Calculate difference from previous item with LINQ

Question

I\'m trying to prepare data for a graph using LINQ.

The problem that i cant solve is how to calculate the \"difference to previous.

the result I expect is <

Answer 1

Modification of Jon Skeet's answer to not skip the first item:

public static IEnumerable SelectWithPrev
    (this IEnumerable source, 
    Func projection)
{
    using (var iterator = source.GetEnumerator())
    {
        var isfirst = true;
        var previous = default(TSource);
        while (iterator.MoveNext())
        {
            yield return projection(iterator.Current, previous, isfirst);
            isfirst = false;
            previous = iterator.Current;
        }
    }
}

A few key differences... passes a third bool parameter to indicate if it is the first element of the enumerable. I also switched the order of the current/previous parameters.

Here's the matching example:

var query = list.SelectWithPrevious((cur, prev, isfirst) =>
    new { 
        ID = cur.ID, 
        Date = cur.Date, 
        DateDiff = (isfirst ? cur.Date : cur.Date - prev.Date).Days);
    });