Hi I have the following code:
if (!_jobs.Any(j => j.Id == emailJob.Id))
{
}
This code should find any elements which satisfy the condition. So I would assume that it should return after finding the first element, something like this:
if (!_jobs.FirstOrDefault(j => j.Id == emailJob.Id) != null)
{
}
Resharper tries to simplify this LINQ expression to:
if (_jobs.All(j => j.Id != emailJob.Id))
{
}
This seems less efficient to me because it has to check that every single element satisifies the inverse condition.
Sorry if I'm just misunderstanding how LINQ works.
Joe
The two versions are mirror approaches and do exactly the same amount of work.
When you say "if none of the items satisfy this condition" (!Any), then all of the items have to be checked in order to get a definite answer.
ReSharper's suggestion is useful because it guides you towards using the method that more clearly shows what is going to happen: All jobs will have to be examined.
Both Any and All will stop looking immediately upon the condition failing.
If you are looking for more than just taking our word or anecdotal evidence, you can see from the source that this is what it is doing.
There is an extra inversion in the All method after the predicate is applied. This is a relative 0 performance impact, so code readability is the main concern.
If there is any job that does match the emailJob id, then the .Any() approach can abort early. By the same token, the .All() approach can stop working as soon as it finds a condition that's false, which will happen at the same job. The efficiency should be about the same.
Related
What is the difference between these two Linq queries:
var result = ResultLists().Where( c=> c.code == "abc").FirstOrDefault();
// vs.
var result = ResultLists().FirstOrDefault( c => c.code == "abc");
Are the semantics exactly the same?
Iff sematically equal, does the predicate form of FirstOrDefault offer any theoretical or practical performance benefit over Where() plus plain FirstOrDefault()?
Either is fine.
They both run lazily - if the source list has a million items, but the tenth item matches then both will only iterate 10 items from the source.
Performance should be almost identical and any difference would be totally insignificant.
The second one. All other things being equal, the iterator in the second case can stop as soon as it finds a match, where the first one must find all that match, and then pick the first of those.
Nice discussion, all the above answers are correct.
I didn't run any performance test, whereas on the bases of my experience FirstOrDefault() sometimes faster and optimize as compare to Where().FirstOrDefault().
I recently fixed the memory overflow/performance issue ("neural-network algorithm") and fix was changing Where(x->...).FirstOrDefault() to simply FirstOrDefault(x->..).
I was ignoring the editor's recommendation to change Where(x->...).FirstOrDefault() to simply FirstOrDefault(x->..).
So I believe the correct answer to the above question is
The second option is the best approach in all cases
Where is actually a deferred execution - it means, the evaluation of an expression is delayed until its realized value is actually required. It greatly improves performance by avoiding unnecessary execution.
Where looks kind of like this, and returns a new IEnumerable
foreach (var item in enumerable)
{
if (condition)
{
yield return item;
}
}
FirstOrDefault() returns <T> and not throw any exception or return null when there is no result
I am trying to get a count of this:
Model.Version.Where(model => model.revision != Model.revision).Count();
However it tells me in VS that I cannot use Lambda expressions.
The model is of type Documents, which is keyed to Version.
I need the count of any documents in the version table for the model documents where the revision is greater than the model revision.
This will either be a 0 or a 1, could sometimes be higher than 1 I suppose.
What am I doing wrong?
if (Model.Version.Where(model => model.revision > Model.revision).Count() > 0)
{
// do something
}
As others have said, your real code should be fine: it sounds like the problem was only that you were trying to execute this in the debugger instead of in normal code. Personally I'm always somewhat leery of trying to take things too far in the debugger - it can be useful of course, but if things behave unexpectedly, I'd always see whether the same code works as part of a real program, rather than assuming there's something fundamentally wrong with the approach. The debugger has to work under rather different constraints than the normal compilation and execution process.
Likewise, as others have said, it's better to use Any() than Count() > 0. However, cleaner yet is to use the overload of Any accepting a predicate:
if (Model.Version.Any(model => model.revision > Model.revision))
{
...
}
Note however that that's not quite the same as your initial predicate, which was asking for any versions which had a different revision rather than a higher revision. You may want:
if (Model.Version.Any(model => model.revision != Model.revision))
{
...
}
It's worth noting that in LINQ to Objects, using Any can have very real performance benefits over using Count() > 0. In providers which convert the query to a different form (e.g. SQL) there may be no performance benefit, but there's a clarity benefit in saying exactly what you're interested in - you don't really care about the count, you only care if there are any matching items.
I got in a discussion with two colleagues regarding a setup for an iteration over an IEnumerable (the contents of which will not be altered in any way during the operation). There are three conflicting theories on which is the optimal approach. Both the others (and me as well) are very certain and that got me unsure, so for the sake of clarity, I want to check with an external source.
The scenario is as follows. We had the code below as a starting point and discovered that some of the hazaas need not to be acted upon. So, starting with the code below, we started to add a blocker for the action.
foreach(Hazaa hazaa in hazaas) ;
My suggestion is as follows.
foreach(Hazaa hazaa in hazaas.Where(element => condition)) ;
One of the guys wants to resolve it by a more explicit form, claiming that LINQ is not appropriate in this case (not sure why it'd be so but he seems to be very convinced). He's solution is this.
foreach(Hazaa hazaa in hazaas) ;
if(condition) ;
The other contra-suggestion is supported by the claim that Where risks to repeat the filtering process needlessly and that it's more certain to minimize the computational workload by picking the appropriate elements once for all by Select.
foreach(Hazaa hazaa in hazaas.Select(element => condition)) ;
I argue that the first is obsolete, since LINQ can handle data objects quite well.
I also believe that Select-ing is in this case equivalently fast to Where-ing and no needless steps will be taken (e.g. the evaluation of the condition on the elements will only be performed once). If anything, it should be faster using Where because we won't be creating an extra instance of anything.
Who's right?
Select is inappropriate. It doesn't filter anything.
if is a possible solution, but Where is just as explicit.
Where executes the condition exactly once per item, just as the if. Additionally, it is important to note that the call to Where doesn't iterate the list. So, using Where you iterate the list exactly once, just like when using if.
I think you are discussing with one person that didn't understand LINQ - the guy that wants to use Select - and one that doesn't like the functional aspect of LINQ.
I would go with Where.
The .Where() and the if(condition) approach will be the same.
But since LinQ is nicely readable i'd prefer that.
The approach with .Select() is nonsense, since it will not return the Hazaa-Object, but an IEnumerable<Boolean>
To be clear about the functions:
myEnumerable.Where(a => isTrueFor(a)) //This is filtering
myEnumerable.Select(a => a.b) //This is projection
Where() will run a function, which returns a Boolean foreach item of the enumerable and return this item depending on the result of the Boolean function
Select() will run a function for every item in the list and return the result of the function without doing any filtering.
I'm trying to wrap my head around what the C# compiler does when I'm chaining linq methods, particularly when chaining the same method multiple times.
Simple example: Let's say I'm trying to filter a sequence of ints based on two conditions.
The most obvious thing to do is something like this:
IEnumerable<int> Method1(IEnumerable<int> input)
{
return input.Where(i => i % 3 == 0 && i % 5 == 0);
}
But we could also chain the where methods, with a single condition in each:
IEnumerable<int> Method2(IEnumerable<int> input)
{
return input.Where(i => i % 3 == 0).Where(i => i % 5 == 0);
}
I had a look at the IL in Reflector; it is obviously different for the two methods, but analysing it further is beyond my knowledge at the moment :)
I would like to find out:
a) what the compiler does differently in each instance, and why.
b) are there any performance implications (not trying to micro-optimize; just curious!)
The answer to (a) is short, but I'll go into more detail below:
The compiler doesn't actually do the chaining - it happens at runtime, through the normal organization of the objects! There's far less magic here than what might appear at first glance - Jon Skeet recently completed the "Where clause" step in his blog series, Re-implementing LINQ to Objects. I'd recommend reading through that.
In very short terms, what happens is this: each time you call the Where extension method, it returns a new WhereEnumerable object that has two things - a reference to the previous IEnumerable (the one you called Where on), and the lambda you provided.
When you start iterating over this WhereEnumerable (for example, in a foreach later down in your code), internally it simply begins iterating on the IEnumerable that it has referenced.
"This foreach just asked me for the next element in my sequence, so I'm turning around and asking you for the next element in your sequence".
That goes all the way down the chain until we hit the origin, which is actually some kind of array or storage of real elements. As each Enumerable then says "OK, here's my element" passing it back up the chain, it also applies its own custom logic. For a Where, it applies the lambda to see if the element passes the criteria. If so, it allows it to continue on to the next caller. If it fails, it stops at that point, turns back to its referenced Enumerable, and asks for the next element.
This keeps happening until everyone's MoveNext returns false, which means the enumeration is complete and there are no more elements.
To answer (b), there's always a difference, but here it's far too trivial to bother with. Don't worry about it :)
The first will use one iterator, the second will use two. That is, the first sets up a pipeline with one stage, the second will involve two stages.
Two iterators have a slight performance disadvantage to one.
So I've been using LINQ for a while, and I have a question.
I have a collection of objects. They fall into two categories based on their property values. I need to set a different property one way for one group, and one way for the other:
foreach(MyItem currentItem in myItemCollection)
{
if (currentItem.myProp == "CATEGORY_ONE")
{
currentItem.value = 1;
}
else if (currentItem.myProp == "CATEGORY_TWO")
{
currentItem.value = 2;
}
}
Alternately, I could do something like:
myItemCollection.Where(currentItem=>currentItem.myProp == "CATEGORY_ONE").ForEach(item=>item.value = 1);
myItemCollection.Where(currentItem=>currentItem.myProp == "CATEGORY_TWO").ForEach(item=>item.value = 2);
I would think the first one is faster, but figured it couldn't hurt to check.
Iterating through the collection only once (and not calling any delegates, and not using as many iterators) is likely to be slightly faster, but I very much doubt that it'll be significant.
Write the most readable code which does the job, and only worry about performance at the micro level (i.e. where it's easy to change) when it's a problem.
I think the first piece of code is more readable in this case. Less LINQy, but more readable.
How about doing it like that?
myItemCollection.ForEach(item => item.value = item.myProp == "CATEGORY_ONE" ? 1 : 2);
Real Answer
Only a profiler will really tell you which one is faster.
Fuzzy Answer
The first one is most likely faster in terms of raw speed. There are two reasons why
The list is only iterated a single time
The second one erquires 2 delegate invocations for every element in the list.
The real question though is "does the speed difference between the two solutions matter?" That is the only question that is relevant to your application. And only profiling can really give you much data on this.