A group blog from members of the VB team
- Visual Studio 2008 (Beta2 or Higher)
Categories: LINQ-To-Objects
Instructions:
- Open Visual Studio 2008, and Click ‘File/New Project’. Find and double-click the ‘Console Application’ Icon
- Add the following code:
Module Module1
Sub Main()
Dim primes = GetPrimesInRange(30, 50)
For Each n In primes
Console.WriteLine(n)
Next
Console.ReadLine()
End Sub
Function GetPrimesInRange(ByVal low, ByVal high) As IEnumerable(Of Integer)
Dim range = Enumerable.Range(1, high)
Return From num In range _
From num2 In range _
Where num Mod num2 = 0 _
Group By num Into Count() _
Where Count = 2 And num > low _
Select num
End Function
End Module
- The grouping is used to select only those numbers that have exactly two factors (i.e. they're prime)
Doesn't that only work if the low value is 0 or 1 ?
And isn't this a really innefficient way of doing this as there is no early exit for a given value once the count > 2.
BTW: I haven't got Beta 2 (what up with that ?) but
the code:
Where Count = 2 _
looks strange to me. Why does Count have () after it, yet when you refer to it on the next line it is as a single value not an array ?
I have to agree with the first comment. I have a general feeling about LINQ that it basically looks impossible to debug and is not intuitive. I have started to play with it in beta 1 and I have tried ScottGu's LINQ to SQL examples and every time I work with them I feel like they are useless in an enterprise application. For simple applications, they seem useful for querying simple collections. But in every example that I see an anonymous method and LINQ in I just fell yucky when I imagine trying to debug someone else's I-Used-A-Cool-Feature code.
Cheers,
Aaron
Thanks Bill, I've updated the sample so it works for any given range now. This isn't really intended to be a textbook example of calculating primes, so I'll happily concede that it's grossly inefficient :)
As far as the Count thing goes...there's parentheses on the first one because we're calling the Count() function, but we don't need it on the second one because that's a property. Since we didn't explicitly name our Group, the compiler inferred it's name to be Count. If you want to name the group you could do this instead:
Group By num Into Factors = Count() _
Where Factors = 2 And num > low _
Thanks!
Jonathan
The VB team just started a LINQ Cookbook series. These are great because they show you how you can easily
I agree that this example takes some time to understand. I believe LINQ is great when you just need a simple query. If you need something debuggable, you could do the same with extension methods from the System.Query.Sequence class and some explicitly defined lambdas. 3-4 times more code though.
It is great, however, that LINQ can do such things.
引用:LINQ菜谱,菜单3:在给定的范围内找到所有的素数(JonathanAneja) [原文作者]：JonathanAneja
[原文链接]：LINQCookbook,Reci...