嗯,我遇到的许多情况下,有一个IEnumerable是不够的。但是我不能确定的上述方法调用的性能。
我真正想问的是:
时了ToList /的ToArray的表现:
在一个O(n)的操作,它复制了IEnumerable到一个新的数组/列表?如果我叫了LINQ延伸方法的名单上,它有一个O(1)性能,如果我叫了ToList但为O(n),如果调用的ToArray(如果我原来的名单是一个数组相反)?
有些神奇发生,表现为O(1)?
大概意思是O(n),对吧?
解决方案是了ToList
/ 的ToArray
为O(n)操作的性能,它复制了的IEnumerable
来一个新的数组/列表?
是的。 了ToList
会更有效,因为它并不需要修剪内部缓冲器到合适的长度第一。
如果我打电话给名单上的LINQ延伸的方法,它有一个O(1)性能,如果我叫了ToList但为O(n),如果调用的ToArray(如果我原来的名单是一个数组相反)?
没有。对于这两个电话,一个的新的集合始终创建;这就是原始集合的浅表副本。这是更有效地调用了ToList
或的ToArray
任何的ICollection< T>
不是一个简单的的IEnumerable< T>
不落实的ICollection< T>
不过,与集合中的长度是已知的开始。 (这是通过检测执行时,虽然,你并不需要担心编译时类型)
大概意思是O(n),对吧?
假设哈希是明智的,这是O(N),是的。基本上,它正是你可能希望它的方式创建一个新的字典。
您可能需要阅读相应的职位,我Edulinq博客系列:
ToList ToArray ToDictionaryWell I encounter many situations where having an IEnumerable is not enough. However I'm unsure about the performance of the above method calls.
What I really want to ask is:
Is the performance of ToList/ToArray:
an O(n) operation which copies the IEnumerable to a new array/List ?If I called a linq extention method on a list, it has an O(1) performance if I call ToList but O(n) if call ToArray (and the opposite if my original list was an array) ?
Some magic happens and the performance is O(1)?
Probably to Dictionary is O(n), right ?
解决方案Is the performance of ToList
/ToArray
an O(n) operation which copies the IEnumerable
to a new array/List ?
Yes. ToList
is slightly more efficient, as it doesn't need to trim the internal buffer to the right length first.
If I called a linq extention method on a list, it has an O(1) performance if I call ToList but O(n) if call ToArray (and the opposite if my original list was an array) ?
No. For both calls, a new collection is always created; that's a shallow copy of the original collection. It's more efficient to call ToList
or ToArray
on any ICollection<T>
than on a simple IEnumerable<T>
which doesn't implement ICollection<T>
though, as with a collection the length is known to start with. (This is detected at execution time though; you don't need to worry about the compile-time type.)
Probably to Dictionary is O(n), right ?
Assuming the hash is sensible, it's O(N), yes. Basically it creates a new dictionary in exactly the way you'd probably expect it to.
You might want to read the corresponding posts in my Edulinq blog series:
ToList
ToArray
ToDictionary
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