I have a enumeration of objects :
public IOrderedEnumerable<RentContract> Contracts {
get { return RentContracts.OrderByDescending(rc => rc.DateCreated); }
}
I have to compare a given RentContract instance with its previous RenContract instance on the list to highlight changes between the two objects, which is the most correct method to get the previous element ?
This is not possible directly. You can do it like this:
var input = new SomeClass[10]; //test data
var zipped = input.Zip(new SomeClass[1].Concat(input), (a, b) => { a, b });
var result = zipped.Where(x => x.b == null || x.a.DateCreated < x.b.DateCreated.AddHours(-1)); //some example
This solution is zipping the sequence with itself, but offset by one null element.
Related
I'm looking to combine many IObservable<bool> streams such that when the latest value for all of them is true, a true is emitted, and otherwise a false is emitted.
CombinedLast would allow me to build something like this for two streams easily, but a) I'm not sure the API easily allows thousands of streams to be combined and b) I'm not sure how efficient it would be even if it could.
All is kinda similar to what I want except I'm assuming that works over a single sequence and once false cannot dynamically changes back to true.
Also I need the values to be "distinct until changed", although the DistintUntilChanged operator may not be efficient for this?
I'm hoping for an O(1) algorithm.
A good approach for combining the latest is to start with a IObservable<IObservable<T>> and turn it in to a IObservable<T[]>. This becomes a very dynamic way to combine as many values you need.
Here's an extension method to do this:
public static IObservable<T[]> CombineLatest<T>(this IObservable<IObservable<T>> sources)
{
return
sources.Publish(ss =>
Observable.Create<T[]>(o =>
{
var composite = new CompositeDisposable();
var list = new List<T>();
composite.Add(
ss.Subscribe(source =>
{
var index = list.Count;
list.Add(default(T));
composite.Add(source.Subscribe(x => list[index] = x));
}));
composite.Add(ss.Merge().Select(x => list.ToArray()).Subscribe(o));
return composite;
}));
}
This nicely creates and tracks all subscriptions and uses a closure to define the index that each subscription needs to use to update its value in the list that is used for output.
If you use it like this:
var sources = new Subject<IObservable<bool>>();
var output = sources.CombineLatest();
output.Subscribe(x => Console.WriteLine(x));
var s1 = new Subject<bool>();
sources.OnNext(s1);
s1.OnNext(true);
var s2 = new Subject<bool>();
sources.OnNext(s2);
s2.OnNext(false);
var s3 = new Subject<bool>();
sources.OnNext(s3);
s3.OnNext(true);
s2.OnNext(true);
s1.OnNext(false);
Then you get this output:
If you change the definition of output to var output = sources.CombineLatest().Select(xs => xs.Aggregate((x, y) => x & y)); then you get the output that I think you're after:
True
False
False
True
False
I don't know how to do this in a classically functional way and still achieve O(1). This used mutable state, and is O(1) for observing each message, but O(n) for memory:
public IObservable<bool> CombineBooleans(this IObservable<bool>[] source)
{
return source.Select((o, i) => o.Select(b => (value: b, index: i)))
.Merge()
.Scan((array: new bool[source.Length], countFalse: source.Length), (state, item) =>
{
var countFalse = state.countFalse;
if (state.array[item.index] == item.value)
return (state.array, countFalse); //nothing to change, emit same state
else if (state.array[item.index]) //previous/current state is true, becoming false
{
countFalse++;
state.array[item.index] = false;
}
else //previous/current state is false, becoming true
{
countFalse--;
state.array[item.index] = true;
}
return (state.array, countFalse);
})
.Scan((countFalse: source.Length, oldCountFalse: source.Length), (state, item) => (countFalse: item.countFalse, oldCountFalse: state.countFalse))
.SelectMany(state =>
state.countFalse == 1 && state.oldCountFalse == 0
? Observable.Return(false)
: state.countFalse == 0 && state.oldCountFalse == 1
? Observable.Return(true)
: Observable.Empty<bool>()
)
.Publish()
.RefCount();
}
EDIT: Added .Publish().Refcount() to eliminate multiple-subscriber bugs.
I am trying to look up record using if I have the key then use Find if not use Where
private ApplicationDbContext db = new ApplicationDbContext();
public bool DeactivatePrice(int priceId = 0, string sponsorUserName = "")
{
var prices = db.BeveragePrices;
// if we have an id then find
if (priceId != 0)
{
prices = prices.Find(priceId);
}
else
{
prices = prices.Where(b => b.UserCreated == sponsorUserName);
}
if (prices != null)
{
// do something
}
return true;
I get the following error for
prices = prices.Find(priceId);
Cannot convert app.Model.BeveragePrices from system.data.entity.dbset
I am copying the pattern from this answer but something must be different.
Seems you forgot to put a predicate inside the Find function call. Also you need to do ToList on the collection. The second option is a lot more efficient. The first one gets the whole collection before selection.
Another note commented by #Alla is that the find returns a single element. So I assume another declaration had been made for 'price' in the first option I state down here.
price = prices.ToList.Find(b => b.PriceId == priceId);
Or
prices = prices.Select(b => b.PriceId == priceId);
I assume the field name is PriceId.
I have two tables with relationship many-to-many. Let's say A and B tables.
I also have List<List<int>> TagIdList with ids of B table's elements.
How can I find every elements from table A, who have all TagIdList[i] elements? I need just ids from table A, so it doesn't have to be all TASKS rows from table.
Example:
A: TASKS:
id: 1,2,3,4,5,6
B: TAGS:
id: 1,2,3,4
A-B links:
1-2; 1-3; 2-1; 2-2; 5-3; 5-4; 6-1; 6-6;
List<List<int>> TagIdList //(ids from TAGS)
TagIdList[0]= {2,3}
TagIdList[1]= {1}
TagIdList[2]= {2,6}
Result: (ids from TASKS)
i=0; -> 1
i=1; -> 2,6
i=2; -> null
I've tried:
List<int> tags = model.TagIdList[i].IdList; //I've got it from my View
List<TASKS> tasks = myEntity.TASKS.Where(t => t.TAGS == tags).ToList();
And I can't get tasks, because there was an error: Unable to create a constant value of type. Only primitive types are supported in this context.
Any ideas?
Your problem is here: myEntity.TASKS.Where(t => t.TAGS == tags)
If i understand the question correct you need something like this:
A compare method for two lists of int (Taken from here)
public static bool ScrambledEquals<T>(IEnumerable<T> list1, IEnumerable<T> list2) {
var cnt = new Dictionary<T, int>();
foreach (T s in list1) {
if (cnt.ContainsKey(s)) {
cnt[s]++;
} else {
cnt.Add(s, 1);
}
}
foreach (T s in list2) {
if (cnt.ContainsKey(s)) {
cnt[s]--;
} else {
return false;
}
}
return cnt.Values.All(c => c == 0);
}
Than use this method inside linq expression:
myEntity.TASKS.AsEnumerable().Where(t => ScrambledEquals<int>(t.TAGS.Select(tag=>tag.id).ToList(),tags))
I found solution. It's maybe not ideal, but works.
List<int> tags = model.TagIdList[i].IdList;
List<List<int>> whole_list = new List<List<int>>();
foreach (var t in tags) //I'm looking for every tasks' ids in every given tag
{
var temp = myEntity.TAGS.Find(t).TASKS.Select(task => task.Id).ToList();
whole_list.Add(temp);
}
//now I want first list from whole_list to compare with the other lists
List<int> collection = whole_list[0]; //collection it's tasks
whole_list.Remove(collection);
//I'm taking a part which is common to all lists from whole_list
foreach (List<int> k in whole_list)
{
var temp = collection.Intersect(k);
collection = temp.ToList();
}
//the collection is now what I wanted for TagIdList[i] - every task which has every tags from TagIdList[i].IdList
In the following image you can see where i put the breakpoint and then debugged two step. You can also see that both assignments worked great they have the same count and are the same.
However if I do the following. Run the exact same call but only break on the third line directly then this happnes
set.QuestionSet.Questions should have count of 8 BEFORE the assigment, so it seems it's not properly assigned for some reason. I suspect this has something to do with how I fetch my data from DB.
Question and QuestionSet are normal POCOs and here is the code for the entire method.
public IEnumerable<QuestionSet> SearchAndFilterQuestionsAndSets(string searchString, int nrPerPage, int page, out int totalSearchCount)
{
searchString = searchString.ToLower();
List<QuestionSet> finalList = new List<QuestionSet>();
var result = ActiveContext.QuestionSets
.Select(x => new
{
QuestionSet = x,
Questions = x.Questions.Where(
y =>
y.Description.ToLower().Contains(searchString)
).OrderBy(
z => z.Description
)
})
.ToList();
foreach (var set in result)
{
//If our search matched the set itself we load all questions
if (set.QuestionSet.Name.ToLower().Contains(searchString))
{
//we dont bring empty sets
if (set.QuestionSet.Questions.Count() > 0)
{
set.QuestionSet.Questions = set.QuestionSet.Questions.ToList<Question>().OrderBy(x => x.Description).ToList<Question>();
finalList.Add(set.QuestionSet);
}
}
//We had one or more questions matching the search term
else if (set.Questions.Count() > 0)
{
var b = set.Questions.ToList<Question>();
set.QuestionSet.Questions = set.Questions.ToList<Question>();
finalList.Add(set.QuestionSet);
}
}
totalSearchCount = finalList.Count();
return finalList.Skip((page - 1) * nrPerPage).Take(nrPerPage);
}
UPDATE
If I do this instead in the failing else if
var a = new QuestionSet();
a.Id = set.QuestionSet.Id;
a.Name = set.QuestionSet.Name;
a.Questions = set.Questions.ToList<Question>();
finalList.Add(a);
Then it works, so the problem lies within the anonymous object, but why does it work when i step through with debugger and not otherwise?? call me puzzled.
Could be something to do with Late binding of anonymous types
I have two value streams and one selector stream and I'd like to produce a result stream that alternates between the value streams based on the selector. The code below gives the right result, but I don't like it.
Does anyone have anything neater?
var valueStreamA = new BehaviorSubject<int>(0);
var valueStreamB = new BehaviorSubject<int>(100);
var selectorStream = new BehaviorSubject<bool>(true);
var filteredA = valueStreamA .CombineLatest(selectorStream, (a, c) => new { A = a, C = c })
.Where(ac => ac.C)
.Select(ac => ac.A);
var filteredB = valueStreamB.CombineLatest(selectorStream, (b, c) => new { B = b, C = c })
.Where(bc => !bc.C)
.Select(bc => bc.B);
var result = Observable.Merge(filteredA, filteredB);
result.Subscribe(Console.WriteLine);
valueStreamA.OnNext(1);
valueStreamB.OnNext(101);
selectorStream.OnNext(false);
valueStreamA.OnNext(2);
valueStreamB.OnNext(102);
selectorStream.OnNext(true);
This productes the following output:
0
1
101
102
2
I'd do something like this:
var a = new BehaviorSubject<int>(0);
var b = new BehaviorSubject<int>(100);
var c = new BehaviorSubject<bool>(true);
var valueStreamA = a as IObservable<int>;
var valueStreamB = b as IObservable<int>;
var selector = c as IObservable<bool>;
var result = selector
// for every change in the selector...
.DistinctUntilChanged()
// select one of the two value streams
.Select(change => change ? valueStreamA : valueStreamB)
// and flatten the resulting wrapped observable
.Switch();
result.Subscribe(Console.WriteLine);
a.OnNext(1);
b.OnNext(101);
c.OnNext(false);
a.OnNext(2);
b.OnNext(102);
c.OnNext(true);
Could do something like:
var xs = Observable.Interval(TimeSpan.FromSeconds(1)).Select(_ => Feeds.Xs);
var ys = Observable.Interval(TimeSpan.FromSeconds(1)).Select(_ => Feeds.Ys);
var selectorSubject = new Subject<Feeds>();
var query = from selector in selectorSubject
select from merged in xs.Merge(ys)
where merged == selector
select merged;
query.Switch().Subscribe(Console.WriteLine);
OnNext into your 'selectorSubject' to change it.
There are a few differences to your example, but easy to get around:
Your question involved a selector of type bool, whereas I have been lazy and reused the Feeds enum in order to allow me to do an easy equality check (where merged == selector).
You of course could simply do (where selector ? merged == Xs : merged == Ys), or something like that to evaluate each merged item and discard ones you don't care about (depending on your selector).
Specifically, you would probably want to select not just the integer, but an identifier of the feed. Consider using something like Tuple.Create(), so you get that info with each update:
{A - 1}, {B - 101} etc. Your where can then do:
where selector ? merged.Item1 == A : merged.Item1 == B //this maps 'true' to feed A
I also used a Switch, which will cause my sample streams to restart because they are not published.
You probably want to publish yours and Connect them (make them 'hot'), so a Switch like mine doesn't cause any new side effects in the subscription. You have a subject (which is hot), but the 'behaviour' part will replace the value you passed into the constructor. Publishing and connecting would prevent that.
Shout if you are still confused. This isn't a full answer, but might give you enough to think about.
Howard.
Now much closer to your original question:
void Main()
{
var valueStreamA = new BehaviorSubject<int>(0);
var valueStreamB = new BehaviorSubject<int>(100);
var selectorStreamA = valueStreamA.Select(id => Tuple.Create("A", id)).Publish();
var selectorStreamB = valueStreamB.Select(id => Tuple.Create("B", id)).Publish();
var selectorStream = new BehaviorSubject<bool>(true);
var query = from selector in selectorStream
select from merged in selectorStreamA.Merge(selectorStreamB)
where selector == true ? merged.Item1 == "A" : merged.Item1 == "B"
select merged.Item2;
query.Switch().Subscribe(Console.WriteLine);
selectorStreamA.Connect();
selectorStreamB.Connect();
//First we get 0 output (because we are already using stream A, and it has a first value)
valueStreamA.OnNext(1); //This is output, because our selector remains as 'A'
valueStreamB.OnNext(101); //This is ignored - because we don't take from B
selectorStream.OnNext(false); //Switch to B
valueStreamA.OnNext(2); //Ignored - we are now using B only
valueStreamB.OnNext(102); //This is output
selectorStream.OnNext(true); //Switch back to A.
}
Outputs:
0
1
102