Remove items earlier than a fix date and replace the first occurrence

Question

Based on a list of a typed class like

class CostCenter
{
    string center;
    string percentage;
    DateTime start;
}

I have a list of these elements like this:

• {"a", 70, "2019/11/10"}
• {"b", 30, "2019/11/10"}
• {"A", 40, "2018/10/05"}
• {"B", 60, "2018/10/05"}
• {"xx", 10, "2010/01/01"}
• {"yy", 90, "2010/01/01"}
• {"100", 50, "2009/07/03"}
• {"101", 50, "2009/07/03"}

taking into account that I want to get rid of all of the elements (list) that have a start date earlier or equal than a fixed(_earliestDate), also this fixed data will replace the earliest match. In this example the earliest date is "2010/11/01", therefore "xx" and "yy" start date is replaced to "2010/11/01". The output should be like this:

• {"a", 70, "2019/11/10"}
• {"b", 30, "2019/11/10"}
• {"A", 40, "2018/10/05"}
• {"B", 60, "2018/10/05"}
• {"xx", 10, "2010/11/01"}
• {"yy", 90, "2010/11/01"}

I have created the following code:

string dateToReplace = null;
bool earliestFound = false;
foreach (var item in list.OrderByDescending(x => DateTime.Parse(x.start)).ToList())
{
    if (earliestFound) //already marked, safe to remove others
    {
        list.Remove(item);
        continue;
    }

    if (!string.IsNullOrEmpty(dateToReplace)) //remaining matches
    {
        if (string.Equals(item.start, dateToReplace))
        {
            item.start = _earliestDate;
            continue;
        }
        else
        {
            earliestFound = true;
            list.Remove(item);
            continue;
        }
    }
    if (item.start <= _earliestDate) //first earliest or same, more required
    {
        dateToReplace = item.start;
        item.start = _earliestDate;
    }   
}

dateToReplace and earliestFound will be used as flags in the code to determine when to loop.

Somehow I think it is not the best option, do you have any suggestion on how to make it easier to understand or efficient?

Answer 1

To address the first part of your question : Removing items earlier that a given date.

Kiss is your best friend. either you reasign the list after a filtering or you remove all the item you don't want based on a predicate.

var inputs = new List<CostCenter> {
    new CostCenter("a", "70", "2019/11/10"),
    new CostCenter("b", "30", "2019/11/10"),
    new CostCenter("A", "40", "2018/10/05"),
    new CostCenter("B", "60", "2018/10/05"),
    new CostCenter("xx","10", "2010/01/01"),
    new CostCenter("yy", "90", "2010/01/01"),
    new CostCenter("100", "50", "2009/07/03"),
    new CostCenter("101", "50", "2009/07/03"),
};

var result1 = inputs.Where(x => x.start > DateTime.Parse("2010/01/01")).ToList();
var result2 = inputs.RemoveAll(x=> x.start <= DateTime.Parse("2010/01/01"));

Answer 2

First, you need to filter out all outdated items. Then, you can just take the first start, which is the earliest one, and then replace all with _earliestDate.

if (list == null)
    throw new ArgumentNullException(...);

// Get rid of unused items and sort:
var actual = list
    .Where(item => item.start >= _earliestDate)
    .OrderByDescending(item => item.start)
    .ToArray();

// If there are no matching items, then return empty array or throw exception
// It depends on your requirements
if (actual.Length == 0)
    return actual; 

// Take the least date in resulted array...
var earliestInActual = actual[actual.Length - 1]; 

// ...and replace it with _earliestDate
for (int i = actual.Length - 1; i >= 0; i--)
{
    if (actual[i].start != earliestInActual)
        break;

    item.start = _earliestDate;  
}

return actual;

Please, keep in mind that it will change original objects, therefore start dates will e updated in the original list as well.

Answer 3

Removing items from a list while using a foreach loop will cause it to lose it's position, you can however use a for loop or just do another loop removing all the flagged ones.

DateTime cutOffDate = DateTime.Parse("2010/11/01");

List<CostCenter> removeList = new List<CostCenter>();

foreach (var item in list.OrderByDescending(x => x.start).ToList())
{
    if (item.start <= cutOffDate) {
        removeList.Add(item);
    }
}

foreach(var remove in removeList)
{
    if (list.Contains(remove)) {
        list.Remove(remove);
    }
}

Answer 4

We can create new filtered list with given criteria

        DateTime threshold = new DateTime(2010, 11, 01);
        List<CostCenter> result = cc.Where(x => x.start.Date > threshold.Date).ToList();

POC : .netFiddle

Answer 5

For a List where 99% of the elements has to be removed, you will currently iterate throught the whole list to find all those elments and removing them.
Instead of clearing the List select only the part you wan't and stop as soon as you have Date inferior to earliestDate. With a "TakeWhile" approach just add a flag to know when to edit the element to fix the date.

public static IEnumerable<CostCenter> TakeWhileAndOneAfter
    (IEnumerable<CostCenter> source, DateTime splitDate )
{
    DateTime? earliestDate = null;
    bool earliestFound = false;

    foreach (CostCenter element in source)
    {
        if (!earliestFound && !(element.start > splitDate))
        {
            earliestFound = true;
            earliestDate = element.start;
        }

        if (earliestFound)
        {
            if (element.start < earliestDate)
            {
                break;
            }
            else
            {
                element.start = splitDate;
            }
        }
        yield return element;
    }
}