及时计算组合 [英] Just in time computation of combinations

问题描述

我想对N个变量的三元组的每个组合做一些事情:

I want to do something with every combination of ternaries for N variables:

示例1:

T
F
U

示例2:

TT
FT
UT
TF
FF
UF
UU

是否有一种计算方法，但仅根据需要:例如:

Is there a way to compute this but only as needed: For example:

    var combinator = new Combinator<string>(2, {"T","F","U"});
    List<String> tt = combinator.Next(); 
    //tt contains {"T","T"}

推荐答案

您可以在迭代器方法中实现它:

You can implement it in an iterator method:

private IEnumerable<List<T>> Combinations<T>(int n, T[] values)
{
    if (n == 0) yield return new List<T>();
    else
    {
        foreach (var list in Combinations(n - 1, values))
            foreach (var item in values)
            {
                var items = new List<T>(list);
                items.Add(item);
                yield return items;
            }
    }
}

这会创建所有组合，但会以一种懒惰的方式进行.

This creates all combinations, but does it in a lazy way.

如果需要，可以这样创建Combinator类:

If you want you can create Combinator class like this:

class Combinator<T>
{
    IEnumerator<List<T>> enumerator;

    public Combinator(int n, T[] values)
    {
        enumerator = Combinations(n, values).GetEnumerator();
    }

    public List<T> Next()
    {
        return enumerator.MoveNext() ? enumerator.Current : null;
    }

    private IEnumerable<List<T>> Combinations<T>(int n, T[] values) { ... }
}

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