我有一个字符串列表
AAPL,28/03/2012,88.34,88.778,87.187,88.231,163682382
AAPL,29/03/2012,87.54,88.08,86.747,87.123,151551216
FB,30/03/2012,86.967,87.223,85.42,85.65,182255227
现在我想删除最后一条记录,如果它不包含使用LINQ的AAPL(符号名称)。
下面我写了我的代码,其中包含多行,但我想把它作为单行代码,
fileLines = System.IO.File.ReadAllLines(fileName).AsParallel().Skip(1).ToList();
var lastLine = fileLines.Last();
if (!lastLine.Contains(item.sym))
{
fileLines.RemoveAt(fileLines.Count - 1);
}
那么如何在单行linq查询中完成所有这些操作呢?
您可以使用三元运算符来决定要连接的尾部,如下所示。
fileLines
= fileLines.Take(fileLines.Count())
.Concat(fileLines.Last().Contains(item.sym) ? Enumerable.Empty
: new string[]{ item.sym });
您可以按照以下方式将其制定得更加简约。
fileLines
= System.IO.File.ReadAllLines(fileName)
.AsParallel()
.Skip(1)
.Take(fileLines.Count())
.Concat(fileLines.Last().Contains(item.sym) ? Enumerable.Empty
: new string[]{ item.sym });
.ToList();
话虽如此,这样的努力是值得怀疑的。延迟评估Linq扩展方法的积累很难调试。
我知道你需要简化过滤操作,从我在你的案例中看到,你只缺少一条信息(即当前项是否是枚举集合中的最后一项),这将有助于你定义你的谓词。我现在要写的东西似乎不是“简单的单行”;然而,它将是一个可重用的扩展,它将提供这条信息(以及更多),而无需执行额外的和不必要的循环或迭代。
最终产品将是:
IEnumerable<string> fileLines = System.IO.File.ReadLines(fileName).RichWhere((item, originalIndex, countedIndex, hasMoreItems) => hasMoreItems || item.StartsWith("AAPL"));
我写的类似LINQ的扩展是由微软的Enumerable在ReferenceSource启发的:
public delegate bool RichPredicate<T>(T item, int originalIndex, int countedIndex, bool hasMoreItems);
public static class EnumerableExtensions
{
/// <remarks>
/// This was contributed by Aly El-Haddad as an answer to this Stackoverflow.com question:
/// https://stackoverflow.com/q/54829095/3602352
/// </remarks>
public static IEnumerable<T> RichWhere<T>(this IEnumerable<T> source, RichPredicate<T> predicate)
{
return new RichWhereIterator<T>(source, predicate);
}
private class RichWhereIterator<T> : IEnumerable<T>, IEnumerator<T>
{
private readonly int threadId;
private readonly IEnumerable<T> source;
private readonly RichPredicate<T> predicate;
private IEnumerator<T> enumerator;
private int state;
private int countedIndex = -1;
private int originalIndex = -1;
private bool hasMoreItems;
public RichWhereIterator(IEnumerable<T> source, RichPredicate<T> predicate)
{
threadId = Thread.CurrentThread.ManagedThreadId;
this.source = source ?? throw new ArgumentNullException(nameof(source));
this.predicate = predicate ?? ((item, originalIndex, countedIndex, hasMoreItems) => true);
}
public T Current { get; private set; }
object IEnumerator.Current => Current;
public void Dispose()
{
if (enumerator is IDisposable disposable)
disposable.Dispose();
enumerator = null;
originalIndex = -1;
countedIndex = -1;
hasMoreItems = false;
Current = default(T);
state = -1;
}
public bool MoveNext()
{
switch (state)
{
case 1:
enumerator = source.GetEnumerator();
if (!(hasMoreItems = enumerator.MoveNext()))
{
Dispose();
break;
}
++originalIndex;
state = 2;
goto case 2;
case 2:
if (!hasMoreItems) //last predicate returned true and that was the last item
{
Dispose();
break;
}
T current = enumerator.Current;
hasMoreItems = enumerator.MoveNext();
++originalIndex;
if (predicate(current, originalIndex - 1, countedIndex + 1, hasMoreItems))
{
++countedIndex;
Current = current;
return true;
}
else if (hasMoreItems)
{ goto case 2; }
//predicate returned false and there're no more items
Dispose();
break;
}
return false;
}
public void Reset()
{
Current = default(T);
hasMoreItems = false;
originalIndex = -1;
countedIndex = -1;
state = 1;
}
public IEnumerator<T> GetEnumerator()
{
if (threadId == Thread.CurrentThread.ManagedThreadId && state == 0)
{
state = 1;
return this;
}
return new RichWhereIterator<T>(source, predicate) { state = 1 };
}
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
}
}
RichPredicate<T>
,可以被认为是Func<T, int, int, bool, bool>
提供有关每个项目的信息:
IEnumerable<T>
源中的索引(直接传递给RichWhere
的索引)。IEnumerable<T>
源中的最后一项。