如何生成列表元素的组合<T>在 .NET 4.0 中 [英] How to Generate Combinations of Elements of a List<T> in .NET 4.0
问题描述
我有一个与回答的问题相似但不完全相同的问题 此处.
I have a question that is similar, but not identical, to the one answered here.
我想要一个函数从 n 个元素的列表中生成所有 k 元素的组合.请注意,我正在寻找组合,而不是排列,并且我们需要一个解决方案来改变 k(即,对循环进行硬编码是不行的).
I would like a function generate all of the k-combinations of elements from a List of n elements. Note that I am looking for combinations, not permutations, and that we need a solution for varying k (i.e., hard-coding the loops is a no-no).
我正在寻找一种解决方案,a) 优雅,b) 可以在 VB10/.Net 4.0 中编码.
I am looking for a solution that is a) elegant, and b) can be coded in VB10/.Net 4.0.
这意味着 a) 需要 LINQ 的解决方案是可以的,b) 那些使用 C#yield"命令的不是.
This means a) solutions requiring LINQ are ok, b) those using the C# "yield" command are not.
组合的顺序并不重要(即字典序、格雷码、你有什么),如果两者发生冲突,则优雅优先于性能.
The order of the combinations is not important (i.e., lexicographical, Gray-code, what-have-you) and elegance is favored over performance, if the two are in conflict.
(OCaml 和 C# 解决方案此处 将是完美的,如果它们可以在 VB10 中编码.)
(The OCaml and C# solutions here would be perfect, if they could be coded in VB10.)
推荐答案
在 C# 中生成组合列表作为 k 个元素的数组的代码:
Code in C# that produces list of combinations as arrays of k elements:
public static class ListExtensions
{
public static IEnumerable<T[]> Combinations<T>(this IEnumerable<T> elements, int k)
{
List<T[]> result = new List<T[]>();
if (k == 0)
{
// single combination: empty set
result.Add(new T[0]);
}
else
{
int current = 1;
foreach (T element in elements)
{
// combine each element with (k - 1)-combinations of subsequent elements
result.AddRange(elements
.Skip(current++)
.Combinations(k - 1)
.Select(combination => (new T[] { element }).Concat(combination).ToArray())
);
}
}
return result;
}
}
此处使用的集合初始值设定项语法在 VB 2010 中可用 (来源).
Collection initializer syntax used here is available in VB 2010 (source).
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