如何“压缩"或“旋转"可变数量的列表?

问题描述

如果我有一个包含任意数量列表的列表，如下所示:

If I have a list containing an arbitrary number of lists, like so:

var myList = new List<List<string>>()
{
    new List<string>() { "a", "b", "c", "d" },
    new List<string>() { "1", "2", "3", "4" },
    new List<string>() { "w", "x", "y", "z" },
    // ...etc...
};

...有没有办法以某种方式将列表压缩"或旋转"成这样的东西?

...is there any way to somehow "zip" or "rotate" the lists into something like this?

{
    { "a", "1", "w", ... },
    { "b", "2", "x", ... },
    { "c", "3", "y", ... },
    { "d", "4", "z", ... }
}

显而易见的解决方案是做这样的事情:

The obvious solution would be to do something like this:

public static IEnumerable<IEnumerable<T>> Rotate<T>(this IEnumerable<IEnumerable<T>> list)
{
    for (int i = 0; i < list.Min(x => x.Count()); i++)
    {
        yield return list.Select(x => x.ElementAt(i));
    }
}

// snip

var newList = myList.Rotate();

...但我想知道是否有更简洁的方法，使用 linq 或其他方式?

...but I was wondering if there was a cleaner way of doing so, using linq or otherwise?

推荐答案

您可以滚动自己的 ZipMany 实例，该实例手动迭代每个枚举.在投影每个序列后，与使用 GroupBy 的序列相比，这可能在更大的序列上表现更好:

You can roll your own ZipMany instance which manually iterates each of the enumerations. This will likely perform better on larger sequences than those using GroupBy after projecting each sequence:

public static IEnumerable<TResult> ZipMany<TSource, TResult>(
    IEnumerable<IEnumerable<TSource>> source,
    Func<IEnumerable<TSource>, TResult> selector)
{
   // ToList is necessary to avoid deferred execution
   var enumerators = source.Select(seq => seq.GetEnumerator()).ToList();
   try
   {
     while (true)
     {
       foreach (var e in enumerators)
       {
           bool b = e.MoveNext();
           if (!b) yield break;
       }
       // Again, ToList (or ToArray) is necessary to avoid deferred execution
       yield return selector(enumerators.Select(e => e.Current).ToList());
     }
   }
   finally
   {
       foreach (var e in enumerators)
         e.Dispose();
   }
}

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