and the third element and carries forward. etc. Example 1. Summing numbers

var nums = new[]{1,2,3,4};
var sum = nums.Aggregate( (a,b) => a + b);
Console.WriteLine(sum); // output: 10 (1+2+3+4)

This adds 1 and 2 to make 3. Then adds 3 (result of previous) and 3 (next element in sequence) to make 6. Then adds 6 and 4 to make 10. Example 2. create a csv from an array of strings

var chars = new []{"a","b","c", "d"};
var csv = chars.Aggregate( (a,b) => a + ',' + b);
Console.WriteLine(csv); // Output a,b,c,d

This works in much the same way. Concatenate a a comma and b to make a,b. Then concatenates a,b with a comma and c to make a,b,c. and so on. Example 3. Multiplying numbers using a seed For completeness, there is an overload of Aggregate which takes a seed value.

var multipliers = new []{10,20,30,40};
var multiplied = multipliers.Aggregate(5, (a,b) => a * b);
Console.WriteLine(multiplied); //Output 1200000 ((((5*10)*20)*30)*40)

Much like the above examples, this starts with a value of 5 and multiplies it by the first element of the sequence 10 giving a result of 50. This result is carried forward and multiplied by the next number in the sequence 20 to give a result of 1000. This continues through the remaining 2 element of the sequence. Live examples: http://rextester.com/ZXZ64749 Docs: http://msdn.microsoft.com/en-us/library/bb548651.aspx Addendum Example 2, above, uses string concatenation to create a list of values separated by a comma. This is a simplistic way to explain the use of Aggregate which was the intention of this answer. However, if using this technique to actually create a large amount of comma separated data, it would be more appropriate to use a StringBuilder, and this is entirely compatible with Aggregate using the seeded overload to initiate the StringBuilder.

var chars = new []{"a","b","c", "d"};
var csv = chars.Aggregate(new StringBuilder(), (a,b) => {
    if(a.Length>0)
        a.Append(",");
    a.Append(b);
    return a;
});
Console.WriteLine(csv);

Updated example: http://rextester.com/YZCVXV6464 Source

Using LINQ to remove elements from a List

Question

Say that I have LINQ query such as:

var authors = from x in authorsList
              where x.firstname == "Bob"
              select x;

Given that authorsList is of type List, how can I delete the Author elements from authorsList that are returned by the query into authors? Or, put another way, how can I delete all of the Bob's from authorsList? Note: This is a simplified example for the purposes of the question.

Answer

Well, it would be easier to exclude them in the first place:

authorsList = authorsList.Where(x => x.FirstName != "Bob").ToList();

However, that would just change the value of authorsList instead of removing the authors from the previous collection. Alternatively, you can use RemoveAll:

authorsList.RemoveAll(x => x.FirstName == "Bob");

If you really need to do it based on another collection, I'd use a HashSet, RemoveAll and Contains:

var setToRemove = new HashSet(authors);
authorsList.RemoveAll(x => setToRemove.Contains(x));

Source

When to use .First and when to use .FirstOrDefault

Question

I've searched around and haven't really found a clear answer as to when you'd want to use .First and when you'd want to use .FirstOrDefault with LINQ. When would you want to use .First? Only when you'd want to catch the exception if no results where returned?

var result = List.Where(x => x == "foo").First();

And when would you want to use .FirstOrDefault? When you'd always want the default type if no result?

var result = List.Where(x => x == "foo").FirstOrDefault();

And for that matter, what about Take?

var result = List.Where(x => x == "foo").Take(1);

Answer

I would use First() when I know or expect the sequence to have at least one element. In other words, when it is an exceptional occurrence when the sequence is empty. Use FirstOrDefault() when you know that you will need to check whether there was an element or not. In other words, when it is legal for the sequence to be empty. You should not rely on exception handling for the check. (It is bad practice and might hurt performance). Finally, the difference between First() and Take() is that First() returns the element itself, while Take() returns a sequence of elements that contains exactly one element. (If you pass 1 as the parameter). Source

What is the Java equivalent for LINQ?

Question

see subj

Answer

LINQ to Objects - JAVA 8 has added the Stream API which adds support for functional-style operations on streams of values: Package java.util.stream Java 8 Explained: Applying Lambdas to Java Collections Source

Multiple Order By

Question

I have two tables, movies and categories, and I get an ordered list by categoryID first and then by Name. The movie table has three columns, ID, Name, and CategoryID. The category table two has columns, ID, and Name. I tried something like the following, but it didn't work.

var movies = _db.Movies.OrderBy( m => { m.CategoryID, m.Name })

Answer

This should work for you:

Var movies = _db.Movies.OrderBy(c => c.Category).ThenBy(n => n.Name)

Source

LINQ query on a DataTable

Question

I'm trying to perform a LINQ query on a DataTable object and bizarrely I am finding that performing such queries on DataTables is not straightforward. For example:

var results = from myRow in myDataTable
where results.Field("RowNo") == 1
select results;

This is not allowed. How do I get something like this working?

Answer

You can't query against the DataTable's Rows collection, since DataRowCollection doesn't implement IEnumerable. You need to use the AsEnumerable() extension for DataTable. Like so:

var results = from myRow in myDataTable.AsEnumerable()
where myRow.Field("RowNo") == 1
select myRow;

If you need to convert IEnumerable to a DataTable, use the CopyToDataTable() extension. Source

Group By Multiple Columns and Sum

Question

How can I do GroupBy Multiple Columns in LINQ Something similar to this in SQL:

SELECT * FROM  GROUP BY ,

How can I convert this to LINQ:

QuantityBreakdown
(
    MaterialID int,
    ProductID int,
    Quantity float
)

INSERT INTO @QuantityBreakdown (MaterialID, ProductID, Quantity)
SELECT MaterialID, ProductID, SUM(Quantity)
FROM @Transactions
GROUP BY MaterialID, ProductID

Answer

Use an anonymous type like group x by new { x.Column1, x.Column2 }

var query = (from t in Transactions
             group t by new {t.MaterialID, t.ProductID}
             into grp
                    select new
                    {
                        grp.Key.MaterialID,
                        grp.Key.ProductID,
                        Quantity = grp.Sum(t => t.Quantity)
                    }).ToList();

Source

Sublists with Group by

Question

Let's suppose if we have a class like

class Person { 
    internal int PersonID; 
    internal string car  ; 
}

Now I have a list of this class: List persons; Now this list can have instances multiple same PersonIDs, for ex.

persons[0] = new Person { PersonID = 1, car = "Ferrari" }; 
persons[1] = new Person { PersonID = 1, car = "BMW"     }; 
persons[2] = new Person { PersonID = 2, car = "Audi"    }; 

Is there a way I can group by personID and get the list of all the cars he has? For ex. expected result would be

class Result { 
   int PersonID;
   List cars; 
}

Could someone please point me in the right direction?

Answer

Absolutely - you basically want:

var results = from p in persons
              group p.car by p.PersonId into g
              select new { PersonID = g.Key, Cars = g.ToList() };

Or as a non-query expression:

var results = persons.GroupBy( p => p.PersonId, 
                               p => p.car,
                               (key, g) => new { 
                                                 PersonId = key, 
                                                 Cars = g.ToList() 
                                                }
                              );

Basically the contents of the group (when view as an IEnumerable) is a sequence of whatever values were in the projection (p.car in this case) present for the given key Source

Returning IEnumerable vs IQueryable

Question

What is the difference between returning IQueryable vs IEnumerable? Will both be deferred execution and when should one be preferred over the other?

IQueryable custs = from c in db.Customers
where c.City == ""
select c;

IEnumerable custs = from c in db.Customers
where c.City == ""
select c;

Answer

Yes, both will give you deferred execution. The difference is that IQueryable is the interface that allows LINQ-to-SQL (LINQ.-to-anything really) to work. So if you further refine your query on an IQueryable, that query will be executed in the database, if possible. For the IEnumerable case, it will be LINQ-to-object, meaning that all objects matching the original query will have to be loaded into memory from the database. Code:

IQueryable custs = ...;
// Later on...
var goldCustomers = custs.Where(c => c.IsGold);

That code will execute SQL to only select gold customers. The following code, on the other hand, will execute the original query in the database, then filtering out the non-gold customers in the memory:

IEnumerable custs = ...;
// Later on...
var goldCustomers = custs.Where(c => c.IsGold);

This is quite an important difference, and working on IQueryable can in many cases save you from returning too many rows from the database. Another prime example is doing paging: If you use Take and Skip on IQueryable, you will only get the number of rows requested; doing that on an IEnumerable will cause all of your rows to be loaded in memory. Source

Linq Distinct on a particular Property

Question

I am playing with Linq to learn about it but I can't figure out how to use Distinct when I do not have a simple list (a simple list of integers is pretty easy to do, this is not the question). What if want to use Distinct on a list of an Object on ONE or MORE Properties of the object? Example: If an object is Person, with Property Id. How can I get all Person and use Distinct on them with the property Id of the object?

Person1: Id=1, Name="Test1"
Person2: Id=1, Name="Test1"
Person3: Id=2, Name="Test2"

How can I get just Person1 and Person3? Is that possible? If it's not possible with Linq, what would be the best way to have a list of Person depending on some of its Properties in .Net 3.5?

Answer

What you need is a "distinct-by" effectively. I don't believe it's part of LINQ as it stands, although it's fairly easy to write:

public static IEnumerable DistinctBy
    (this IEnumerable source, Func keySelector)
{
    HashSet seenKeys = new HashSet();
    foreach (TSource element in source)
    {
        if (seenKeys.Add(keySelector(element)))
        {
            yield return element;
        }
    }
}

So to find the distinct values using just the Id property, you could use:

var query = people.DistinctBy(p => p.Id);

And to use multiple properties, you can use anonymous types, which implement equality appropriately:

var query = people.DistinctBy(p => new { p.Id, p.Name });

Source

MoreLINQ - enhances LINQ with a lot of methods

This project enhances LINQ to Objects with the following methods: Acquire, AssertCount, Batch, Concat, Consume, DistinctBy, EquiZip, ExceptBy, Fold, ForEach, Generate, GenerateByIndex, GroupAdjacent, Index, MaxBy, MinBy, OrderedMerge, Pad, Pairwise, Pipe, Prepend, PreScan, Scan, SingleOrFallback, SkipUntil, Split, TakeEvery, TakeLast, TakeUntil, ToDataTable, ToDelimitedString, ToHashSet, Trace, Zip, ZipLongest. To install MoreLINQ, run the following command in the Package Manager Console PM> Install-Package morelinq source

LEFT OUTER JOIN in LINQ

Question

How to perform left outer join in C# LINQ to objects without using join-on-equals-into clauses? Is there any way to do that with where clause? Correct problem: For inner join is easy and I have a solution like this

List innerFinal = (from l in lefts from r in rights where l.Key == r.Key
                             select new JoinPair { LeftId = l.Id, RightId = r.Id})

but for left outer join I need a solution. Mine is something like this but it's not working

List< JoinPair> leftFinal = (from l in lefts from r in rights
                             select new JoinPair { 
                                            LeftId = l.Id, 
                                            RightId = ((l.Key==r.Key) ? r.Id : 0
                                        })

where JoinPair is a class:

public class JoinPair { long leftId; long rightId; }

Answer

As stated on: 101 LINQ Samples - Left outer join

var q =
    from c in categories
    join p in products on c equals p.Category into ps
    from p in ps.DefaultIfEmpty()
    select new { Category = c, ProductName = p == null ? "(No products)" : p.ProductName };

Source

LINQ equivalent of foreach for IEnumerable

Question

I'd like to do the equivalent of the following in LINQ, but I can't figure out how:

IEnumerable items = GetItems();
items.ForEach(i => i.DoStuff());

What is the real syntax?

Answer

1. LINQ is all about query data, while ForEach method is about manipulation (cause side effects)! 2. There is no ForEach extension for IEnumerable; only for List. So you could do

items.ToList().ForEach(i => i.DoStuff());

Alternatively, write your own ForEach extension method:

public static void ForEach(this IEnumerable enumeration, Action action)
{
    foreach(T item in enumeration)
    {
        action(item);
    }
}

Source

Retrieving Property name from lambda expression

Question

Is there a better way to get the Property name when passed in via a lambda expression?

Answer

Here's an update to method proposed by Cameron. The first parameter is not required.

public PropertyInfo GetPropertyInfo(
    Expression> propertyLambda)
{
    Type type = typeof(TSource);

    MemberExpression member = propertyLambda.Body as MemberExpression;
    if (member == null)
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a method, not a property.",
            propertyLambda.ToString()));

    PropertyInfo propInfo = member.Member as PropertyInfo;
    if (propInfo == null)
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a field, not a property.",
            propertyLambda.ToString()));

    if (type != propInfo.ReflectedType &&
        !type.IsSubclassOf(propInfo.ReflectedType))
        throw new ArgumentException(string.Format(
            "Expresion '{0}' refers to a property that is not from type {1}.",
            propertyLambda.ToString(),
            type));

    return propInfo;
}

Usage

var propertyInfo = GetPropertyInfo(u => u.UserID);

Source

Split List into Sublists

Question

: I believe this is another easy one for you LINQ masters out there. Is there any way I can separe a List into several separate lists of SomeObject, using the item index as the delimiter of each split? Let me exemplify: I have a List and I need a List<List> or List[], so that each of these resulting lists will contain a group of 3 items of the original list (sequentially). eg.: Original List: [a, g, e, w, p, s, q, f, x, y, i, m, c] Resulting lists: [a, g, e], [w, p, s], [q, f, x], [y, i, m], [c] I'd also need the resulting lists size to be a parameter of this function. Is it possible??

Answer

: completely lazy: works on infinite enumerables no intermediate copying/buffering O(n) execution time works also when child sequences are only partially consumed

public static IEnumerable Chunks(this IEnumerable enumerable,
                                                    int chunkSize)
{
    if (chunkSize < 1) throw new ArgumentException("chunkSize must be positive");

    using (var e = enumerable.GetEnumerator())
    while (e.MoveNext())
    {
        var remaining = chunkSize;    // elements remaining in the current chunk
        var innerMoveNext = new Func(() => --remaining > 0 && e.MoveNext());

        yield return e.GetChunk(innerMoveNext);
        while (innerMoveNext()) {/* discard elements skipped by inner iterator */}
    }
}

private static IEnumerable GetChunk(this IEnumerator e,
                                          Func innerMoveNext)
{
    do yield return e.Current;
    while (innerMoveNext());
}

Example Usage

var src = new [] {1, 2, 3, 4, 5, 6}; 

var c3 = src.Chunks(3);      // {{1, 2, 3}, {4, 5, 6}}; 
var c4 = src.Chunks(4);      // {{1, 2, 3, 4}, {5, 6}}; 

var sum   = c3.Select(c => c.Sum());    // {6, 15}
var count = c3.Count();                 // 2
var take2 = c3.Select(c => c.Take(2));  // {{1, 2}, {4, 5}}

Explanations The code works by nesting two yield based iterators. The outer iterator must keep track of how many elements have been effectively consumed by the inner (chunk) iterator. This is done by a closing over remaining with innerMoveNext(). Unconsumed elements of a chunk are discarded before the next chunk is yielded by the outer iterator. This is necessary because otherwise you get inconsistent results, when the inner enumerables are not (completely) consumed (e.g. c3.Count() would return 6). stackoverflow.com/q/419019#20953521

HOWTO: access an administrative share on a computer

If you cannot access an administrative share on a computer, apply EnableAdminShares.reg file and restart "Server" windows service.

Enjoy!