I was asking this to myself for quite few days and it is time as I need a help on this..
I have sample code that looks like
int nTestCase = Convert.ToInt32(Console.ReadLine());
string[] inputStrings = new string[nTestCase];
for (int i =0; i<nTestCase;i++)
{
inputStrings[i] = Console.ReadLine();
}
Is there any possibility to avoid extensive for loop over here. Just a thought of replacing it with Lambda expresssions?
this question might sound crazy, but I just want to know whether Lambda can handle Console.Read for this scenario?
int nTestCase = Convert.ToInt32(Console.ReadLine());
string[] inputStrings = Enumerable.Range(0, nTestCase)
.Select(x => Console.ReadLine())
.ToArray();
or even
string[] inputStrings = Enumerable.Range(0, Convert.ToInt32(Console.ReadLine()))
.Select(x => Console.ReadLine())
.ToArray();
Related
I have a problem with a query. I have a List with int and want to use it to get the values from my dictionary. The dictionary-keys are int and some of them have the value of the list-items. My question is how i get the objects out of the dictionary, thats keys matces the list items. Was programming JAVA the last years and now struggling with LINQ :(
Thanks in advance
Problem solved. Thank you all :)
No idea how to close this topic. I am reading stackoverflow since one year, but this was my first post.
You can use Linq to join list items with dictionary KeyValuePair entries on entry key. And then select entry value from each joined pair:
var values = from l in list
join kvp in dictionary on l equals kvp.Key
select kvp.Value;
Lambda syntax:
var values = list.Join(dictionary, l => l, kvp => kvp.Key, (l,kvp) => kvp.Value);
Basically:
var value = dictionary[integerKey];
Or:
if (dictionary.TryGetValue(integerKey, out value)) {
}
You can also create an extension method:
public static class DictionaryExtensions
{
public static IEnumerable<TValue> FilterValuesBy<TKey, TValue>(this IDictionary<TKey, TValue> dictionary, IEnumerable<TKey> filter)
{
if (dictionary == null) throw new ArgumentNullException("dictionary");
if (filter == null) throw new ArgumentNullException("filter");
var coll = filter as ICollection<TKey> ?? new HashSet<TKey>(filter);
return dictionary.Where(kvp => coll.Contains(kvp.Key)).Select(kvp => kvp.Value);
}
}
Usage:
class Program
{
static void Main()
{
var dict = Enumerable.Range(0, 10).ToDictionary(x => x);
var filter = Enumerable.Range(0, 2);
foreach (var i in dict.FilterValuesBy(filter))
{
Console.WriteLine(i);
}
Console.ReadLine();
}
}
Simple Linq method chain:
var dict = Enumerable.Range(0, 10).ToDictionary(x => x);
var filter = Enumerable.Range(0, 2).ToList();
var filtered = dict.Where(x => filter.Contains(x.Key)).Select(x => x.Value).ToList();
I have the following code in my repository:
public PagedResult<Post> GetAllPublishedByTag(int tagId, int start, int max)
{
var query = Database.Set<Post>().Where(p => p.IsPublished)
.OrderByDescending(p => p.CreatedAt)
.Skip(start)
.Take(max);
int total = query.Count();
var result = query.ToList();
return new PagedResult<Post>(result, total);
}
This will give me all published posts. But what I want is selecting all published posts for a certain tag. My model is setup in such a way that tags have a many to many relationship to posts. I tried to slightly modify the above code but this did not work:
public PagedResult<Post> GetAllPublishedByTag(Tag tag, int start, int max)
{
var query = Database.Set<Post>().Where(p => p.Tags.Contains(tag) && p.IsPublished)
.OrderByDescending(p => p.CreatedAt)
.Skip(start)
.Take(max);
int total = query.Count();
var result = query.ToList();
return new PagedResult<Post>(result, total);
}
I would prefer to pass in the tagId (as per the first code example) as opposed to the tag object but not sure how to correctly write the LINQ statement.
var query = Database.Set<Post>().Where(p => p.Tags.Any(t => t.Id == tagId) && p.IsPublished)
.OrderByDescending(p => p.CreatedAt)
.Skip(start)
.Take(max);
Side Note: I believe you may have issues with your pagination, as the variable total is calculated after skip/take are called.
I have a list of prices ordered by date. I need to select all monotonously decreasing values. The following code works:
public static List<DataPoint> SelectDecreasingValues(List<DataPoint> dataPoints)
{
var ret = new List<DataPoint>(dataPoints.Count);
var previousPrice = dataPoints[0].Price;
for (int i = 0; i < dataPoints.Count; i++)
{
if (dataPoints[i].Price <= previousPrice)
{
ret.Add(dataPoints[i]);
previousPrice = dataPoints[i].Price;
}
}
return ret;
}
However, is there a shorter/cleaner way to accomplish it with Linq?
This code is equivalent:
previousPrice = dataPoints[0].Price;
var ret = dataPoints.Where(x => {
if(x.Price <= previousPrice)
{ previousPrice = x.Price; return true;}
return false;
}).ToList();
However, if you don't need to have a list, go with plain enumerables and drop the ToList at the end. That way you can make use of the deferred execution feature built into LINQ.
The following code is also equivalent:
DataPoint previous = dataPoints.FirstOrDefault();
var ret = dataPoints.Where(x => x.Price <= previous.Price)
.Select(x => previous = x).ToList();
This works because of the deferred execution in LINQ. For each item in dataPoints it will first execute the Where part and then the Select part and only then will it move to the second item in dataPoints.
You need to decide which version you want to use. The second one is not as intention revealing as the first one, because you need to know about the internal workings of LINQ.
public IEnumerable<T> WhereMonotonicDecreasing<T>(
IEnumerable<T> source,
Func<T, IComparable> keySelector)
{
IComparable key;
bool first = true;
foreach(T t in source)
{
if (first)
{
key = keySelector(t);
yield return t;
first = false;
}
else
{
IComparable newKey = keySelector(t);
if (newKey.CompareTo(key) < 0)
{
key = newKey;
yield return t;
}
}
}
}
Called by:
dataPoints.WhereMonotonicDecreasing(x => x.Price);
previousPrice = dataPoints[0];
dataPoints.Where(p => p.Price <= previousPrice.Price)
.Select(p => previousPrice = p);
You can then use .ToList() if you really need one.
How about (untested):
return dataPoints.Take(1)
.Concat(dataPoints.Skip(1)
.Zip(dataPoints,
(next, previous) =>
new { Next = next, Previous = previous })
.Where(a => a.Next.Price <= a.Previous.Price)
.Select(a => a.Next))
.ToList();
Essentially, this overlays a "one-deferred" version of the sequence over the sequence to produce "next, previous" tuples and then applies the relevant filters on those tuples. The Take(1) is to pick the first item of the sequence, which it appears you always want.
If you don't care for the readability of the variable names, you could shorten it to:
return dataPoints.Take(1)
.Concat(dataPoints.Skip(1)
.Zip(dataPoints, Tuple.Create)
.Where(a => a.Item1.Price <= a.Item2.Price)
.Select(a => a.Item1))
.ToList();
I faced a rather stupid performance issue in my code. After a small investigation, i have found that AsQueryable method i used to cast my generic list slows down the code up to 8000 times.
So the the question is, why is that?
Here is the example
class Program
{
static void Main(string[] args)
{
var c = new ContainerTest();
c.FillList();
var s = Environment.TickCount;
for (int i = 0; i < 10000; ++i)
{
c.TestLinq(true);
}
var e = Environment.TickCount;
Console.WriteLine("TestLinq AsQueryable - {0}", e - s);
s = Environment.TickCount;
for (int i = 0; i < 10000; ++i)
{
c.TestLinq(false);
}
e = Environment.TickCount;
Console.WriteLine("TestLinq as List - {0}", e - s);
Console.WriteLine("Press enter to finish");
Console.ReadLine();
}
}
class ContainerTest
{
private readonly List<int> _list = new List<int>();
private IQueryable<int> _q;
public void FillList()
{
_list.Clear();
for (int i = 0; i < 10; ++i)
{
_list.Add(i);
}
_q = _list.AsQueryable();
}
public Tuple<int, int> TestLinq(bool useAsQ)
{
var upperBorder = useAsQ ? _q.FirstOrDefault(i => i > 7) : _list.FirstOrDefault(i => i > 7);
var lowerBorder = useAsQ ? _q.TakeWhile(i => i < 7).LastOrDefault() : _list.TakeWhile(i => i < 7).LastOrDefault();
return new Tuple<int, int>(upperBorder, lowerBorder);
}
}
UPD As i understand, i have to avoid AsQueryable method as much as possible(if it's not in the line of inheritance of the container), because i'll get immediately performance issue
"and avoid the moor in those hours of darkness when the powers of evil are exalted"
Just faced the same issue.
The thing is that IQueryable<T> takes Expression<Func<T, Bool>> as parameter for filtering in Where()/FirstOrDefault() calls - as opposed of just the Func<T, Bool> pre-compiled delegate taken in simple IEnumerable's correspondent methods.
That means there will be a compile phase to transform the Expression into a delegate. And this costs quite a lot.
Now you need that in a loop (just I did)? You'll get in some trouble...
PS: It seems .NET Core/.NET 5 improves this significantly. Unfortunately, our projects are not there yet...
at least use LINQ with List too
manual implementation will always be faster than LINQ
EDIT
you know that both test doesn't give the same result
Because AsQueryable returns an IQueryable, which has a completely different set of extension methods for the LINQ standard query operators from the one intended for things like List.
Queryable collections are meant to have a backing store of an RDBMS or something similar, and you are building a different, more complex code expression tree when you call IQueryable.FirstOrDefault() as opposed to List<>.FirstOrDefault().
I have got a collection. The coll has strings:
Location="Theater=1, Name=regal, Area=Area1"
Location="Theater=34, Name=Karm, Area=Area4445"
and so on. I have to extract just the Name bit from the string. For example, here I have to extract the text 'regal' and group the query. Then display the result as
Name=regal Count 33
Name=Karm Count 22
I am struggling with the query:
Collection.Location.GroupBy(????);(what to add here)
Which is the most short and precise way to do it?
Yet another Linq + Regex approach:
string[] Location = {
"Theater=2, Name=regal, Area=Area1",
"Theater=2, Name=regal, Area=Area1",
"Theater=34, Name=Karm, Area=Area4445"
};
var test = Location.Select(
x => Regex.Match(x, "^.*Name=(.*),.*$")
.Groups[1].Value)
.GroupBy(x => x)
.Select(x=> new {Name = x.Key, Count = x.Count()});
Query result for tested strings
Once you've extracted the string, just group by it and count the results:
var query = from location in locations
let name = ExtractNameFromLocation(location)
group 1 by name in grouped
select new { Name=grouped.Key, Count=grouped.Count() };
That's not particularly efficient, however. It has to do all the grouping before it does any counting. Have a look at this VJ article for an extension method for LINQ to Objects,
and this one about Push LINQ which a somewhat different way of looking at LINQ.
EDIT: ExtractNameFromLocation would be the code taken from answers to your other question, e.g.
public static string ExtractNameFromLocation(string location)
{
var name = (from part in location.Split(',')
let pair = part.Split('=')
where pair[0].Trim() == "Name"
select pair[1].Trim()).FirstOrDefault();
return name;
}
Here is another LINQ alternative solution with a working example.
static void Main(string[] args)
{
System.Collections.Generic.List<string> l = new List<string>();
l.Add("Theater=1, Name=regal, Area=Area"); l.Add("Theater=34, Name=Karm, Area=Area4445");
foreach (IGrouping<string, string> g in l.GroupBy(r => extractName(r)))
{
Console.WriteLine( string.Format("Name= {0} Count {1}", g.Key, g.Count()) );
}
}
private static string extractName(string dirty)
{
System.Text.RegularExpressions.Match m =
System.Text.RegularExpressions.Regex.Match(
dirty, #"(?<=Name=)[a-zA-Z0-9_ ]+(?=,)");
return m.Success ? m.Value : "";
}