[英]Linq continuous date takewhile
主要问题:仅当连续且仅当它们与某些属性匹配时,才如何按日期对元素进行分组?
详细资料 :
给定这种类型的对象:
public class MyObj
{
public DateTime Date { get; set; }
public string ConditionalValue1 { get; set; }
public int ConditionalValue2 { get; set; }
public int OtherValue { get; set; }
//for test purposes only, i would like to avoid doing this
public bool CompareFields(MyObj toBeCompared)
{
return ConditionalValue1 == toBeCompared.ConditionalValue1 &&
ConditionalValue2 == toBeCompared.ConditionalValue2;
}
}
我有它们的列表,填充如下:
//list is ordered for semplicity, but it won't be by default
//list is a result of a group by too
var myList = new[]
{
new MyObj { Date = new DateTime(2009, 10, 20), ConditionalValue1 = "A", ConditionalValue2 = 1, OtherValue = 1 },
new MyObj { Date = new DateTime(2009, 10, 21), ConditionalValue1 = "A", ConditionalValue2 = 1, OtherValue = 2 },
new MyObj { Date = new DateTime(2009, 10, 22), ConditionalValue1 = "B", ConditionalValue2 = 1, OtherValue = 3 },
new MyObj { Date = new DateTime(2009, 10, 23), ConditionalValue1 = "A", ConditionalValue2 = 2, OtherValue = 4 },
new MyObj { Date = new DateTime(2009, 10, 24), ConditionalValue1 = "A", ConditionalValue2 = 2, OtherValue = 5 },
new MyObj { Date = new DateTime(2009, 10, 25), ConditionalValue1 = "A", ConditionalValue2 = 1, OtherValue = 6 },
//Note the hole for day 26
new MyObj { Date = new DateTime(2009, 10, 27), ConditionalValue1 = "A", ConditionalValue2 = 1, OtherValue = 7},
};
想要的 (伪代码):
List =
{ ListOfObjects }, //First has date = "2009/10/20", last has = "2009/10/21"
{ ListOfObjects }, //First has date = "2009/10/22", last has = "2009/10/22" //reason: doesn't match Value1
{ ListOfObjects }, //First has date = "2009/10/23", last has = "2009/10/24"
{ ListOfObjects }, //First has date = "2009/10/25", last has = "2009/10/25" //reason: doesn't match Value2
{ ListOfObjects } //First has date = "2009/10/27", last has = "2009/10/27" //reason: there is no "2009/10/26" object
到目前为止的测试 :
//i pick the first element, to be compared with the next ones
var firstInList = myList.First();
//take while date is consequent and fields are compared
var partialList = myList
.TakeWhile(
(obj, index) => obj.Date == firstInList.Date.AddDays(index) &&
obj.CompareFields(firstInList)
);
这部分起作用,因为它当然只会为第一个匹配的元素返回正确的结果。
我可以Skip(count)
列表直到0个元素,但是我想使用IEqualityComparer
而不是CompareFields
方法来使用group by
。
您可能可以使用Enumerable.GroupBy 。 但是,我觉得它可能会遍历整个场景。 如果是这样,您总是可以创建自己的SequencedGroupBy扩展方法。
警告:不适用于并行扩展(AsParallel)
DateTime lastGroupDate = DateTime.MinValue;
DateTime lastDate = DateTime.MinValue;
IEnumerable<IGrouping<DateTime, MyObj>> groups = myList
.GroupBy(o =>
{
if (lastGroupDate != DateTime.MinValue &&
o.Date.AddDays(-1) == lastDate)
{
lastDate = o.Date;
}
else
{
lastGroupDate = o.Date;
lastDate = o.Date;
}
return lastGroupDate;
}
);
foreach(IGrouping<DateTime, MyObj> grouping in groups)
{
// grouping.Key == the grouped date / first in sequence
foreach(MyObj obj in grouping)
{
// obj.Date == actual date
}
}
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