I've been trying to figure out if there was a way to use IEnumerable's Sum method to subtract two values instead of adding them up?
IEnumerable<T> list = first
.Concat(second)
.GroupBy(x => new { x.crita, x.critb})
.Select(y => new T
{
//other fields//
vara= y.Sum(z => z.vara),
varb= y.Sum(z => z.varb),
varc= y.Sum(z => z.varc),
vard= y.Sum(z => z.vard)
});
I'm using this method to get sums between the two IEnumerables I've joined, but I was hoping to be able to subtract them too.
Any help would be really appreciated.
Also, if someone could tell me how to get products and quotients, that'd be awesome!
Aggregate is the elder brother of Sum that can do subtract and other aggregations too. On the other hand if you want to do element-by-element subtraction, you can use general Select projections.
Reading your comment, one easy trick would be to create a projection of your second IEnumerable that negates vara, varb etc. and then use the same Sum function to compute the aggregates of groups. So basically you would just be doing a + (-b) instead of a - b. Something on the following lines:
IEnumerable<T> list = first
.Concat(second.Select(n => new T() { vara = -n.vara...} )
.GroupBy(x => new { x.crita, x.critb})
.Select(y => new T
{
//other fields//
vara= y.Sum(z => z.vara),
varb= y.Sum(z => z.varb),
varc= y.Sum(z => z.varc),
vard= y.Sum(z => z.vard)
});
Related
So I come with a List list and first make it to a dictionary for faster searches.
Dictionary<Guid, Int32> dictionaryFromList = list
.ToDictionary(x => x.Item1, y => y.Item2);
Now Im loading all the other info in another Dictionary from the cache:
Dictionary<Guid, Model> modelDictionary = _cache
.Where(x => dictionaryFromList.ContainsKey(x.Id))
.Select(y => new {y.Id, y })
.ToDictionary(t => t.Id, t=> t.y);
Now I need two different things:
1. I need to insert more data into some of the Models in the modelDictionary
2. I need to insert the Int32 from the dictionaryFromList into the modelDictionary
My approach for 1. was the following:
HashSet<Guid> toLoadIds = new HashSet<Guid>(modelDictionary
.Where(x => !x.Value.IsLoaded)
.Select(x => x.Key));
context.myTable
.Where(x => toLoadIds.Contains(x.Id))
.Select(x => new {x.value1, x.value2, x.value3, x.value4, x.value5, x.value6 }));
I selected the values now afaik but how should I get them into the right model in the modelDictionary?
For the 2. one I tried doing this:
dictionaryFromList.Select(y => modelDictionary[y.Key].myValue = y.Value);
But it seems like nothing is working properly :(
The previous solution for the 2. was the following when modelDictionary was stil a List
modelDictionary.ForEach(x => x.myValue = dictionaryFromList[x.AddressId]);
When you write:
dictionaryFromList.Select(y => modelDictionary[y.Key].myValue = y.Value);
the execution is deferred until someone runs through the returned select iterator. But you do not pick it up, so no-one does that.
You could do:
dictionaryFromList.Select(y => modelDictionary[y.Key].myValue = y.Value)
.LastOrDefault();
where the final call will force the iteration of the select iterator. But I find that ugly. Why not simply use foreach?
foreach (var y in dictionaryFromList) { modelDictionary[y.Key].myValue = y.Value; }
How can the below linq Query be modified such that i can get a Rounded figure of values.
var result=GetStudentsWithTheirMarks()
.OrderByDescending(x => Math.Round(x.PercentageScore)).Take(5)
.OrderBy(x => x.PercentageScore);
Please ignore the presence of two order by clause as this is done for with a purpose.
GetStudentsWithThierMarks returns Student List with their FirstName and PercentageScore.
I believe in the above query Math.Round is only applicable when during order by operation so final result still contains values in decimal places whereas i am only interested to see rounded figures with integer values. I just cant figure out the syntax.
You just need a Select :
var result= GetStudentsWithTheirMarks()
.OrderByDescending(x => Math.Round(x.PercentageScore))
.Take(5)
.OrderBy(x => x.PercentageScore)
.Select(x => Math.Round(x.PercentageScore));
You can store this value in an anonymous type:
var result = GetStudentsWithTheirMarks()
.Select(s => new
{
Student = s,
RoundedPercentageScore = Math.Round(s.PercentageScore)
})
.OrderByDescending(x => x.RoundedPercentageScore )
.Take(5)
.OrderBy(x => x.Student.PercentageScore);
Now you can access it in this way:
foreach(var x in result)
Console.WriteLine("RoundedPercentageScore: " x.RoundedPercentageScore);
This is related to my other question here. James World presented a solution as follows:
// idStream is an IObservable<int> of the input stream of IDs
// alarmInterval is a Func<int, TimeSpan> that gets the interval given the ID
var idAlarmStream = idStream
.GroupByUntil(key => key, grp => grp.Throttle(alarmInterval(grp.Key)))
.SelectMany(grp => grp.IgnoreElements().Concat(Observable.Return(grp.Key)));
<edit 2:
Question: How do I start the timers immediately without waiting for the first events to arrive? That's the root problem in my question, I guess. For that end, I planned on sending off dummy objects with the IDs I know should be there. But as I write in following, I ended up with some other problems. Nevertheless, I'd think solving that too would be interesting.
Forwards with the other interesting parts then! Now, if I'd like to group a complex object like the following and group by the key as follows (won't compile)
var idAlarmStream = idStream
.Select(i => new { Id = i, IsTest = true })
.GroupByUntil(key => key.Id, grp => grp.Throttle(alarmInterval(grp.Key)))
.SelectMany(grp => grp.IgnoreElements().Concat(Observable.Return(grp.Key)));
then I get into trouble. I'm unable to modify the part about SelectMany, Concat and Observable.Return so that the query would work as before. For instance, if I make query as
var idAlarmStream = idStream
.Select(i => new { Id = i, IsTest = true })
.GroupByUntil(key => key.Id, grp => grp.Throttle(alarmInterval(grp.Key)))
.SelectMany(grp => grp.IgnoreElements().Concat(Observable.Return(grp.Key.First())))
.Subscribe(i => Console.WriteLine(i.Id + "-" + i.IsTest);
Then two events are needed before an output can be observed in the Subscribe. It's the effect of the call to First, I gather. Furthermore, I woul like to use the complex object attributes in the call to alarmInterval too.
Can someone offer an explanation what's going on, perhaps even a solution? The problem in going with unmodified solution is that the grouping doesn't look Ids alone for the key value, but also the IsTest field.
<edit: As a note, the problem probably could be solved firsly by creating an explicit class or struct and then that implements a custom IEquatable and secondly then using James' code as-is so that grouping would happen by IDs alone. It feels like hack though.
Also, if you want to count the number of times you've seen an item before the alarm goes off you can do it like this, taking advantage of the counter overload in Select.
var idAlarmStream = idStream
.Select(i => new { Id = i, IsTest = true })
.GroupByUntil(key => key.Id, grp => grp.Throttle(alarmInterval(grp.Key))
.SelectMany(grp => grp.Select((count, alarm) => new { count, alarm }).TakeLast(1));
Note, this will be 0 for the first (seed) item - which is probably what you want anyway.
You are creating an anonymous type in your Select. Lets call it A1. I will assume your idStream is an IObservable. Since this is the Key in the GroupByUntil you do not need to worry about key comparison - int equality is fine.
The GroupByUntil is an IObservable<IGroupedObservable<int, A1>>.
The SelectMany as written is trying to be an IObservable<A1>. You need to just Concat(Observable.Return(grp.Key)) here - but the the type of the Key and the type of the Group elements must match or the SelectMany won't work. So the key would have to be an A1 too. Anonymous types use structural equality and the return type would be stream of A1 - but you can't declare that as a public return type.
If you just want the Id, you should add a .Select(x => x.Id) after the Throttle:
var idAlarmStream = idStream
.Select(i => new { Id = i, IsTest = true })
.GroupByUntil(key => key.Id, grp => grp.Throttle(alarmInterval(grp.Key)
.Select(x => x.Id))
.SelectMany(grp => grp.IgnoreElements().Concat(Observable.Return(grp.Key)));
If you want A1 instead - you'll need to create a concrete type that implements Equality.
EDIT
I've not tested it, but you could also flatten it more simply like this, I think this is easier! It is outputing A1 though, so you'll have to deal with that if you need to return the stream somewhere.
var idAlarmStream = idStream
.Select(i => new { Id = i, IsTest = true })
.GroupByUntil(key => key.Id, grp => grp.Throttle(alarmInterval(grp.Key))
.SelectMany(grp => grp.TakeLast(1));
I'm trying to do a GroupBy and then OrderBy to a list I have. Here is my code so far:
reportList.GroupBy(x => x.Type).ToDictionary(y=>y.Key, z=>z.OrderBy(a=>a.Lost));
With the help of the last question I asked on linq I think the ToDictionary is probably unneeded, but without it I don't know how to access the inner value.
To be clear, I need to GroupBy the Type property and want the inner groups I get to be OrderBy the Lost property (an integer). I want to know if there is a better, more efficient way or at the least better then what I've done.
An explanation and not just an answer would be very much appreciated.
Yes, there is better approach. Do not use random names (x,y,z,a) for variables:
reportList.GroupBy(r => r.Type)
.ToDictionary(g => g.Key, g => g.OrderBy(r => r.Lost));
You can even use long names to make code more descriptive (depends on context in which you are creating query)
reportList.GroupBy(report => report.Type)
.ToDictionary(group => group.Key,
group => group.OrderBy(report => report.Lost));
Your code does basically the following things:
Group elements by type
Convert the GroupBy result into a dictionary where the values of the dictionary are IEnumerables coming from a call to OrderBy
As far as the code correctness it is perfectly fine IMO, but maybe can be improved in term of efficiency (even if depends on your needs).
In fact, with your code, the values of your dictionary are lazily evaluated each time you enumerate them, resulting in a call to OrderBy method.
Probably you could perform it once and store the result in this way:
var dict = reportList
.GroupBy(x => x.Type)
.ToDictionary(y => y.Key, z => z.OrderBy(a => a.Lost).ToList());
// note the ToList call
or in this way:
var dict = reportList.OrderBy(a => a.Lost)
.GroupBy(x => x.Type)
.ToDictionary(y => y.Key, z => z);
// here we order then we group,
// since GroupBy guarantees to preserve the original order
Looks fine to me. If you use an anonymous type instead of a Dictionary, you could probably improve the readability of the code that uses the results of this query.
reportList.GroupBy(r => r.Type)
.Select(g => new { Type = g.Key, Reports = g.OrderBy(r => r.Lost) });
Based on my previous question, I've trying now to have them in the following order using the same approach, OrderByDescending and ThenBy
Original (can be in any random order):
1:1
0:0
0:1
2:1
1:0
2:0
Output
2:0
1:0
0:0
2:1
1:1
0:1
as you can see, a is descending, and b being ascending. But I'm still not getting the right sort. Any ideas why? Thanks
Think to what you would do manually:
First you must sort the values by the 2nd part in ascending order
Then you must sort values having the same 2nd part, using the 1st part in descending order
Translated in LINQ it's pretty the same:
var sorted = arrayList
.Cast<string>()
.Select(x => x.Split(':'))
.OrderBy(x => x[1])
.ThenByDescending(x => x[0])
.Select(x => x[0] + ":" + x[1]);
To clarify a bit more, ThenBy/ThenByDescending methods are used to sort elements that are equal in the previous OrderBy/OrderByDescending, hence the code :)
arrayList.ToList().Select(i => { var split = i.Split(":".ToArray(),2));
return new { a = Int32.Parse(split[0]),
b = Int32.Parse(split[1}) };
})
.OrderByDescending(i => i.a)
.ThenBy(i => i.b)
From your question it is not clear whether you want the order-by's reversed (just swap them).
Work from there, perhaps rejoining
.Select(i => String.Format("{0}:{1}", i.a, i.b));
Good luck